Home

- AbcLinuxu.cz

1. eee 7 3 4 Grub Boot Loader Setup lt lt lt 4 aosan aa reor eean 8 Current Status of the GNU Hurd 8 1 Hurd p kernel Usage History lt lt lt lt lt a 8 2 Known Flaws and Limitations lt lt lt lt lt lt lt lt lt lt lt lt o 8 3 Linux related L4 Based Projects 9 Epilogue 24 24 24 25 25 25 26 27 27 27 27 28 28 30 30 30 31 31 32 32 33 34 34 34 34 35 35 36 37 38 40 40 40 41 41 42 42 44 45 46 Chapter 1 Prologue The most important program that runs on a computer is an operating system OS Every general purpose computer have an operating system to run other programs Operating systems perform basic tasks such as recognizing input from the keyboard sending output to the display screen keeping track of files and directories on the disk and controlling peripheral devices such as disk drives and printers Operating systems provide a software platform on the top of which other programs called application programs can run The application programs must be written to run on the top of a particular operating system Your choice of the operating system therefore determines a great extent of applications you can run The GNU Hurd is a set of servers or daemons in UNIX speak that work on the top of a microkernel u kernel for short Together they form a base of the GNU operating system It has been under development since 1991 by the GNU Pro
2. An active translator is a running translator process as introduced above You can set and remove active translators using the settrans a command The a option is necessary to tell settrans that you want to modify the active translator The settrans command takes three kind of arguments First you can set options for the settrans command itself like a to modify the active translator Then you set the inode you want to modify Remember that a translator is always associated with an inode in the directory hierarchy You can only modify one inode at a time If you do not specify any more arguments settrans will try to remove an existing translator How hard it tries depends on the force options you specify if the translator is in use by any process you will get device or resource busy error message unless you force it to go away 16 But if you specify further arguments it will be interpreted as a command line to run the translator This means the next argument is the filename of the translator executable Further arguments are options to the translator and not to the settrans command For example to mount an ext2fs partition you can run settrans a c mnt hurd ext2fs dev hd2s5 The c option will create the mount point for you if it doesn t exist already This does not need to be a directory by the way To unmount you would try settrans a mnt Passive Translators A passive translator is set and modified with the same sy
3. Homepage http os inf tu dresden de fiasco ux 45 Chapter 9 Epilogue This project describes the GNU Hurd operating system The Hurd project exists over fifteen years but its development is very very slow and stable version hasn t been released yet That is because main goals of the Hurd are especially stability security and freedom but it is not so easy to implement all these features together The Hurd is really different from other UNIX and UNIX like systems You can find a lot of stable and secure UNIXes but if you take the last mentioned goal freedom it is unique just for the Hurd The freedom here is understood as freedom of user so he is limited as less as possible so he can really experiment under OS as he wants for example to write his own filesystem server or device driver and he does not need administrator permitions and even do not need to reboot his computer On the other side the Hurd s concept of servers causes the whole system to be a bit slower The interprocess communication is in a trouble here and optimising it can greatly help Therefore there is an effort to replace original GNU Mach p kernel L4Ka Pistachio u kernel has solved the interprocess communication problem but core developers furthermore need new features that are required for successful implementation of the Hurd according to system vision so there is a discussion about another u kernel that will be more suitable for the Hurd There are thoughts abou
4. A translator for invoking host specific translators ifsock A server which only handles S_IFSOCK filesystem nodes for filesystems which don t do it themselves acting as a hook upon which to hang UNIX domain socket addresses pfinet and pflocal implement the socket API init The initialisation server for system boot procedures and basic runtime configura tions iso9660fs A server for iso9660 type filesystems commonly used on compact disks Server does the same as iso9660fs static only that iso9660fs static is a statically linked executable 18 mach defpager A standalone version of the Mach default pager magic A translator that returns the magic retry result MAGIC Normal end users proba bly need not to know much about it since it is used for example to facilitate terminal I O new fifo Alternative translator for named pipes nfs A translator that supports Sun s Network File System null The kitchen sink A server for lots of free space and countless numbers of zeroes implements dev null and dev zero password The Hurd standard password server pfinet A server for TCP IP which implements the IPv4 PF INET protocol family pflocal This server implements UNIX domain sockets Needed for pipes for example proc The process server that assigns PID s and process structures to tasks and manages all the process level stuff like wait bits of fork C library support proxy defpa
5. IDE 1st partition hd0s1 single user Before you boot the system you should check boot parameters of this item It should be like this root hd0 0 kernel boot gnumach gz s root device hd0s1 module hurd ext2fs static multiboot command line fkernel command line X host priv port fhost port device master port device port X exec server task fexec task T typed root task create task resume module lib ld so 1 hurd exec exec task task create First command indicates filesystem second command path to the kernel on that root fs with single user flag and device name for fs after boot Last two commands loads the root filesystem server and the exec server This is done using Grub s boot module capability The var are filled in by GNU Mach For more informations about server s options try servername help Note that we need to choose single user mode because Hurd installation is incomplete and system need to be set up before boot to multi user mode Once you are presented with a shell prompt after boot and any time that the Hurd is in single user mode it is necessary to set the terminal type export TERM mach Be warned that Ctrl C and family will not work in single user mode We can now run the native install script This will configure the packages and set up several important translators and device files native install Before the script terminates it need to choose timezon
6. accessible only by the kernel Each thread is further associated with a page fault handler and an exception handler These are separate threads that are set up to handle page faults and other exceptions caused by the thread Each thread in an address space has its own stack A thread s stack address is explicitly specified during thread creation It is up to the thread to determine where to place its heap 26 this is a design issue but typically several threads in the same address space will share a heap L4 also distinguishes between privileged and non privileged threads Any thread be longing to the same address space as one of the initial threads created by the kernel upon boot time is treated as privileged Some system calls can only be executed by privileged threads Threads can be created as active or inactive threads Inactive threads do not execute but can be activated by active threads that execute in the same address space They are typically used during the creation of address spaces A thread created active starts executing immediately after it is created The first thing it does is execute a short receive operation waiting for a message from its pager This message will provide the thread with an instruction and stack pointer and allow it to start executing its code 5 4 1 Tasks A task is the set of of threads sharing an address space The terms task and address space are often used interchangeably although strictly speaki
7. installation of the Debian GNU Hurd to x86 computer It is to date March 31 2006 and the guide expects K10 version of the distribution 7 1 About Debian GNU Hurd The GNU Hurd is a totally new operating system being put together by the GNU group In fact the GNU Hurd is the final component which makes it possible to built an entirely GNU OS and Debian GNU Hurd is going to be one such possibly even the first GNU OS Debian is a free operating system Currently stable version of Debian is only available with Linux kernel but there are a non Linux ports such as Debian GNU NetBSD Debian GNU kFreeBSD and Debian GNU Hurd as well Because Debian GNU Hurd is still under active development therefore hasn t been officially released yet and won t be for some time 7 2 Native Installation 7 2 1 Introduction This section is focused to the Debian GNU Hurd installation on x86 computer without any operating system You can have already some OS installed but you need about 2 GB it depends whatever you want to install of free space on your hard drive There are many ways how to install Hurd You can use e a set of four CD s e a one DVD e or a mini CD for network installation The best way is to use the first CD from the set for the first part of install process and then choose network install for the second one part That is because Hurd is still unstable and TIf you have some it is highly recommended to do complete backup of y
8. it no longer has a valid mapping for that region Figure 5 5 shows an example of the situation before and after a region of address space A has been granted to address space B Unlike revocation of mappings a granter cannot revoke a grant as it no longer has access to the page Before Grant After Grant Address Space B Address Space A Address Space A Physical Memory A Physical Memory im Figure 5 5 A region of one address space is granted to another Address Space B Note that for security reasons when mapping or granting memory the receiver must always explicitly agree to accept maps or grants Mapping and granting of memory are implemented using L4 s IPC mechanism In order to map memory a mapper sends the intended mappee a message containing a map item specifying the region of memory to be mapped The mappee must explicitly specify that it 29 is willing to receive mappings It also specifies where in its own address space the memory should be mapped The receiver does not actually have to do anything with the received map item or grant item The mapping is performed as a side effect of the IPC Note that although mappings apply to an address space map messages are sent via IPC to a thread in that address space 5 7 Interprocess Communication IPC Message passing is the basic interprocess communication IPC mechanism in L4 It allows L4 threads in separate address spaces to communicate by sending
9. of other threads in the same address space which may be phys ically possible if some are mapped to memory locations is illegal and can have undefined effects on all threads of the current address space 5 3 2 Address Space An address space contains all the data that is directly accessible by a thread It consists of a set of mappings from virtual to physical memory This set of mappings is partial in the sense that many mappings may be undefined making the corresponding virtual memory inaccessible Figure 5 1 shows an example of how the virtual memory in an address space may map onto physical memory The regions of virtual memory that are not mapped are inaccessible to threads running in that address space Typically an address space will contain a number of standard regions These include the text data heap and stack regions The text region contains program code the data region contains preinitialised data used by the program the heap region is used for dynamically allocated data and the stack region is used for storing temporary data during execution 25 Mapped Memory Unmapped Memory Address Space Mapping Physical Memory Figure 5 1 Mapping of virtual memory to physical memory The text and data regions usually have a fixed size and do not change during execution of a program The heap and stack regions on the other hand can grow or shrink while the program is executing Note that by convention on most architectures the he
10. order to be able to handle a received message the receiver must explicitly agree to accept messages of that type The acceptor is used to specify which typed items will be accepted when a message is received If an acceptor specifies that map or grant items are accepted then it also specifies where the associated memory will be mapped in the receiver s address space If an acceptor specifies that string items are accepted then any string items received are placed in the buffer registers BRs BRs are registers in the sense that they are per thread objects and can only be addressed directly not indirectly through pointers BRs are static objects like TCRs i e they keep their values until explicitly modified BRs can be mapped to either special registers or to memory locations There are 34 BRs numbered from BRy to BR33 inclusive The acceptor is always placed in BR Any string items received are placed in the BRs starting at BR 5 7 4 Send and Receive Messages are sent and received through the IPC system call IPC is the fundamental operation for inter process communication and synchronization It can be used for intra and inter address space communication All communication is synchronous and unbuffered a message is transferred from the sender to the recipient if and only if the recipient has invoked a corresponding IPC operation The sender blocks until this happens or until a period specified by the sender has elapsed without the destina
11. receive phase are included the destination thread is specified as a local thread ID the destination thread runs on the same processor there is a short receive timeout and the IPC includes no map grant operations These restrictions allow LIPC to be up to an order of magnitude faster than regular IPC 31 Chapter 6 GNU Mach and L4 Differences As you can see in previous chapters 4 and 5 there are some differences between these two p kernels Currently the Hurd runs on Mach but as mentioned in chapter 3 there is an effort to replace Mach with L4 or another u kernel because Mach has some serious lacks 6 1 GNU Mach GNU Mach is big and slow more than 100 system calls Mach runs only on 1A32 systems IPC e A Mach IPC message is sent to a task instead of a thread This is because ports are shared by all the threads in a task e IPC is asynchronous messages sent by IPCs are queued in kernel level buffers Mach acts as a central object server providing not only the facility but also defining the policy This policy makes IPC convenient but slow Message buffering requires in copy and out copy Flexible data types require costly message analysis The port right system is very heavyweight VMM e Mach acts as a central virtual memory management server not only providing access to the physical memory but also defining the policy of virtual memory management It doesn t know enough about ho
12. threads are now supported 2003 L4Ka Pistachio 0 1 is released 2003 Ext2fs without the 2GB limit in alpha stage 2004 Ext2fs without the 2gb limit reach release candidate 2005 Ext2fs without the 2gb in Debian GNU Hurd 2005 First program running on L4Hurd 2005 Initial Gnome port 2005 Debian GNU Hurd K10 series is released 14 3 5 Servers and Translators 3 5 1 Translator Concept Before we take a closer look at translators let us consider regular filesystems A filesystem is a store for a hierarchical tree of directories and files You access directories and files by a special character string the path Furthermore there are symbolic links to refer to one file at several places in the tree there are hard links to give one and the same file several names There are also special device files for communication with the hardware device drivers of the kernel and there are mount points to include other stores in the directory tree Then there are obscure objects like FIFOs Although these objects are very different they share some common properties for example they have all an owner and a group associated with them as well as access rights permissions This information is written in inodes This is actually a further commonality Every object has exactly one inode associated with it hard links are somewhat special as they share one and the same inode Sometimes the inode has further information stored in it For example the inode can co
13. very cheap When hardware is involved a translator usually starts to communicate with the kernel to get the data from the hardware However if no hardware access is necessary the kernel does not need to be involved For example dev zero does not require hardware access and can therefore be implemented completely in user space Symbolic Links A symbolic link can be seen as a translator Accessing the symbolic link would start up the translator which would forward the request to the filesystem that contains the file the link points to However for better performance filesystems that have native support for symbolic links can take advantage of this feature and implement symbolic links differently Internally accessing a symbolic link would not start a new translator process However to the user it would still look as if a passive translator is involved see below for an explanation what a passive translator is Because the Hurd ships with a symlink translator any filesystem server that provides support for translators automatically has support for symlinks and firmlinks and device files etc This means you can get a working filesystem very fast and add native support for symlinks and other features later 3 5 3 Passive and Active Translators There are two types of translators passive and active They are really completely different things so don t mix them up but they have a close relation to each other Active Translators
14. view The data is simply copied to the receiver The p kernel associates no semantics with it The typed items section contains typed data such as string items map items and grant items Map items and grant items were introduced earlier and are used to map and grant memory String items are used for passing string data by reference 5 7 2 Message Registers IPC messages are transferred using message registers MRs A sender writes a message into the message registers associated with its own thread and a receiver reads the message out of the message registers associated with its thread Each thread has 64 MRs numbered M Ro to MRgz3 inclusive A message can use some or all MRs to transfer untyped words and typed items The message tag is always transfered in M Rg MRs are transient read once virtual registers Once an MR has been read its value is undefined until the MR is written again The send phase of an IPC implicitly reads all MRs the receive phase writes the received message into MRs MRs can be implemented as either special purpose hardware registers general mem ory locations or general purpose hardware registers It is generally up to the particular L4 implementation and the hardware that it is implemented on whether any MRs are 30 implemented as hardware registers and if so which ones For example in the MIPS imple mentation M R to M Ry are implemented as hardware registers 5 7 3 Acceptor and Buffer Registers In
15. BRNO UNIVERSITY OF TECHNOLOGY FACULTY OF INFORMATION TECHNOLOGY q if System GNU Hurd on Architecture x86 Bachelor Project 2006 Zbyn k Michl System GNU Hurd on Architecture x86 Submitted in partial fulfillment of the requirements for the degree of Bachelor of Information Technology at Brno University of Technology Faculty of Information Technology April 27 2006 Zbyn k Michl 2006 The author thereby grants persmission to reproduce and distribute copies of this document in whole or in part to Brno University of Technology Faculty of Information Technology Declaration I hereby declare that I have created this project on my own and under the supervision of Ing Tom s Ka p rek I have listed all information sources which I used Zbyn k Michl April 27 2006 Abstract This project focuses on the GNU Hurd operating system The first part presents concept of the Hurd Mach and L4 microkernels that create a base of the whole system The second part describes a few installation procedures of the Hurd on x86 architecture Current status of the Hurd and presumption of the future development is also mentioned Keywords Coyotos Debian GNU GPL Grub Hurd Hurd NG kernel kernel space L4 L4 Sec Mach microkernel ngHurd operating system Pistachio POSIX server translator UNIX user space Acknowledgements First I would like to thank my supervisor In
16. Me RAR ek olo K oe do k eS 5 4 2 Identifying Threads and Address Spaces lt lt DE Communication cados c de pen a e a bo A 5 5 1 Communication Within an Address Space 5 5 2 Communication Between Address Spaces lt lt lt 44 D6 Memory Mapping loro rss pa a o ee e Bn el Je 6 5 7 Interprocess Communication IPC lt 44 Sahel o be Soho Micke SM eee ee BO ak Ve ny Ree ee bee ee Bate ca Ojo Message Registers os a RR ae 5 7 3 Acceptor and Buffer Registers lt lt e 000 5 5 74 Send and Receive 2 64 eee we u dk eba d 6 GNU Mach and L4 Differences EL GNU Mach comida Sel OR e eee 800K a a ode 6 Be LE oko E eee Be Eas Oe ee ee NA 7 GNU Hurd Installation 71 About Debian GNU Hurd 25455284444 Soh ad BREE d iei 7 2 Native Installation lt lt lt lt lt lt lt lt 4 lt 44 maL Introduction z ecs s ovocn ds wwe e a eS a oku e daa ale CDs Preparations lt lt dg Bod wade A Obj bn e k E a 7 2 3 Base System Installation Stage One 7 2 4 Base System Installation Stage Two 2 7 2 0 System Configutali d ecos osas a ke Pao Final WORKS o 38 js aa a te a Sew a BAe oe eA ee k Te Cross Installati n s s 2244 00 424 45 be eee wae he eae fol Introduction csc a 5258 a eRe aa 192 Install Preparation 26 6 2 ao ar dol enk 7 3 3 Base System Installation lt lt lt lt lt lt lt lt lt lt lt 4
17. There is shown Hurd s u kernel usage history and Hurd s releases overview Next you can find in this chapter some flaws and limitations of current Hurd system Some Linux related projects are mentioned as well Chapter 2 Monolithic Kernels and u kernels 2 1 What is a Kernel Since the operating system provides an hardware abstraction layer and since it must provide resource sharing every access to the hardware from user programs should be done through the operating system To enforce this in a secure way and prevent malicious or buggy applications to mess up with the hardware a protection is needed at the hardware level Hardware must provide at least two execution levels Kernel mode In this mode the software has access to all the instructions and every piece of hardware User mode In this mode the software is restricted and cannot execute some instruc tions and is denied access to some hardware like some area of the main memory or direct access to the IDE bus So we define two spaces at software level Kernel space Code running in the kernel mode is said to be inside the kernel space User space Every other programs running in user mode is said to be in user space 2 1 1 Monolithic Kernel Based Systems This is the traditional design of UNIX systems Every part which is to be accessed by most programs which cannot be put in a library is in the kernel space e Device drivers e Scheduler e Memo
18. address space it is easiest and most ef ficient to use shared memory Threads in the same address space automatically share memory so they do not have to make use of L4 s memory mapping facilities As long as both threads agree on which shared memory region or variables to use they are free to communicate in this way 27 When threads communicate using shared memory it is necessary to avoid race condi tions This is best done by enforcing mutual exclusive access to shared memory Note that L4 does not provide any mutual exclusion primitives such as semaphores to do this it is expected that these are provided at user level possibly using mechanisms provided by the underlying hardware Various implementations of user level mutual exclusion primitives are available It is possible for threads in the same address space to communicate using IPC The main use for this is thread synchronisation Addressing a thread in the same address space can be done using local or global thread Ids 5 5 2 Communication Between Address Spaces When communicating between address spaces i e when threads in different address spaces communicate with each other both the use of shared memory and IPC are valid approaches IPC is generally used for smaller messages and synchronisation while shared memory is used to exchange larger amounts of data Communicating between address spaces using shared memory requires that all commu nicating threads have acce
19. ague Czech Republic 2004 online cit 2006 04 23 URL http avc sh cvut cz archiv index php id 1041krid 39koffset Og select 0penWeekend Known Limitations and Fixes of Hurd The Hurd Wiki online cit 2006 04 23 URL http hurd gnufans org bin view Hurd KnownHurdLimits GOLUB D B DEAN R W FORIN A RASHID R F Unix as an Application Program In Proceedings of the Summer 1990 USENIX Conference June 1990 online cit 2006 04 23 URL http citeseer ist psu edu golub90unix html KUZ I L4 User Manual API Version X 2 2004 National ICT Australia online Version 1 9 September 23 2004 cit 2006 04 23 URL http ertos nicta com au publications papers L4UM_x 2 pdf 47 10 11 12 The L4 Microkernel Family Operating Systems Group TU Dresden online cit 2006 04 28 URL http os inf tu dresden de L4 MIGNOT G L The GNU Hurd In Libre Software Meeting July 5 9 Dijon France 2005 online cit 2006 04 23 URL http kilobug free fr hurd pres en abstract abstract pdf URL http kilobug free fr hurd pres en slides slides html BRINKMANN M The GNU Hurd Lessons and Perspective In Open Weekend 2004 Collection of Talks Prague Czech Republic 2004 p 37 40 online rev 2004 10 03 cit 2006 04 23 URL http www openweekend cz download sbornik_ow2004 pdf 48
20. ap grows up toward high memory while the stack grows down toward lower memory Figure 5 2 shows a typical arrangement of these regions in an address space Note that L4 does not enforce any specific layout of address spaces The layout is generally determined by a combination of the compiler and higher level operating system services built on top of L4 Address Space e is Jr 0x0 Oxfffff Figure 5 2 A typical address space layout 5 4 Threads A thread is the basic execution abstraction in L4 L4 threads are light weight and cheap to manage The light weight thread concept together with fast IPC are the keys to the efficiency of L4 and OS personalities running on top of L4 A thread is always associated with a specific address space The relationship between threads and address spaces is 1 n that is a thread is associated with exactly one address space while any address space can have many threads associated with it Each thread has its own set of virtual registers called the thread control registers TCRs These virtual registers are static they keep their values until explicitly modified and store the thread s private state e g parameters for IPCs scheduling information the thread s identifiers etc They are stored in an area of the address space called the thread control block TCB The TCB is actually split into two parts there is the user TCB UTCB which is accessible by the thread and the kernel TCB KTCB which is
21. atures Mach does guite a lot of things for a u kernel e It handles tasks as containers a task contains memory areas threads IPC rights A complex IPC system The virtual memory layer with an LRU decision algorithm e A basic scheduler e Device drivers 43 GNU Mach GNU Mach was derived from OSF Mach The last stable version is 1 3 and maybe it is the last of the 1 x branch The new 2 0 version might be released sometime in the future uses the OSKit framework for device drivers allowing nearly all drivers from Linux 2 2 to be ported easily 21 4 4 Interprocess Communication IPC IPC with Mach is based on ports A port is kernel managed entity which can be seen as a message queue with one task having a receive right on the port and any number of tasks having send rights to the port The task having the receive right can read the messages sent by the tasks having send rights Receive rights can be given to another task and send rights can be given destroyed or duplicated rights are sent across tasks using IPCs There is also a special send once right which can be used only once and is often used when waiting for an answer Communication across ports is asynchronous a sending thread is not blocked until the receiving task received the message This implies that all IPC messages must be copied either logically or physically inside the kernel and queued there which slows down the IPC a lot 4 5 Memory Handling
22. ce Figure 2 1 Design of monolithic kernel based system 2 1 2 kernel Based Systems Principles Only parts which really require to be in a privileged mode are in kernel space IPC Inter Process Communication Basic scheduler or scheduling primitives Basic memory handling Basic I O primitives 11 Many critical parts are now running in user space e The main part of scheduler e Memory handling e File systems e Network stacks Mono server Systems A single user space program server handles everything that belonged to the kernel The two most common examples are MachOS and L Linux a port of Linux as a user space server on top of the L4 microkernel This split allows better hardware independence of the operating system itself a slightly easier development and a limited improvement in overall security since the code running in the user space the server cannot directly access the hardware But this design still has most drawbacks of monolithic systems if the monolithic server crashes the whole system crashes it is impossible to add code to it without being root changing most of the code requires a reboot etc Multi server Systems All features are now split into a set of communicating processes with each of them handling only a very specific task like a TCP IP server or an ext2fs server This modularity allows to replace components easily an easier developement a far better fault tolerance since a crash of o
23. cfdisk for that Partition table should look like this Name Flags Part Type FS Type Size MB hda1 Boot Primary Linux ext3 1800 00 hda2 Primary Linux swap 128 00 hda3 Primary Linux 1800 00 On hda1 our GNU Linux is installed on hda2 there is swap that we can share with both systems and hda3 will be used for GNU Hurd Now we can put ext2 file system on created partition mke2fs b 4096 o hurd dev hda3 40 The o hurd is necessary so that the system know that it is safe to setup translators Review etc crosshurd sources list gnu Package sources should be the same as for native installation see content of the etc apt sources list file in 7 2 6 Installing More Packages Because the list of required packages for the GNU Hurd in crosshurd package is quite outdated it is necessary to edit etc crosshurd packages common file and replace these names of packages libdb2 libdb3 libreadline4 readline common slang libslangl Now we need to mount formatted partition mkdir hurd mount t ext2 dev hda3 hurd 7 3 3 Base System Installation Finally we can start GNU Hurd installation cd hurd crosshurd Choose the target directory to be hurd the target system to be gnu leave the target CPU at 1586 and say yes for a usr gt symlink 7 3 4 Grub Boot Loader Setup It is necessary to add GNU Hurd to Grub boot menu after installation We can use prepared record from the usr share doc grub examples m
24. disk I chose this variant Name Flags Part Type FS Type Size MB hda1 Boot Primary Linux 1800 00 hda2 Primary Linux swap 128 00 If you have already swap partition you can share it with another OS e g Linux and you don t have to create another one Note that partition names in cfdisk are in the Linux format because installer uses Linux kernel GNU Mach that Debian GNU Hurd uses enumerates disks starting at zero IDE drives are prefixed with hd while SCSI disks are prefixed with sd Like Linux drives are number by their position on the controller For instance the primary master is hdO and the secondary slave is hd3 Partitions use the BSD slice naming convention and append sM to the drive name to indicate a given partition M is a one not zero based index For example Mach s hd0s1 hd0s2 corresponds to Linux s hdal hda2 The next step is Initialize and Activate a Swap Partition We can skip bad blocks scan and then choose dev hda2 as a swap partition Then Initialize a GNU Hurd Partition Again skip bad blocks scan use dev hda1 and then mount this partition as root The last step is Install the Base System We select CD ROM installation method and Choose from the list Then the installer copy base system to your hard drive And finally Reboot the system 35 7 2 4 Base System Installation Stage Two Now we replace CD1 with the Grub CD and boot from it Then shows Grub boot menu We choose fourth item GNU Hurd
25. e so we can choose 8 for Europe and type e g Prague Then the script will indicate that it needs to be run a second time Follow its instructions and reboot using the reboot command Again go into single user mode and put into the command line export TERM mach native install In debconf configuration you can choose default values Dialog and then Medium priority Maybe installer ask you for another questions but everything is nicely described Installation of the base system and configuration of basic packages is now complete 36 7 2 5 System Configuration Other File Systems Next edit etc fstab to add any additional filesystems as well as swap space So we add at least our swap space partition that we already created After that etc fstab will look like this lt file system gt lt mount point gt lt type gt lt options gt lt dump gt lt pass gt dev hd0s1 ext2 rw 1 1 dev hd0s2 none swap none 0 0 Note that nano is the only editor installed by the the base distribution You can use simply cat as well Remember to create any devices using the MAKEDEV command cd dev MAKEDEV hdO hd0s1 hd0s2 hd2 hd0 is whole hard drive primary master hd0s1 is our root filesystem hd0s2 is our swap partition and hd2 is CD ROM drive you can replace it with your CD ROM device name To mount an NFS filesystem hurd nfs translator is used When run as non root the translator will connect to the server using a port abo
26. e B uses virtual address 0x2001000 A mapper that is the thread making the memory available retains full control of the mapped region of memory In particular the mapper is free to revoke a mapping at any time Revoking a mapping is called unmapping After a mapping has been revoked the receiver of that mapping the mappee can no longer access the mapped memory This is shown in figure 5 4 Here a thread in address space B can no longer access the region of virtual memory that used to contain the mapped memory Access to mapped memory is limited by the access permissions set by the mapper These access permissions specify read permission write permission and execute permission and determine how the mappee can access the mapped memory Note that a mapper cannot grant access rights that it does not itself have Thus if a thread does not have write access 28 0x2001000 Address Space B 0x1000000 Address Space A Physical Memory Figure 5 3 Two address spaces sharing a region of memory Address Space B Address Space A Physical Memory Figure 5 4 A shared region of memory is unmapped to a particular region of memory it cannot map that memory with write permission into another address space It is also possible for a region of one address space to be granted to another address space Granting differs from mapping in that after the grant has succeeded the granter loses access to that region of its address space i e
27. enu 1st file and copy it to the boot grub menu 1st file The record should be the same as in 7 2 4 only with another root device name in our case hd0s3 Note that the record is prepared for mutli user mode so it is recommended leave it as is When you need to boot to single user mode you can manually add s option to the kernel in the boot menu It is time to reboot the system In the boot menu we choose GNU Hurd that we added and boot to single user mode using s as mentioned above Next procedure is the same as for native installation only with a few differences We can go to 7 2 4 Base System Installation Stage Two in Native Installation section Before doing so please read next lines Pay attention that in Native Installation GNU Hurd is on hd0s1 But now we have GNU Linux on hd0s1 and GNU Hurd is on hd0s3 So in the next installation procedure you need substitute hd0s1 by hd0s3 Because Grub boot loader is already installed and configured on the hard drive you don t need the Grub CD that is required in Native Installation section and you can certainly skip 7 2 6 The Grub Installation If you don t known it is item with GNU also known as GNU Hurd title 41 Chapter 8 Current Status of the GNU Hurd 8 1 Hurd u kernel Usage History The first u kernel that GNU Hurd has used was GNU Mach which is based on the Mach 3 u kernel Developers has choosen it this way because no other u kernel had been more suitab
28. focused on the L4Ka Pistachio implementation which GNU Hurd uses Name CPU Impl Lng L4Ka Pistachio Pentium or better A64 PowerPC Alpha C 64 bit MIPS NICTA L4 embedded ARM x86 and MIPS C Fiasco i486 or better StrongARM Linux user C mode L4 emulation P4 x86 MIPS PowerPC ARM C L4 for PowerPC PowerPC 603e C L4Ka Hazelnut Pentium or better StrongARM C L4 MIPS MIPS R4x00 C L4 Alpha Alpha AXP 21264 Assembler L4 x86 i486 or better Assembler Table 5 1 Various implementations of the L4 u kernel 5 2 Design Philosophy The most fundamental task of an operating system is to provide secure sharing of resources A p kernel should to be as small as possible Hence the main design criterion of the u kernel is minimality with respect to security A service feature is to be included in the u kernel if and only if it impossible to provide that service outside the kernel without loss of security The idea is that once we make things small and do it well performance will look after itself 24 A strict application of this rule has some surprising consequences Take for example device drivers Some device drivers access physical memory e g DMA and can therefore break security They need to be trusted This does not however mean that they need to be in the kernel If they do not need to execute privileged instructions and if the kernel can provide sufficie
29. g Tom s Kasp rek for the opportunity to work on this project and for the time to discuss problems with him My special thanks go to my family for the possibility to study in Brno and for their constant support all the time as well as to my friends for their encouragement Abstrakt Tato pr ce se zab v opera n m syst mem GNU Hurd Prvn st pr ce je zam ena na koncept Hurdu d le na mikroj dra Mach a L4 kter tvo z klad cel ho syst mu Druh st popisuje n kolik instala n ch postup Hurdu na architektu e x86 Pr ce tak sleduje aktu ln stav Hurdu a sna se odhadnout dal budoucnost v voje Kl ov slova Coyotos Debian GNU GPL Grub Hurd Hurd NG j dro L4 L4 Sec Mach mikroj dro ngHurd opera n syst m Pistachio POSIX prostor j dra server transl tor UNIX u i vatelsk prostor Contents Contents 1 Prologue 2 Monolithic Kernels and u kernels 21 Whatisa Kernel lt s oo s m Xda ee ee eko aaa as B 2 1 1 Monolithic Kernel Based Systems lt lt 4 444 2 1 2 kernel Based Systems lt lt 4 3 The GNU Hurd J1 The GNU Proj cai n dom Zoe ra e A Be wl 3 2 The Hurd s Definition s sared os oea eona Se a a RGR GEES 3 2 1 Vocabulary 2 024455 bore aa be a AA 3 3 The Hurd Goals 2 i sw pad ks ke da te 4 a a eee ec ga The Hurd s History eop a he a RY A A A AA 3 5 Servers and Translators gt lt lt sc 4 4 lt kce k 8 aaaea Ge we ee aa a
30. ger This translator allows access to control interfaces of Mach default pager storeio The storage translator for devices and other stores streamio A translator for stream devices symlink A translator for symbolic links for filesystems which don t support it them selves term The terminal translator that implements a POSIX terminal tmpfs A translator which provides tmp filesystem ufs A server for ufs type filesystems It does the same as ufs static only that ufs static is a statically linked executable usermux A translator for invoking user specific translators 3 6 Security Infrastructure 3 6 1 Authentication Tokens The security on GNU Hurd is based on authentication tokens A token is the right for an application to perform a specific set of tasks Tokens are handled by the auth server a server trusted by all other pograms and enforcing that no one lies on which token they have Through auth tokens can be given destroyed or created Tokens can exist for anything for example you can imagine a token is able to bind ports below 1024 Tokens are comparable to Kerberos tickets or POSIX capatibilities 19 3 6 2 POSIX Compatibility POSIX compatibility with regard to authentication is implemented via specific tokens UIDs and GIDs are only a kind of tokens that auth can deliver It is therefore possible for an application to have several UIDs tokens and as such being able to act
31. have set a passive translator you can still set a different active translator Only if the translator is automatically started because there was no active translator the time the inode was accessed the passive translator is considered 3 5 4 Managing Translators As mentioned above you can use settrans to set and alter passive and active translators There are a lot of options to change the behaviour of settrans in case something goes wrong and to conditionalize its action Here are some common usages e settrans c mnt hurd ext2fs dev hd2s5 mounts a partition the translator will stay across reboots e settrans a mnt hurd ext2fs dummy fs mounts a filesystem inside a data file the translator will go away if it dies e settrans fg nfs data forces a translator to go away You can use the showtrans command to see if a translator is attached to an inode This will only show you the passive translator though You can change the options of an active filesystem translator with fsysopts without actually restarting it This is very convenient For example you can do what is called 17 remounting a partition read only under Linux simply by running fsysopts mntpoint readonly The running active translator will change its behaviour according to your request if possible fsysopts mntpoint without a parameter shows you the current set tings 3 5 5 The GNU Hurd s Translators and Servers auth The authentication server wh
32. he translator active we will immediately see any error messages on stderr The latter saves the translator and arguments in the node so it can be transparently restarted later i e making the setting persistent across reboots The options are followed by the node to which the translator is to be attached then the program i e translator to run and any arguments to give it The i option is the interface pfinet will listen on a is the IP address m is the network mask and g is the gateway Be sure to add name servers to the etc resolv conf file in our case nameserver 192 168 1 1 To test the configuration we can ping the gateway ping c3 192 168 1 1 Help on settrans can be obtained by passing it the help option Help on a specific translator can be gotten by invoking it from the command line with the same argument e g hurd pfinet help 7 2 6 Final Words The Hurd Console Besides the Mach console you encountered during installation the GNU Hurd features a powerful user space console providing virtual terminals Currently you have to start the Hurd console manually with the following command console d vga d pc_kbd d pc_mouse d generic_speaker c dev vcs Inside the Hurd console you can switch between virtual terminals via Alt F1 Alt F2 and so on Ctri Alt Backspace detachs the Hurd console and brings you back to the Mach console from where you can reattach again with the above command The Grub Ins
33. hy the information about translators is stored in the inode Translators are responsible to serve all file system operations that involve the inode they are attached to Because they are not restricted to the usual set of objects device file link etc they are free to return anything that makes sense to the programmer One could imagine a translator that behaves like a directory when accessed by cd or 1s and at the same time behaves like a file when accessed by cat 3 5 2 Translator Examples Mount Points A mount point can be seen as an inode that has a special translator attached to it Its pur pose would be to translate filesystem operations on the mount point in filesystem operations on another store let s say another partition 15 Indeed this is how filesystems are implemented under the Hurd A filesystem is a trans lator This translator takes a store as its argument and is able to serve all filesystem operations transparently Device Files There are many different device files and in systems with a monolithical kernel they are all provided by the kernel itself In the Hurd all device files are provided by translators One translator can provide support for many similar device files for example all hard disk partitions This way the number of actual translators needed is quite small However note that for each device file accessed a separate translator task is started Because the Hurd is heavily multi threaded this is
34. ich passes credentials when two mutually untrusting servers communicate In a sense each auth server establishes a domain of trust console A translator that provides virtual consoles crash The server gets active whenever a task gets a fatal error signal for example because it violates memory boundaries segmentation fault The crash server has three modes of operation suspending the process group pgrp of the offending task killing it or dumping a core file exec The execute server that manages the creation of a new process image from the image file ext2fs A server that manages ext2 type filesystems It does the same as ext2fs static only that ext2fs static is a statically linked executable fakeroot A translator for faking privileged access to an underlying filesystem fatfs A translator that manages FAT type filesystems fifo A translator that implements named pipes firmlink The firmlink translator is sort of halfway between a symbolic link and a hard link ftpfs A translator for FTP filesystems fwd When accessed the fwd translator forwards requests to another translator It is used in the fifo and symlink translators The idea is so that you don t get a jillion servers for such trivial things fwd is used to coordinate having one server handle several different nodes conveniently hello A translator providing a warm greeting hello mt is the same but multi threaded hostmux
35. iguration install inetutils tools package and use inetutils ifconfig command e run the Hurd console automatically on bootup follow instructions in the file etc default hurd console e use df command you have to specify filesystem e g df e find out memory info try vmstat e read kernel log use cat dev klog gt kernel log and read the file It is because log is readable only once e change the options or show the current settings of an active filesystem translator use fsysopts More tips and FAQs you can find at http www gnu org software hurd faq en html 7 3 Cross Installation 7 3 1 Introduction This section explains the Debian GNU Hurd installation on x86 computer with installed GNU Linux system We expect configured Debian GNU Linux 3 1r1 Sarge with the Grub boot loader and 2 GB of free space on the hard drive like in the native installation it depends whatever you want to install We need also some supported network card Like in the first case check your hardware at http www nongnu org thug gnumach_hardware html Don t forget that the GNU Hurd currently runs only on 1432 machines 7 3 2 Install Preparation First of all we need to install crosshurd package apt get install crosshurd Note that crosshurd can now be used to install not only GNU Hurd but also GNU Linux GNU kFreeBSD and GNU kNetBSD but it is outside this guide Next it is necessary to create a partition You can use e g
36. in GNU Hurd with Mach 4 5 1 Paging with Mach Mach takes the decision of which page to keep and which page to discard move to back end storage in time of memory pressure using a Least Recently Used algorithm LRU but the pagers are in user space When a page has to be evicted from memory Mach sends an IPC to the user space pager associated with the page it is up to the pager to save it somewhere When a page fault occurs Mach asks to the pager to bring back the page to memory and then resumes the faulting application 4 5 2 Some Applications User space pagers can be used to implement different kind of backing stores hard disks compressed memory a remote computer through networking Several pagers can handle different parts of the address space of the same task The Use in diskfs The library used by regular filesystem translators like ext2fs iso9660fs fatfs works by mapping in memory all the metadata and then using only pointer indirection to access to the metadata The GNU Mach VM handles all the caching and a user space pager is used to load and write pages to from the backing store The problem is that for filesystems with metadata spread all over the partition like ext2fs the whole partition has to be mapped into memory That is why many diskfs translators are limited to 2GB partitions on IA 32 and that s why fatfs isn t limited to 2 GB the FAT is located at the beginning of the partition so the only
37. ject and is distributed as free software under the GPL The Hurd aims to surpass UNIX kernels in functionality security and stability while remaining largely compatible with them This is done by having the Hurd track the POSIX specification while avoiding arbitrary restrictions on the user Chapter 2 describes monolithic kernel based systems and u kernel based systems Es pecially differences between mono server and mutli server systems are explained Chapter 3 presents the GNU Hurd system It covers its history Hurd s goals and description of active and passive translators in detail Hurd s security infrastructure is mentioned as well Chapter 4 focuses on the Mach p kernel The Mach key features interprocess com munication memory handling device drivers and others are mentioned there Chapter 5 describes the L4 p kernel in detail You can find table with various imple mentations of the L4 there sections about address spaces threads memory mapping and interprocess communication Chapter 6 presents GNU Mach and L4 p kernel differences Mainly interprocess com munication virtual memory management and device drivers differences are described Chapter 7 focuses on Debian GNU Hurd installation on the x86 architecture We describe native installation procedure on computer without OS and cross installation on computer with Debian GNU Linux installed Chapter 8 describes current status of the GNU Hurd
38. l Translator Concepte o 294 ala e ee ee A a a 39 2 Translator Examipl s es s 2 ea mog Pann e e RA eee 3 5 3 Passive and Active Translators lt 4 4 44 3 5 4 Managing Translators o e o coos saa aoo a we a ea 3 5 5 The GNU Hurd s Translators and Servers lt lt 3 6 Security Infrastructure lt 5 ocios caps U Eee 808 dak a dh 3 6 1 Authentication Tokens lt lt lt lt 44 44 4 4 e 20 2 POSIX Compatibility cs xo aapa hadaa RA a 3 6 3 Some Applications gt s sco onas apaa sars lt oa ss 4 The Mach u kernel 41 About N ch 2 226 awa G aa Ede ob ae es ae a a 42 Mach Key Features x csa soa cp soa aa ao BORCE aa g kk BB Ad GNU MISC e acae sph A p k ee a AR AA 4 4 Interprocess Communication IPC e 4 5 Memory Handling in GNU Hurd with Mach aoaaa aeaa 40 1 Pasme with Maei lt e 2 04 25 Bee a o kok n EDR ee 45 2 pome Applications lt lt sx seseg tpw adost keea db E eea AG Dewee Drivers lt k 5 sms Ae eee a a de e a 10 10 10 11 13 13 13 13 13 14 15 15 15 16 17 18 19 19 20 20 5 The L4 p kernel Ol About LA coa Goce a A a O ee Ao vk 5 2 Design Philosophy s lt ra u soe ra a a Sa ck dd kok v 5 3 Registers and Address Spaces lt lt lt lt lt 4 4 lt 4 ee ee Daet RERS sapp o 4 Bie aes oe A Boe Ree gees B82 Addr ss Space cocoa SRE a Aa A O al o 4 epos oe k Rob Ro Hod e E ew BR OR ee ae buebn GAl Washes c g esas pco Be
39. le for the Hurd But GNU Mach has had some lacks and mainly performance problems so core developers have created new experimental branch of the Hurd based on L4Ka Pistachio u kernel Although L4Ka Pistachio has been much faster then GNU Mach it still hasn t got new features to offer That is why Hurd on L4Ka Pistachio has been stalled and core developers think about new gu kernel again It will be probably Coyotos which is a successor to the EROS system or L4 Sec which is another implementation of the L4 u kernel But both u kernels are still in the design stage and can t run so it is quite difficult to make a decision which one is better for the Hurd now New experimental branch with new u kernel should be called ngHurd or Hurd NG Developers Marcus Brinkmann and Neal H Walfield have started work on a proposition paper and a technical specification of the ngHurd These documents should make a decision of a new u kernel easier One of the design goals of GNU Hurd was to be platform architecture u kernel in dependent but currently there are experimental ports only as mentioned above Porting Hurd to another p kernel for the first time will help to be able to do that in the future again Development of the Hurd on L4Ka Pistachio will probably continue only in the case when Coyotos and L4 Sec fail Probably the Hurd on GNU Mach 1 x will continue its development until more stable kernel is developed The GNU Hurd s kernel usage hist
40. limitation is to have the FAT be less than 2 GB Two possible solutions for this problem are either to set up a tree of intelligent special ized pagers or to use a cache of mappings creating and destroying mappings of meta data as needed The current implementation of ext2fs in Debian GNU Hurd see section 7 1 contains a patch from Ognyan to cover this problem It implements a caching of mappings with static mapping of fixed metadata since in ext2 some metadata are in fixed places and others can be anywhere in the filesystem 22 4 6 Device Drivers The Mach kernel provides all low level device support Each device is represented as a port to which messages can be sent to transfer data or control the device Data is transferred through read and write operations the request and reply messages are exported separately allowing both synchronous and asynchronous styles of I O The external memory object protocol allows a user to map the frame buffer for a graphics device directly into its address space 23 Chapter 5 The L4 u kernel 5 1 About L4 Originally L4 is the name of a second generation u kernel designed and implemented around 1995 by Jochen Liedtke running on 1486 and Pentium CPUs However there are numerous implementations of the L4 API on several hardware ar chitectures You can see overview of various implementations in table 5 1 the last one is Liedtke s original implementation Note that this chapter is
41. messages to each other This message passing IPC is the heart of L4 It is used to pass data between threads either by value with the u kernel copying the data between two address spaces or by reference through mapping or granting L4 IPC is blocking so it is used for synchronisation each successful IPC operation results in a rendez vouz and wakeup calls timeouts can be spec ified so IPC can be used for timed sleeps It is even used for memory management the u kernel converts a page fault into an IPC to a user level pager exception handling the u kernel converts an exception fault into an IPC to a user level exception handler and interrupt handling the kernel converts an interrupt into an IPC from a pseudo thread to a user level interrupt handler 5 7 1 Messages A message consists of one mandatory and two optional sections The mandatory message tag is followed by the optional untyped words section which is followed by the optional typed items section The message tag contains message control information and a message label The message control information specifies the size of the message and the kind of data contained in it The u kernel associates no semantics with the message label it allows threads to identify a message and is often used to encode a request key or to identify the function to be invoked upon reception of the message The untyped words section holds arbitrary data that is untyped from the p kernel s point of
42. ne of servers cannot corrupt the internal state of any other and far more flexibility for the end user But like always in computer science all those benefits come with several drawbacks the communication between all servers can slow down the whole system and the definition of a strict set of interfaces and protocols for communication between those servers is an extra work to do a Sia KK 2 Server User Space User Space Figure 2 2 Schemes of mono server and multi server based system 12 Chapter 3 The GNU Hurd 3 1 The GNU Project The GNU Project was started by Richard M Stallman in 1983 The goal of the project is to create a full operating system and all applications required to allow any user to be able to use only Free Software to perform whatever task he needs to use a computer for GNU means GNU is Not Unix since GNU takes many ideas from UNIX but does not intend to be only a Free implementation of UNIX but to correct UNIX flaws and weaknesses at the same time 3 2 The Hurd s Definition The GNU Hurd is a set of servers libraries and interfaces running on top of a p kernel and providing the services which used to be inside the kernel 3 2 1 Vocabulary The Hurd The Hurd or the GNU Hurd is the set of servers it is not an operating system and since it runs in user space it is not what we call a kernel Hurd means Hird of Unix Replacing Daemons and Hird means Hurd of Inte
43. ng they are not the same We will try to avoid using the term task where possible prefering to specifically refer to an address space or thread 5 4 2 Identifying Threads and Address Spaces A thread is identified by its unique identifier UID A thread can actually have two identi fiers a local identifier and a global identifier While a global identifer is valid in any address space a local identifier is only valid within the thread s address space That is a global identifier can be used by any thread while a local identifier can only be used by threads that are part of the same task In different address spaces the same local thread ID may identify different and unrelated threads Unlike threads address spaces do not have identifiers Instead an address space is identified by the UID of any thread associated with that address space This means that an address space must always have at least one thread associated with it That thread does not however have to be active 5 5 Communication One of the main activities that threads engage in is to communicate with other threads for example in order to request services from each other in order to share results of computations etc There are two ways that threads communicate using shared memory see section 5 6 or using L4 s Interprocess Communication IPC facilities see section 6 7 5 5 1 Communication Within an Address Space When communicating with threads in the same
44. nt protection to run them at user level then this is what should be done Consequently this is what L4 does According to Liedtke and based on such reasoning a u kernel must provide Address spaces the basis of protection Threads an abstraction of program execution Interprocess communication IPC a mechanism to transfer data between address spaces Unique identifiers UIDs providing context free addressing in IPC operations AI these concepts will be explain in this chapter later 5 3 Registers and Address Spaces 5 3 1 Registers L4 p kernel provides programs with access to virtual registers Virtual registers offer a fast interface to exchange data between the u kernel and user threads They are registers in the sense that they are static per thread objects Depending on the specific processor type they can be mapped to hardware registers or to memory locations Mixed mappings where some virtual registers map to hardware registers while others map to memory are also possible There are three classes of virtual registers Thread Control Registers TCRs Message Registers MRs and Buffer Registers BRs In general virtual registers can only be addressed directly not indirectly through point ers The L4 API provides specific functions for accessing the three different classes of regis ters Loading illegal values into virtual registers overwriting read only virtual registers or accessing virtual registers
45. ntain the target of a symbolic link However these commonalities are usually not exploited in the implementations despite the common programming interface to them All inodes can be accessed through the standard POSIX calls for example read and write For example to add a new object type for example a new link type to a common monolithic UNIX kernel you would need to modify the code for each filesystem separately In the Hurd things work differently Although in the Hurd a special filesystem server can exploit special properties of standard object types like links in the ext2 filesystem with fast links for example it has a general interface to add such features without modifying existing code The trick is to allow a program to be inserted between the actual content of a file and the user accessing this file Such a program is called a translator because it is able to process the incoming requests in many different ways In other words a translator is a Hurd server which provides the basic filesystem interface Translators have very interesting properties From the kernel s point of view they are just another user process This means translators can be run by any user You don t need root privileges to install or modify a translator you only need the access rights for the underlying inode the translator is attached to Many translators don t require an actual file to operate they can provide information by their own means This is w
46. ntax as the active translator just leave away the a so everything said above is true for passive translators too However there is a difference passive translators are not yet started This makes sense because this is what you usually want You don t want the partition mounted unless you really access files on this partition You don t want to bring up the network unless there is some traffic and so on Instead the first time the passive translator is accessed it is automatically read out of the inode and an active translator is started on top of it using the command line that was stored in the inode This is similar to the Linux automounter functionality However it does not come as an additional bonus that you have to set up manually but an integral part of the system So setting passive translators defers starting the translator task until you really need it By the way if the active translator dies for some reason the next time the inode is accessed the translator is restarted There is a further difference active translators can die or get lost As soon as the active translator process is killed for example because you reboot the machine it is lost forever Passive translators are not transient and stay in the inode during reboots until you modify them with the settrans program or delete the inodes they are attached to This means you don t need to maintain a configuration file with your mount points One last point Even if you
47. o gain privileges and be able to answer to the user The huge difference with UNIX systems is that the ftp or ssh server never has root privileges or only at bind time and then drop it completly and never has even the privileges of a normal user before someone gives it a valid login password pair A flaw inside ftp or ssh would only give a shell with very few rights No Auth Programs Programs can discard all the security tokens they have and become noauth programs This can be used to process untrusted contents like a ghostscript interpreter running on contents coming from an untrusted source A security flaw inside the interpreter could not allow a malicious postscript file to damage your own files since the interpreter discarded the UID security token before processing the data 20 Chapter 4 The Mach u kernel 4 1 About Mach Mach was one of the first u kernels It was a project of Carnegie Mellon university to implement a fairly new theory It came with a lot of new concepts e A complex and powerful IPC layer e It was designed for multiprocessor and even clusters e It use external pagers e It was the first system to clearly define the notions of task thread Mach was the first u kernel to be successful taken and improved by OSF 1 and other research groups It was the base of MachOS a mono server operating system with the UX server running on top of the Mach u kernel and providing BSD compatibility 4 2 Mach Key Fe
48. ody is working on it Few people ever got it running and by now there are also problems building it with recent toolchains Homepage http hurd gnufans org bin view Mach OskitMach About Coyotos Coyotos is a secure microkernel based operating system that builds on ideas and experi ences of EROS Extremely Reliable Operating System project Much of the code developed for EROS will migrate directly to Coyotos The Coyotos project has several goals e Correct some of the shortcomings of the earlier EROS design e Demonstrate that an atomic kernel design scales up as well as down They are plan ning to bring up versions of Coyotos on large scale multiprocessors e Provide an efficient Linux compatibility environment for use as a transitional runtime system so that we can explore adapting applications to a more secure API foundation 43 e Construct the kernel and key utilities in a new system programming language BitC with a well defined mechanically specified semantics This will allow us to formally verify security and correctness properties of the system and its key utilities e Develop the proving technology necessary to do useful verification about a project of this sort System is being developed on AMD 64 and Pentium platforms A port is also underway to recent Coldfire processors Support for PowerPC and ARM processors is expected as well Coyotos is still in the design stage and no release is available today Home
49. onvenient The easiest method is to use dselect but first of all it is necessary configure apt We use internet for upgrading and installing new packages as I mentioned in 7 2 1 So we need to edit etc apt sources list The file should look like this deb http ftp cz debian org debian unstable main contrib deb http ftp gnuab org debian unreleased main To use a local Debian mirror visit http www debian org mirror list http ftp gnuab org and its mirrors contain packages that have hacks or patches that have not yet been integrated upstream or in Debian Now we can run dselect and choose the first item Access then apt Source list we have already configured so we can skip this step Update in the main menu will update list of available packages Note that during this process you can see some GPG errors Don t worry about it it is fine When you choose Select the dselect automatically marks common packages to install e g cron manpages gcc wget etc You can select another packages that you need mc bzip2 Now select Install You can see that a lot of common packages will be upgraded or newly installed Before downloading files some authentication warning could appear Don t worry and continue without verification That is all and henceforward the Debian GNU Hurd should be quite usable Useful Tips If you want to e change hostname edit etc hostname 39 e use Linux like ifconfig for network conf
50. ory with Hurd s releases overview is shown in figure 8 1 About OSKit Mach OSKit Mach began as a branch of the GNUMach 1 2 kernel but since the release of GNU Mach 1 3 OSKit Mach has been merged as the new GNUMach 2 x mainline TCoyotos homepage http www coyotos org EROS homepage http www eros os org research effort has ended 3L4 Sec homepage http os inf tu dresden de L4 L4 Sec 4A draft specification of the ngHurd iterfaces can be found at http hurd gnufans org bin view Hurd NextHurd 42 May Apr May Jun May Nov Apr What 1991 1997 1997 1999 2002 2002 2006 next L4Ka Pistachio L4Ka Pistachio stalled OSKit Mach OSKit Mach OSKit Mach initial version future GNU Mach 2 0 stalled GNU Mach initial version GNU Mach v1 0 GNU Mach v1 1 GNU Mach v1 2 GNU Mach v1 3 GNU Mach continue Mach 3 Coyotos probably L4 Sec L4ng Sep Jun Jun 1991 GNU Hurd 1996 GNU Hurd 1997 GNU Hurd 2002 GNU Hurd initial version v0 1 v0 2 v0 3 ee aaaea o Figure 8 1 Hurd s u kernel usage history and Hurd s releases overview GNU Mach 2 0 makes use of the drivers provided by the OSKit from the Flux Research Group The OSKit provided a neat driver base where both FreeBSD and Linux 2 2 12 drivers are made available to Mach and thus the Hurd However OSKit isn t maintained anymore The OSKit Mach version of GNU Mach is more or less defunct today 2006 Nob
51. our system 2We don t use mini CD because it doesn t include boot loader and some other utilities that we need for comfortable installation 34 a lot of packages have a new version then that in released distribution on the CDs So it is useless to install all packages from the CDs and then install new versions of these packages from the internet For installation you need besides 2 GB of free space some supported network card A relatively complete hardware compatibility guide can be found at http www nongnu org thug gnumach_hardware html Don t forget that the GNU Hurd currently runs only on 1432 machines 7 2 2 CDs Preparation CD1 of the latest Debian GNU Hurd distribution can be downloaded from ftp ftp gnuab org pub debian cd current hurd i386 Now we burn the downloaded CD1 and look up the file install grub 94 iso on it This image includes Grub boot loader that we will need for boot Hurd after install to hard drive So we burn it as well Now we have got two CDs installation CD1 and the Grub CD and we can start installation of the Debian GNU Hurd 7 2 3 Base System Installation Stage One When you boot from the CD1 then shows a welcome screen and boot command line We don t need any special parameters so press Enter After boot shows Debian GNU Hurd Installation Main Menu You can setup keyboard in 1st menu entry if you want or use default gwerty us Next is Partition a Hard Disk We select device and then starts cf
52. page http www coyotos org About L4 Sec L4 Sec is a specification of next generation u kernel interface targeted to co host user level servers implementing security critical functionality next to untrusted and potentially malicious software L4 Sec is still in the design stage and no release is available today Homepage http os inf tu dresden de L4 L4 Sec 8 2 Known Flaws and Limitations Note that the following list of known limitations is for the GNU Hurd with the GNU Mach u kernel that is used in the Debian GNU Hurd distribution e 1 5 GB ext2 partition size limit The problem is fixed in the Debian GNU Hurd distribution e Missing device drivers Current GNU Mach drivers are from Linux 2 0 OSKit Mach currently has Linux 2 2 drivers e Swap is highly recommended like on UNIX systems e Minimum of 8 MB RAM has been reported to work but 32 MB is more realistic Don t forget to use plenty of swap space e 768 MB RAM maximum approximately If you have got too much use Grubs s uppermem command to limit the amount seen by Mach e Random devices dev random and dev urandom are not in the main distribution yet These are needed by for instance OpenSSH e There are some system API limits e No interface for userspace non critical drivers e Sound support missing 44 OSKit Mach can provide but no work yet e PPP is currently supported but is not stable e USB support is missing e No scanne
53. rfaces Representing Depth but all of them are spelled like the word herd which is the real meaning of this name the GNU Hurd is a herd of gnus GNU Hurd GNU Hurd or GNU is the full operating system including the u kernel the Hurd the dynamic linker GNU ld the C library GNU libc and other applica tions 3 3 The Hurd s Goals The Hurd is core of the GNU project Every program running on the GNU system rely upon the Hurd to perform most of operating system related tasks The philosophy of the GNU project as defined in the GNU manifesto was the philosophy leading to the Hurd design The goal of the GNU project is to give back the freedom to the users of computer Hurd has been originally developed on the top of GNU Mach currently there is an effort to replace Mach with another u kernel see section 8 1 13 HURD Figure 3 1 The GNU Hurd s logo systems but not only at the license level Every technical limitation which is not strictly necessary is a reduction of user freedom The fact that non privilegied user in a UNIX system cannot mount an ISO image setuid does not count or test his own file system implementation is a reduction of user freedom With its highly modular design the Hurd gives back some additional freedom to the users This is a major idea of the whole GNU project displayed even in the name of the project we want to stay compatible with UNIX programs as much as possible but we want to overth
54. row as much limitations as we can Interfaces between components of the GNU Hurd were clearly defined and fixed as soon as possible during the development Fixed interfaces are very important to allow users to design their own replacement of some part of the system without breaking the other parts It also suppress compatibility problems Interfaces of the GNU Hurd were designed to fill the flaws of UNIX systems For example there is no standard way in a UNIX system to create a file with no name The commonly used method is to create it in tmp and then unlink it without closing the file descriptor The kernel will wait for the file descriptor to be closed before deleting it But during a short amount of time the file has a name and this causes many security holes Another example is notification the most common way to ask the kernel of UNIX system to be informed when a file is changed SIGIO is non portable and not very flexible in recent UNIX kernels other ways can exist but still lack flexibility 3 4 The Hurd s History 1983 Richard Stallman starts the GNU project 1988 Mach 3 is chosen as u kernel 1991 Mach 3 is released under a Free license 1991 Thomas Bushnell BSG founds the Hurd 1994 GNU Hurd boots for the first time 1997 The Hurd version 0 2 is released 1998 Marcus Brinkmann creates Debian GNU Hurd 2002 GNU Mach version 1 3 is released 2002 Debian GNU Hurd has 4 CDs 2002 Port of the Hurd to L4 is started 2002 POSIX
55. rs are currently supported e No ACPI support e No WLAN support e XFree86 is stabilizing e X Org is available in the Debian GNU Hurd distribution e fatfs translator is available and it doesn t have partition limits that get in the way e Ext3 implementation is very unstable e Portability between alternate microkernels e g L4Ka Pistachio e Stability issues 8 3 Linux related L4 Based Projects L Linux L Linux is a port of the Linux kernel to the L4 p kernel API It is a para virtualized Linux running on top of a hypervisor completely without privileges if wanted L Linux runs in user mode on top of the u kernel side by side with other u kernel appli cations such as real time components It is binary compatible with the normal Linux x86 kernel and can be used with any PC based Linux distribution Several releases of L Linux are available based on different Linux versions currently 2 6 16 2 4 30 2 2 26 and 2 0 21 Only 2 6 x branch is currently mantained Homepage http os inf tu dresden de L4 LinuxOnL4 Fiasco UX kernel Fiasco UX is a port of the Fiasco microkernel to the Linux system call interface Due to its special design it runs without kernel level privileges despite the fact that it is a fully functional L4 microkernel Because it uses GNU Linux as host system it can run on any x86 based system as a normal user mode application Supported Linux kernels are from 2 2 x up to 2 6 x earlier kernels untested
56. ry handling e File systems e Network stacks Many system calls are provided to applications more than 250 for Linux 2 4 to allow them to access all those services This design has several flaws and limitations 10 Coding in kernel space is hard since you cannot use common libraries like a full featured libc debugging is harder it is hard to use a source level debugger like gdb rebooting the computer is often needed Remember this is not just a problem of convenience to the developer as debugging is harder as difficulties are stronger it is likely that code is buggier Bugs in one part of the kernel have strong side effects since every function in the kernel has all the privileges a bug in one function can corrupt data structure of another totally unrelated part of the kernel or of any running program Kernels often become very huge more than 100 MB of source code for Linux 2 6 and difficult to maintain The presence of strong side effects makes the whole kernel less modular remember the VM issues of Linux 2 4 some people suggested to make the VM modular allowing the system administrator to choose one VM at compile time but the impact of the VM over the other parts of the code was so huge that the idea was dropped And since kernel code runs with all the privileges at hardware level only the system administrator can be allowed to load a module inside the kernel space 1 TCP IP i User Spa
57. ss to the same memory region This is achieved by having one thread map a region of its address space into the address spaces of the other threads The concept of mapping memory is explained in section 5 6 Once a shared region of memory has been established the threads communicate by simply reading from or writing to the particular memory region Note that as mentioned previously when communicating using shared memory it is necessary to avoid race conditions Communication using IPC requires that the threads send messages to one another A message is sent from a sender thread and addressed to a particular receiver thread Messages can be used to directly share data by sending it back and forth to indirectly share data by sending memory mappings or as a control mechanism e g to synchronise 5 6 Memory Mapping Address spaces can be recursively constructed A thread can map parts of its address space into another thread s address space and thereby share data Figure 5 3 shows an example of two address spaces with a region of address space A mapped into address space B Both the thread running in address space A and the thread running in address space B can access the shared region of memory Note however that the memory may have different virtual addresses in the different address spaces Thus in the example a thread running in address space A accesses the shared memory region using virtual address 0x1000000 while a thread in address spac
58. t Coyotos the successor of the EROS or L4 Sec both not complete yet They are also indications of completely new p kernel that should be created especially for the Hurd It is a pity that only a few developers currently work on the Hurd and furthermore they have not got too much time so development is practically suspended nowadays For sure the GNU Hurd is a good idea and I hope that this great system will be released as a stable version one day 46 Bibliography 1 Introduction to the translator concept DEBIAN ORG The Universal Operating System online cit 2006 04 23 URL http www debian org ports hurd hurd doc translator Preliminary GNU Hurd User Interface Description DEBIAN ORG The Universal Operating System online cit 2006 04 23 URL http www debian org ports hurd hurd doc server Debian GNU Hurd Installation DEBIAN ORG The Universal Operating System online cit 2006 04 23 URL http www debian org ports hurd hurd install Hurd Installation Guide University of Warwick Hurd User Group online Last up date 2006 03 01 cit 2006 04 23 URL http uwhug org uk index pl Hurd Installation Guide WALFIELD N H The Hurd Installation Guide online Last update 2003 01 21 cit 2006 04 23 URL http web walfield org pub people neal papers hurd installation guide english hurd install guide html BRINKMANN M How to Install Hurd In Open Weekend 2004 Video archive Pr
59. tallation It is a good idea to install the Grub boot loader to MBR on the hard drive We need for it the Grub CD that we have in CD ROM drive in my case it is device dev hd2 mkdir cdrom settrans a cdrom hurd iso9660fs dev hd2 cp R cdrom boot grub boot settrans g cdrom 3 servers socket 1 is used for loopback device Setting for network card is always placed in servers socket 2 because that is where glibc will look for it If you would like configure multiple network interfaces for example eth0 and ethl you can use settrans fgap servers socket 2 hurd pfinet i ethO a i ethl a 38 The first one command creates direcory for mount CD ROM the second one sets the active translator mounts CD ROM filesystem third one copies Grub files to the hard drive and the last one asks the active translator to go away unmounts CD ROM Now you can edit boot grub menu lst and change timeout default entry to boot etc The last step that we need is write the Grub to MBR The easiest way is reboot the system again boot from the Grub CD and choose the fifth item Install GRUB into hd0 MBR And that is all Now you can boot from the hard drive Adding Devices By default only a few devices are created in the dev directory Use the MAKEDEV script to create any needed device nodes Installing More Packages There are several ways to add packages Downloading and using dpkg i works but is very inc
60. tion becoming ready to receive Similarly the reciever blocks until a message has been recieved The receiver must also supply all necessary buffers as the kernel does not provide any such storage This unbuffered operation reduces the amount of copying involved and is the key to high performance IPC A single IPC call combines an optional send phase and an optional receive phase Which phases are included is determined by specific parameters to the IPC call It is also possible to specify timeouts for IPCs Different combinations of timeouts and send and receive phases lead to fundamentally different kinds of IPCs For example including both a send and a receive phase with no timeout implements a synchronous IPC that blocks until a reply is received while an IPC that includes only a receive phase and no or rather infinite timeout implements a blocking call that waits for a message to arrive The L4 API provides convenience interfaces to these different kinds of IPC calls To enable implementation specific optimizations two variants of the IPC system call exist normal IPC and lightweight IPC LIPC Functionally both variants are identical but LIPC may be optimized for sending messages to local threads Transparently to the user a kernel implementation can unify both variants or implement differently optimized functions IPC is the default IPC function It must always be used except if all criteria for using LIPC are fulfilled both a send and
61. ve 1023 By default GNU Linux will reject this To tell GNU Linux to accept connections originating from a non reserved port add the insecure option to the export line Here is an example etc exports file assuming the client s IP address is 192 168 1 2 home 192 168 1 2 rw insecure To mount this from a Hurd box and assuming that NFS server s IP address is 192 168 1 1 settrans cgap mount point hurd nfs 192 168 1 1 home Rebooting Finally we can reboot into multi user mode It means that we choose in the Grub boot menu third item GNU Hurd IDE 1st partition hd0s1 multi user Note that multi user entry is the same as singe user that has just one more s option when loading the kernel When the system starts up in multi user mode you can login as root with login gt 1 root The Network To configure the network the pfinet translator must be configured This is done using the settrans command to attach a translator to a given filesystem node When programs access the node by for example sending an RPC the operating system will transparently start the server to handle the request For example we can set up network like this 37 settrans fgap servers socket 2 hurd pfinet i eth0 a 192 168 1 2 m 255 255 255 0 g 192 168 1 1 Here settrans is passed several options The first two fg force any existing translator to go away The next two ap make both active and passive translators By making t
62. w the memory is used to make correct pageout decisions 32 Device Drivers e Mach has built in device drivers That means a bug in a driver can crash the whole system 6 2 L4 L4 is small and very fast Pistachio provides only 11 system calls There are L4 implementations for Alpha AMD64 ARM 1A32 IA64 MIPS and PowerPC architecture IPC e In L4 a message is always sent to a thread e Only primitive security mechanism e Every server will manage itself the objects it provides and the handles to them e L4 IPC is synchronous messages are sent to target threads directly the sending thread blocks until the receiving thread received the message No copying necessary if all arguments fit into the registers Limited support for data types makes message analysis easy VMM e L4 provides only access to the physical pages e Instead of implementing a single VMM server distributed approach was proposed e Every task enters a contract with the memory server about guaranteed physical pages and some extra pages e Guaranteed pages are renegotiated from time to time e Extra pages have to be returned on short notice e Tasks manage their own virtual memory Device Drivers e L4 has no built in device drivers they are implemented in user space An interface for drivers is needed 33 Chapter 7 GNU Hurd Installation This installation guide provides a basic instructions for native and cross
63. with different POSIX identities at the same time Programs can lose or give UIDs at any time during their execution they just need to contact auth and the password server or any other auth trusted server that can manage authentication for example a server that matches a user given and a sysadmin provided certificates A specific addauth command a normal non suided program can give tokens to pro grams If you are running a shell as kilobug you can give the UID kilobug security token to any currently running application with addauth u kilobug p PID The same way programs can drop their tokens lowering their permissions Suid ed Binaries The translator in charge of a filesystem is the one enforcing the suid bit If you run bin ping as a normal user the ext2fs translator managing the filesystem will give the root security token to the ping program before starting it Since a translator cannot give a security token it doesn t have a translator ran by a normal user would not be able to enfore the suid bit and this way there is no security risk in allowing normal users to run translators 3 6 3 Some Applications The Password Server The password server is a very simple around 200 code lines program which can give UID and GID security tokens in exchange of a login password pair A ftp server or an ssh server could then run without any permission and gives the user provided login password to the password server in order t

- AbcLinuxu.cz

Contents

Download Pdf Manuals

Related Search

Related Contents