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Chapter 21: The Linux System Operating System Concepts – 8th Edition, Silberschatz, Galvin and Gagne ©2009 History Linux is a modern, free operating system based on UNIX standards First developed as a small but self-contained kernel in 1991 by Linus Torvalds, with the major design goal of UNIX compatibility Its history has been one of collaboration by many users from all around d th the world, ld corresponding di almost l t exclusively l i l over th the Internet Designed to run efficiently and reliably on common PC hardware, but also runs on a variety of other platforms OS kernel is entirely original, but it can run much existing free UNIX software, resulting in an entire UNIX-compatible operating system free from proprietary code Many, varying Linux Distributions including the kernel, applications, and management tools Operating System Concepts – 8th Edition 21.2 Silberschatz, Galvin and Gagne ©2009 The Linux Kernel Version 0.01 (May 1991) had no networking, ran only on 80386- compatible Intel processors and on PC hardware, had extremely limited device-drive device drive support, and supported only the Minix file system Linux 1.0 (March 1994) included these new features: z z z z z z Support for UNIX’s standard TCP/IP networking protocols BSD-compatible p socket interface for networking gp programming g g Device-driver support for running IP over an Ethernet Enhanced file system Support for a range of SCSI controllers for high-performance f disk access Extra hardware support Version 1.2 (March 1995) was the final PC-only Linux kernel Operating System Concepts – 8th Edition 21.3 Silberschatz, Galvin and Gagne ©2009 Linux 2.0 Released in June 1996, 2.0 added two major new capabilities: z Support for multiple architectures, including a fully 64-bit native Alpha port z Support for multiprocessor architectures Other new features included: z Improved memory-management code z Improved TCP/IP performance z Support for internal kernel threads, for handling dependencies between loadable modules, and for automatic loading of modules on demand z Standardized configuration interface Available for Motorola 68000-series processors, Sun Sparc systems, and for PC and PowerMac systems 2.4 2 4 and 2.6 2 6 increased SMP support support, added journaling file system, system preemptive kernel, 64-bit memory support Operating System Concepts – 8th Edition 21.4 Silberschatz, Galvin and Gagne ©2009 The Linux System Linux uses many tools developed as part of Berkeley’s BSD operating system, MIT’s X Window System, and the Free Software Foundation's GNU project The min system libraries were started by the GNU project, with improvements p p provided by y the Linux community y Linux networking-administration tools were derived from 4.3BSD code; recent BSD derivatives such as Free BSD have borrowed code from Linux in return The Linux system is maintained by a loose network of developers collaborating over the Internet, with a small number of public ftp sites acting as de facto standard repositories Operating System Concepts – 8th Edition 21.5 Silberschatz, Galvin and Gagne ©2009 Linux Distributions Standard, precompiled sets of packages, or distributions, include the basic Linux system, system installation and management utilities, tiliti and d ready-to-install d t i t ll packages k off common UNIX tools t l The first distributions managed these packages by simply providing a means of unpacking all the files into the appropriate places; modern distributions include advanced package management Early distributions included SLS and Slackware z Red Hat and Debian are popular distributions from commercial and noncommercial so sources, rces respecti respectively el The RPM Package file format permits compatibility among the various Linux distributions Operating System Concepts – 8th Edition 21.6 Silberschatz, Galvin and Gagne ©2009 Linux Licensing The Linux kernel is distributed under the GNU General Public License (GPL), the terms of which are set out by the Free Software F Foundation d ti Anyone using Linux, or creating their own derivative of Linux, may not make the derived product proprietary; software released under the GPL may not be redistributed as a binary-only product Operating System Concepts – 8th Edition 21.7 Silberschatz, Galvin and Gagne ©2009 Design Principles Linux is a multiuser, multitasking system with a full set of UNIX- compatible tools Its file system adheres to traditional UNIX semantics, and it fully implements the standard UNIX networking model Main design goals are speed, efficiency, and standardization Linux is designed to be compliant with the relevant POSIX documents; at least two Linux distributions have achieved official POSIX certification The Linux programming interface adheres to the SVR4 UNIX semantics, rather than to BSD behavior Operating System Concepts – 8th Edition 21.8 Silberschatz, Galvin and Gagne ©2009 Components of a Linux System Operating System Concepts – 8th Edition 21.9 Silberschatz, Galvin and Gagne ©2009 Components of a Linux System (Cont) Like most UNIX implementations, Linux is composed of three main bodies of code; the most important distinction between the kernel and d allll other th components t The kernel is responsible for maintaining the important abstractions of the operating system z Kernel code executes in kernel mode with full access to all the physical resources of the computer z All kernel code and data structures are kept in the same single address space Operating System Concepts – 8th Edition 21.10 Silberschatz, Galvin and Gagne ©2009 Components of a Linux System (Cont) The system libraries define a standard set of functions through which applications interact with the kernel, and which implement much h off the th operating-system ti t functionality f ti lit th thatt d does nott need d th the full privileges of kernel code The system utilities perform individual specialized management tasks Operating System Concepts – 8th Edition 21.11 Silberschatz, Galvin and Gagne ©2009 Kernel Modules Sections of kernel code that can be compiled, loaded, and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver, a file system, or a networking protocol The module interface allows third parties to write and distribute, on their own terms, device drivers or file systems that could not be distributed under the GPL Kernel modules allow a Linux system to be set up with a standard minimal kernel standard, kernel, without itho t an any e extra tra de device ice dri drivers ers b built ilt in Three components to Linux module support: z module management z driver registration z conflict resolution Operating System Concepts – 8th Edition 21.12 Silberschatz, Galvin and Gagne ©2009 Process Management UNIX process management separates the creation of processes and the running of a new program into two distinct operations. z The fork system call creates a new process z A new program is run after a call to execve Under UNIX, a process encompasses all the information that the operating system must maintain to track the context of a single execution of a single program Under Linux, process properties fall into three groups: the process’s identity, environment, and context Operating System Concepts – 8th Edition 21.13 Silberschatz, Galvin and Gagne ©2009 Process Identity Process ID (PID). The unique identifier for the process; used to specify processes to the operating system when an application makes a system call to signal, signal modify, modify or wait for another process Credentials. Each process must have an associated user ID and one or more group IDs that determine the process’s rights to access system resources and files Personality. Not traditionally found on UNIX systems, but under Linux each process has an associated personality identifier that can slightly modify the semantics of certain system calls z Used primaril primarily b by em emulation lation libraries to req request est that ssystem stem calls be compatible with certain specific flavors of UNIX Operating System Concepts – 8th Edition 21.14 Silberschatz, Galvin and Gagne ©2009 Process Environment The process’s environment is inherited from its parent, and is composed of two null-terminated vectors: z The argument vector lists the command-line arguments used to invoke the running program; conventionally starts with the name of the program itself z The environment vector is a list of “NAME=VALUE” pairs that associates named environment variables with arbitrary textual values Passing environment en ironment variables ariables among processes and inheriting variables by a process’s children are flexible means of passing information to components of the user-mode system software The environment-variable mechanism provides a customization of the operating system that can be set on a per-process basis, rather than being configured for the system as a whole Operating System Concepts – 8th Edition 21.15 Silberschatz, Galvin and Gagne ©2009 Process Context The (constantly changing) state of a running program at any point in time The scheduling context is the most important part of the process context; it is the information that the scheduler needs to suspend and restart the process The kernel maintains accounting information about the resources currently being b i consumed db by each h process, and d th the ttotal t l resources consumed db by th the process in its lifetime so far The file table is an array of pointers to kernel file structures z When making file f I/O /O system calls, processes refer f to files f by their index into this table Operating System Concepts – 8th Edition 21.16 Silberschatz, Galvin and Gagne ©2009 Process Context (Cont) Whereas the file table lists the existing open files, the file-system context applies to requests to open new files z The current root and default directories to be used for new file searches are stored here The signal-handler table defines the routine in the process’s address space to t be b called ll d when h specific ifi signals i l arrive i The virtual-memory context of a process describes the full contents of the its private address space Operating System Concepts – 8th Edition 21.17 Silberschatz, Galvin and Gagne ©2009 Processes and Threads Linux uses the same internal representation for processes and threads; a thread is simply a new process that happens to share the same address space as its parent A distinction is only made when a new thread is created by the clone system call z f k creates a new process with its own entirely new process context fork z clone creates a new process with its own identity, but that is allowed to share the data structures of its parent Using U i clone gives i an application li ti fifine-grained i d control t l over exactly tl what h t iis shared between two threads Operating System Concepts – 8th Edition 21.18 Silberschatz, Galvin and Gagne ©2009 Kernel Synchronization A request for kernel-mode execution can occur in two ways: z A running program may request an operating system service, either explicitly via a system call, or implicitly, for example, when a page fault occurs z A device driver may deliver a hardware interrupt that causes the CPU to start t t executing ti a kernel-defined k l d fi d h handler dl ffor th thatt iinterrupt t t Kernel synchronization requires a framework that will allow the kernel’s critical sections to run without interruption by another critical section Operating System Concepts – 8th Edition 21.19 Silberschatz, Galvin and Gagne ©2009 Kernel Synchronization (Cont) Linux uses two techniques to protect critical sections: 1. Normal kernel code is nonpreemptible (until 2.6) – when a time interrupt is received while a process is executing a kernel system service routine, the kernel’s need_resched flag is set so that the scheduler will run once the system call has completed and control is about to be returned to user mode – so kernel code gets another turn 2. The second technique applies to critical sections that occur in an interrupt p service routines – By using the processor’s interrupt control hardware to disable interrupts during a critical section, the kernel guarantees that it can proceed without the risk of concurrent access of shared data structures Operating System Concepts – 8th Edition 21.20 Silberschatz, Galvin and Gagne ©2009 Kernel Synchronization (Cont) To avoid performance penalties, Linux’s kernel uses a synchronization architecture that allows long critical sections to run without having interrupts disabled for the critical section’s section s entire duration Interrupt service routines are separated into a top half and a bottom half. z The top half is a normal interrupt service routine, and runs with recursive i iinterrupts t t di disabled bl d z The bottom half is run, with all interrupts enabled, by a miniature scheduler that ensures that bottom halves never interrupt themselves z This architecture is completed by a mechanism for f disabling selected bottom halves while executing normal, foreground kernel code Operating System Concepts – 8th Edition 21.21 Silberschatz, Galvin and Gagne ©2009 Interrupt Protection Levels Each level may be interrupted by code running at a higher level, but will never be interrupted by code running at the same or a lower level User processes can always be preempted by another process when a time-sharing scheduling interrupt occurs Operating System Concepts – 8th Edition 21.22 Silberschatz, Galvin and Gagne ©2009 Process Implementation Same fork()/exec() interface you are familiar with. fork() is actually implemented with more flexible clone() method z can create a task with a virtual memory context (process) or without (thread) Each thread (task) has z unique Program Counter z stack z register set z address space pointer (optional) Operating System Concepts – 8th Edition 21.23 Silberschatz, Galvin and Gagne ©2009 Process Implementation (2) Each task has a process descriptor struct task_struct (include/linux/sched.h, line 335 in 2.4.24 kernel source) Contains all relevant info about a process Stored at the "end" of the kernel stack: z Register-poor Register poor architectures don't don t need Stack High addrs extra register for task_struct The task_struct instance for the current taskk iis available il bl via i the h current pointer i (see include/asm-arm/current.h) current is actually calculated from stack pointer value Operating System Concepts – 8th Edition 21.24 task_struct space Low addrs Silberschatz, Galvin and Gagne ©2009 task_struct fields of interest volatile long state – process state (-1 unrunnable, 0 runnable, >0 stopped) z Actual values are: #define TASK_RUNNING 0 #define TASK_INTERRUPTIBLE 1 #define TASK TASK_UNINTERRUPTIBLE UNINTERRUPTIBLE 2 #define TASK_ZOMBIE 4 #define TASK_STOPPED 8 #define TASK_EXCLUSIVE 32 int sigpending – set if a signal is waiting for this process struct mm_struct *mm – pointer to virtual address space pid_t pid – process ID (default max is 32768) Operating System Concepts – 8th Edition 21.25 Silberschatz, Galvin and Gagne ©2009 task_struct fields of interest (cont) struct task_struct *p_opptr, *p_pptr, *p_cptr, *p_ysptr, *p_osptr; z Process tree pointers: parent, youngest child, younger sibling, older sibling uid_t uid, euid, suid, fsuid; gid_t gid, egid, sgid, fsgid; z user and group IDs struct thread_struct thread z CPU specific p state of this task. see ((include/asm-arm/processor.h p l. 64)) struct fs_struct *fs z file system info (current working directory, etc.) struct files_struct files struct *files z open file info Operating System Concepts – 8th Edition 21.26 Silberschatz, Galvin and Gagne ©2009 Process context Normal program execution occurs in user space (like Nachos) Exception or system call switches execution to kernel space (like Nachos) z still executing in the process user context Interrupt causes context switch to interrupt context z This is a kernel thread with no associated virtual address space Operating System Concepts – 8th Edition 21.27 Silberschatz, Galvin and Gagne ©2009 System Calls User program makes call: e.g. read( ... ) read() is a C library wrapper (glibc, typically) library generates "software interrupt" (0x80 on intel architectures) with the appropriate system call number This causes switch to kernel space and execution of system call handler system_call() (which is actually an assembly asse b y language a guage routine) out e) This looks up the appropriate method to call for the supplied system call number, e.g. sys_read(...) Very similar to Nachos, except the stubs are hidden by a C library Operating System Concepts – 8th Edition 21.28 Silberschatz, Galvin and Gagne ©2009 Adding a new system call Add an entry to the end of the system call table (see linux/arch/armnommu/kernel/calls.S) for your system call: #ifndef NR NR_syscalls syscalls #define NR_syscalls 288 #else __syscall y _start: /* 0 */ /* 5 */ .long SYMBOL_NAME(sys_ni_syscall) .long SYMBOL_NAME(sys_exit) .long SYMBOL_NAME(sys_fork_wrapper) .long SYMBOL_NAME(sys_read) .long SYMBOL_NAME(sys_write) .long SYMBOL_NAME(sys_open) .long SYMBOL_NAME(sys_madvise) .long long SYMBOL NAME(sys fcntl64) SYMBOL_NAME(sys_fcntl64) ... /* 220 */ __syscall_end: .rept NR_syscalls - (__syscall_end - __syscall_start) / 4 .long g SYMBOL_NAME(sys ( y _ni_syscall) y ) .endr Operating System Concepts – 8th Edition 21.29 Silberschatz, Galvin and Gagne ©2009 Adding a new system call For a real system call that was generally useful, you'd do this in the subdirectories for all of the supported architectures. We're only interested in the iPod processor Add the resulting system call number to include/asm-armnommu/unistd.h #define __NR_mincore (__NR_SYSCALL_BASE+219) #define __NR_madvise (__NR_SYSCALL_BASE+220) #define __NR_fcntl64 (__NR_SYSCALL_BASE+221) Now,, your y syscall y has to be compiled p into the kernel. To avoid having g to frob with the Makefile, you can add your code to arch/armnommu/kernel/sys_arm.c, source code for miscellaneous "special" system calls Operating System Concepts – 8th Edition 21.30 Silberschatz, Galvin and Gagne ©2009 Using your new sys call You could modify the source for glibc to add the appropriate library call OR, use the provided _syscalln() macros, where n is in the range 0 to 6 and indicates how many parameters the sys call will pass So, for an imaginary sys call # 42, reopen, with the following signature: long reopen(const char *filename, int flags, int mode) we would add z #define __NR_reopen 42 _syscall3(long, syscall3(long reopen, reopen const char *, * filename, filename int flags, int mode) to the program that wanted to call reopen() See IBM DeveloperWorks article for details Operating System Concepts – 8th Edition 21.31 Silberschatz, Galvin and Gagne ©2009 Kernel programming tips Example from Love: /* * silly_copy - utterly worthless syscall that copies the len bytes from * 'src' to 'dst' using the kernel as an intermediary in the copy for no * good reason. But it i makes for a good example! ! */ / asmlinkage long sys_silly_copy(unsigned long *src, unsigned long *dst, unsigned long len) { unsigned long buf; /* fail if the kernel wordsize and user wordsize do not match */ if (len != sizeof(buf)) return -EINVAL; /* copy src, which is in the user's address space, into buf */ if (copy_from_user(&buf, src, len)) return -EFAULT; /* copy buf into dst, which is in the user's address space */ if (copy_to_user(dst, &buf, len)) return t -EFAULT; EFAULT /* Return amount of data copied */ return len; } Operating System Concepts – 8th Edition 21.32 Silberschatz, Galvin and Gagne ©2009 Compiling/Installing the kernel Unless you have added files, compiling should be simple – just type "make" in the top level of the source tree. There will be an executable called "linux" at the top level of the source tree. This isn't the one you want. Copy arch/armnommu/boot/Image to linux.bin in the root of the Linux partition titi on the th iPod iP d Reboot the iPod – hold down "Menu" on the click wheel and the center button together for several seconds Operating System Concepts – 8th Edition 21.33 Silberschatz, Galvin and Gagne ©2009 Compiling user-level programs You are compiling on an x86 platform and need to produce an executable for the ARM processor on the iPod Department Linux boxes have the appropriate cross-compiler installed: arm- elf-gcc. This is the 2.95 toolchain. See http://www.ipodlinux.org/wiki/Toolchain if you'd like to install on your own system t To compile myuserprog.c: arm-elf-gcc –o myuserprog myuserprog.c –elf2flt The –elf2flt creates the executable format expected by the iPod You will have to copy onto the iPod from a Ubuntu system You can use /home on the iPod's iPod s Linux partition for your user programs. Remember to use sudo when you do your copy Check permissions before copying – should be 755 (all users have read/exec)) arm-elf-gcc produces two executables, use the one without .gdb ext. Operating System Concepts – 8th Edition 21.34 Silberschatz, Galvin and Gagne ©2009 Executing user-level programs One of the top-level choices in podzilla is "File Browser" Use it to browse to your user program With your executable selected, press AND HOLD the center button Select Execute Select "Read Read Output" Output , unless you're you re trying something interactive Press MENU on the clickwheel to exit Operating System Concepts – 8th Edition 21.35 Silberschatz, Galvin and Gagne ©2009 Resources http://www.netlibrary.com/library_home_page.asp - you can get to Robert Love's Linux Kernel Development through here. See Ch. 5 http://www.ibm.com/developerworks/linux/library/l-system-calls/ - Nice article from IBM DeveloperWorks about writing new sys calls http://lxr.linux.no/linux-old+v2.4.24// - Linux Cross reference, searchable version i off the th kkernell source code d http://www.faqs.org/docs/kernel_2_4/lki.html - Linux Kernel Internals by Tigran Aivazian http://www.gelato.unsw.edu.au/~dsw/public-files/kernel-docs/kernel// / / f / / api/index.html - Linux Kernel API Operating System Concepts – 8th Edition 21.36 Silberschatz, Galvin and Gagne ©2009