Introduction to linux kernel by Quontra Solutions

Information about Introduction to linux kernel by Quontra Solutions

Published on October 13, 2014

Author: quontra123



Linux Kernel Introduction: Linux Kernel Introduction Presented By Quontra Solutions Email:[email protected] Contact Us 3427 Vintage cir,SE Smyrna, GA-30080 Phone :  (404)-900-9988 / (404)-990-3007 History: History UNIX: 1969 Thompson & Ritchie AT&T Bell Labs. BSD: 1978 Berkeley Software Distribution. Commercial Vendors: Sun, HP, IBM, SGI, DEC. GNU: 1984 Richard Stallman, FSF. POSIX: 1986 IEEE Portable Operating System unIX. Minix: 1987 Andy Tannenbaum. SVR4: 1989 AT&T and Sun. Linux: 1991 Linus Torvalds Intel 386 (i386). Open Source: GPL. Linux Features: Linux Features UNIX-like operating system. Features: Preemptive multitasking. Virtual memory (protected memory, paging). Shared libraries. Demand loading, dynamic kernel modules. Shared copy-on-write executables. TCP/IP networking. SMP support. Open source. What’s a Kernel?: What’s a Kernel? AKA: executive, system monitor. Controls and mediates access to hardware. Implements and supports fundamental abstractions: Processes, files, devices etc. Schedules / allocates system resources: Memory, CPU, disk, descriptors, etc. Enforces security and protection. Responds to user requests for service (system calls). Etc…etc… Kernel Design Goals: Kernel Design Goals Performance: efficiency, speed. Utilize resources to capacity with low overhead. Stability: robustness, resilience. Uptime, graceful degradation. Capability: features, flexibility, compatibility. Security, protection. Protect users from each other & system from bad users. Portability. Extensibility. Example “Core” Kernel: Example “Core” Kernel Applications System Libraries (libc) System Call Interface Hardware Architecture-Dependent Code I/O Related Process Related Scheduler Memory Management IPC File Systems Networking Device Drivers Modules Architectural Approaches: Architectural Approaches Monolithic. Layered. Modularized. Micro-kernel. Virtual machine. Linux Source Tree Layout : Linux Source Tree Layout /usr/src/linux Documentation arch fs init kernel include ipc drivers net mm lib scripts alpha arm i386 ia64 m68k mips mips64 ppc s390 sh sparc sparc64 acorn atm block cdrom char dio fc4 i2c i2o ide ieee1394 isdn macintosh misc net … adfs affs autofs autofs4 bfs code cramfs devfs devpts efs ext2 fat hfs hpfs … asm-alpha asm-arm asm-generic asm-i386 asm-ia64 asm-m68k asm-mips asm-mips64 linux math-emu net pcmcia scsi video … adfs affs autofs autofs4 bfs code cramfs devfs devpts efs ext2 fat hfs hpfs … 802 appletalk atm ax25 bridge core decnet econet ethernet ipv4 ipv6 ipx irda khttpd lapb … linux/arch: linux/arch Subdirectories for each current port. Each contains kernel , lib , mm , boot and other directories whose contents override code stubs in architecture independent code. lib contains highly-optimized common utility routines such as memcpy, checksums, etc. arch as of 2.4: alpha, arm, i386, ia64, m68k, mips, mips64. ppc, s390, sh, sparc, sparc64. linux/drivers: linux/drivers Largest amount of code in the kernel tree (~1.5M). device, bus, platform and general directories. drivers/char – n_tty.c is the default line discipline. drivers/block – elevator.c, genhd.c, linear.c, ll_rw_blk.c, raidN.c. drivers/net –specific drivers and general routines Space.c and net_init.c. drivers/scsi – scsi_*.c files are generic; sd.c (disk), sr.c (CD-ROM), st.c (tape), sg.c (generic). General: cdrom, ide, isdn, parport, pcmcia, pnp, sound, telephony, video. Buses – fc4, i2c, nubus, pci, sbus, tc, usb. Platforms – acorn, macintosh, s390, sgi. linux/fs: linux/fs Contains: virtual filesystem (VFS) framework. subdirectories for actual filesystems. vfs-related files: exec.c, binfmt_*.c - files for mapping new process images. devices.c, blk_dev.c – device registration, block device support. super.c, filesystems.c. inode.c, dcache.c, namei.c, buffer.c, file_table.c. open.c, read_write.c, select.c, pipe.c, fifo.c. fcntl.c, ioctl.c, locks.c, dquot.c, stat.c. linux/include: linux/include include/asm-*: Architecture-dependent include subdirectories. include/linux: Header info needed both by the kernel and user apps. Usually linked to /usr/include/linux. Kernel-only portions guarded by #ifdefs #ifdef __KERNEL__ /* kernel stuff */ #endif Other directories: math-emu, net, pcmcia, scsi, video. linux/init: linux/init Just two files: version.c, main.c. version.c – contains the version banner that prints at boot. main.c – architecture-independent boot code. start_kernel is the primary entry point. linux/ipc: linux/ipc System V IPC facilities. If disabled at compile-time, util.c exports stubs that simply return –ENOSYS. One file for each facility: sem.c – semaphores. shm.c – shared memory. msg.c – message queues. linux/kernel: linux/kernel The core kernel code. sched.c – “the main kernel file”: scheduler, wait queues, timers, alarms, task queues. Process control: fork.c, exec.c, signal.c, exit.c etc… Kernel module support: kmod.c, ksyms.c, module.c. Other operations: time.c, resource.c, dma.c, softirq.c, itimer.c. printk.c, info.c, panic.c, sysctl.c, sys.c. linux/lib: linux/lib kernel code cannot call standard C library routines. Files: brlock.c – “Big Reader” spinlocks. cmdline.c – kernel command line parsing routines. errno.c – global definition of errno. inflate.c – “gunzip” part of gzip.c used during boot. string.c – portable string code. Usually replaced by optimized, architecture-dependent routines. vsprintf.c – libc replacement. linux/mm: linux/mm Paging and swapping: swap.c, swapfile.c (paging devices), swap_state.c (cache). vmscan.c – paging policies, kswapd. page_io.c – low-level page transfer. Allocation and deallocation: slab.c – slab allocator. page_alloc.c – page-based allocator. vmalloc.c – kernel virtual-memory allocator. Memory mapping: memory.c – paging, fault-handling, page table code. filemap.c – file mapping. mmap.c, mremap.c, mlock.c, mprotect.c. linux/scripts : linux/scripts Scripts for: Menu-based kernel configuration. Kernel patching. Generating kernel documentation. Summary: Summary Linux is a modular, UNIX-like monolithic kernel. Kernel is the heart of the OS that executes with special hardware permission (kernel mode). “Core kernel” provides framework, data structures, support for drivers, modules, subsystems. Architecture dependent source sub-trees live in /arch. Booting and Kernel Initialization: Booting and Kernel Initialization System Lifecycle: Ups & Downs: System Lifecycle: Ups & Downs start_kernel Power on Power off Boot Kernel Init OS Init RUN! Shut down init shutdown LILO sleep? (hlt) Boot Terminology: Boot Terminology Loader: Program that moves bits from disk (usually) to memory and then transfers CPU control to the newly “loaded” bits (executable). Bootloader / Bootstrap: Program that loads the “first program” (the kernel). Boot PROM / PROM Monitor / BIOS: Persistent code that is “already loaded” on power-up. Boot Manager: Program that lets you choose the “first program” to load. LILO: Linux Loader: LILO: Linux Loader A versatile boot manager that supports: Choice of Linux kernels. Boot time kernel parameters. Booting non-Linux kernels. A variety of configurations. Characteristics: Lives in MBR or partition boot sector. Has no knowledge of filesystem structure so… Builds a sector “map file” (block map) to find kernel. /sbin/lilo – “map installer”. /etc/lilo.conf is lilo configuration file. Example lilo.conf File: Example lilo.conf File boot=/dev/hda map=/boot/map install=/boot/boot.b prompt timeout=50 default=linux image=/boot/vmlinuz-2.2.12-20 label=linux initrd=/boot/initrd-2.2.12-20.img read-only root=/dev/hda1 /sbin/init: /sbin/init Ancestor of all processes (except idle/swapper process). Controls transitions between “runlevels”: 0: shutdown 1: single-user 2: multi-user (no NFS) 3: full multi-user 5: X11 6: reboot Executes startup/shutdown scripts for each runlevel. Shutdown: Shutdown Use /bin/shutdown to avoid data loss and filesystem corruption. Shutdown inhibits login, asks init to send SIGTERM to all processes, then SIGKILL. Low-level commands: halt, reboot, poweroff. Use -h, -r or -p options to shutdown instead. Ctrl-Alt-Delete “Vulcan neck pinch”: defined by a line in /etc/inittab. ca::ctrlaltdel:/sbin/shutdown -t3 -r now. Advanced Boot Concepts: Advanced Boot Concepts Initial ramdisk (initrd) – two-stage boot for flexibility: First mount “initial” ramdisk as root. Execute linuxrc to perform additional setup, configuration. Finally mount “real” root and continue. See Documentation/initrd.txt for details. Also see “man initrd”. Net booting: Remote root (Diskless-root-HOWTO). Diskless boot (Diskless-HOWTO). Summary: Summary Bootstrapping a system is a complex, device-dependent process that involves transition from hardware, to firmware, to software. Booting within the constraints of the Intel architecture is especially complex and usually involves firmware support (BIOS) and a boot manager (LILO). /sbin/lilo is a “map installer” that reads configuration information and writes a boot sector and block map files used during boot. start_kernel is Linux “main” and sets up process context before spawning process 0 (idle) and process 1 (init). The init() function performs high-level initialization before exec’ing the user-level init process. System Calls: System Calls System Calls: System Calls Interface between user-level processes and hardware devices. CPU, memory, disks etc. Make programming easier: Let kernel take care of hardware-specific issues. Increase system security: Let kernel check requested service via syscall. Provide portability: Maintain interface but change functional implementation. POSIX APIs: POSIX APIs API = Application Programmer Interface. Function defn specifying how to obtain service. By contrast, a system call is an explicit request to kernel made via a software interrupt. Standard C library ( libc ) contains wrapper routines that make system calls. e.g., malloc, free are libc routines that use the brk system call. POSIX-compliant = having a standard set of APIs. Non-UNIX systems can be POSIX-compliant if they offer the required set of APIs. Linux System Calls (1): Linux System Calls (1) Invoked by executing int $0x80 . Programmed exception vector number 128. CPU switches to kernel mode & executes a kernel function. Calling process passes syscall number identifying system call in eax register (on Intel processors). Syscall handler responsible for: Saving registers on kernel mode stack. Invoking syscall service routine. Exiting by calling ret_from_sys_call() . Linux System Calls (2): Linux System Calls (2) System call dispatch table: Associates syscall number with corresponding service routine. Stored in sys_call_table array having up to NR_syscall entries (usually 256 maximum). nth entry contains service routine address of syscall n. Initializing System Calls: Initializing System Calls trap_init() called during kernel initialization sets up the IDT (interrupt descriptor table) entry corresponding to vector 128: set_system_gate(0x80, &system_call); A system gate descriptor is placed in the IDT, identifying address of system_call routine. Does not disable maskable interrupts. Sets the descriptor privilege level (DPL) to 3: Allows User Mode processes to invoke exception handlers (i.e. syscall routines). The system_call() Function: The system_call() Function Saves syscall number & CPU registers used by exception handler on the stack, except those automatically saved by control unit. Checks for valid system call. Invokes specific service routine associated with syscall number (contained in eax ): call *sys_call_table(0, %eax, 4) Return code of system call is stored in eax . Parameter Passing: Parameter Passing On the 32-bit Intel 80x86: 6 registers are used to store syscall parameters. eax (syscall number). ebx , ecx , edx , esi , edi store parameters to syscall service routine, identified by syscall number. Wrapper Routines: Wrapper Routines Kernel code (e.g., kernel threads) cannot use library routines. _syscall0 … _syscall5 macros define wrapper routines for system calls with up to 5 parameters. e.g., _syscall3(int,write,int,fd, const char *,buf,unsigned int,count) Example: “Hello, world!”: Example: “Hello, world!” Linux Files Relating to Syscalls: Linux Files Relating to Syscalls Main files: arch/i386/kernel/entry.S System call and low-level fault handling routines. include/asm-i386/unistd.h System call numbers and macros. kernel/sys.c System call service routines. arch/i386/kernel/entry.S: arch/i386/kernel/entry.S Add system calls by appending entry to sys_call_table: .long SYMBOL_NAME(sys_my_system_call) include/asm-i386/unistd.h: include/asm-i386/unistd.h Each system call needs a number in the system call table: e.g., #define __NR_write 4 #define __NR_my_system_call nnn , where nnn is next free entry in system call table. kernel/sys.c: kernel/sys.c Service routine bodies are defined here: e.g., asmlinkage retval sys_my_system_call (parameters) { body of service routine; return retval; } Kernel Modules: Kernel Modules Kernel Modules: Kernel Modules See A. Rubini, “Device Drivers”, Chapter 2. Modules can be compiled and dynamically linked into kernel address space. Useful for device drivers that need not always be resident until needed. Keeps core kernel “footprint” small. Can be used to “extend” functionality of kernel too! Example: “Hello, world!”: Example: “Hello, world!” #define MODULE #include <linux/module.h> int init_module(void) { printk(“<1>Hello, world!\n”); return 0; } void cleanup_module(void) { printk(“<1>Goodbye cruel world \n”); } Using Modules: Using Modules Module object file is installed in running kernel using insmod module_name . Loads module into kernel address space and links unresolved symbols in module to symbol table of running kernel. The Kernel Symbol Table: The Kernel Symbol Table Symbols accessible to kernel-loadable modules appear in /proc/ksyms . register_symtab registers a symbol table in the kernel’s main table. Real hackers export symbols from the kernel by modifying kernel/ksyms.c  Project Suggestions (1): Project Suggestions (1) Real-Time thread library. Scheduler activations in Linux. A Linux “upcall” mechanism. Real-Time memory allocator / garbage collector. A distributed shared memory system. A QoS-based socket library. An event-based mechanism for implementing adaptive systems. DWCS packet scheduling. A heap-based priority scheduler for Linux. Project Suggestions (2): Project Suggestions (2) m S resolution timers for Linux. Porting the Bandwidth-Broker to Linux. A QoS Management framework like QuO or Dionisys. A Real-Time communications protocol. A feedback-control system for flow/error/rate/congestion control. “Active Messages” for Linux. A thread continuation mechanism. A thread migration / load-balancing system. Thank You : Thank You

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