Project #2, Linux Kernel Modifications CS-502 Fall 20061 Programming Project #2 Linux Kernel Hacking CS-502 Operating Systems Fall 2006.

Project #2, Linux Kernel Modifications

CS-502 Fall 2006 1

Programming Project #2Linux Kernel Hacking

CS-502 Operating SystemsFall 2006


CS-502 Fall 2006 2

Objective

• To learn how to work with an operating system kernel

• To understand some of the constraints and techniques of programming in a kernel (versus user space)


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Method

• To build and install a new Linux kernel

• To add a new system call to the Linux kernel

• To get useful information from the data structures of a Linux kernel


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Background – User vs. Kernel mode

• Hardware provides two modes– Indicated by bit in PSW

• Allows OS to protect itself & system components against – Faulty and malicious processes

• Some instructions designated as privileged– Only executable in kernel mode

• System call, all traps, & interrupts change mode to kernel– return from system call resets it to user


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Transition from User to Kernel Mode


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Accessing the Kernel via System Call

• Normally embedded within a library routine• User API never uses system calls directly

• System call mechanism is machine specific• Different CPU architectures make system calls in

different ways

• System call numbers different for various architectures

• Even for same operating system & version!• E.g., poll system call is #167 on PowerPC but

#168 on Intel 386 platforms (in SUSE Linux 9.3)


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Inside Kernel, the OS can …

• Read and modify data structures not in user address space

• Control devices forbidden to user processes

• Invoke operating system functions not available to user processes

• …


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In this project, we will …

• Add a new system call to the Linux kernel– It does nothing except announce its presence

• Upgrade that system call to provide information about the process– Information not readily available via existing system

calls

• Follow Linux naming & numbering conventions


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In this project, we won’t …

• … bother to make a library routine to encapsulate our systems calls

• … try to support them on all machine architectures


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To get started

• Find kernel sources in /usr/srclinux-2.6.11.4-20alinux-2.6.11.4-21.13linux-2.6.11.4-21.14

• Clone kernel source tree by linked copycp –al /usr/src/linux-2.6.11.4-20a myKernel

– Creates a linked copy of original tree in new directory called myKernel


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Digression on Unix Hard Links

• Directory entries point to files• Two entries may point to same file!

• Same or different directories• Same or different name• Permissions attached to file, not directory• Called hard links (as opposed to symbolic links)

• Modifications to file seen via all hard links

• mv and rm commands change directories, not files!• File goes away when all directory entries (i.e., hard

links) to that file are deleted.


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Cloned Source Tree

• All directories are copied

• All directory entries in copy are linked back to files in original source tree

• To modify a file:–• Unlink it (via mv or rm)

• Replace it with modified copy in that directory

• Original is preserved intact in other directory

• Note: Versions of Linux kernel sources are linked copies of each other


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To Modify a File

• mv file.c file.c~

• Edit file.c~ in your favorite editor• Save as file.c• (Eventually) delete file.c~

• EMACS and patch do this automatically

• Most other editors require you to do it manually


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Part 1: Build Linux Kernel

• Make configuration– As ordinary user

• Build source tree– As ordinary user

• Install– Needs root privileges


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Make configuration

• In a command shell at top of your cloned directory, do one of:–– make config

• Very long and tedious– make menuconfig

• Still somewhat long and tedious; also hokey– make xconfig

• Nice– make gconfig

• Really nice, but our virtual machines are missing some libraries to support this


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Make configuration (continued)

• Edit “General Setup” to name your “Local version”

• No other edits necessary at this time.• Save and quit

• If need to rebuild, use•make oldconfig

to reuse same configuration


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To Build Kernel

• make > make-output.txt

• Execute at top of your kernel tree

• Takes• 40 minutes on csopt4• ~ one hour on 3 gigahertz Pentium

• Rebuilds after small edits are much faster

• Changing .h files can cause longer rebuilds


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To Install Kernel

• Requires root privileges– sudo make install modules_install

• Puts kernel, initrd file, symbols in /boot– Changes links for vmlinuz, initrd

• Adds entries to /boot/grub/menu.lst– So you can select which kernel to boot

• Some apparent bugs in this process– Use YaST to repair manually


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Running Your Kernel

• Restart your virtual machine

• Click in boot window

• Use arrow keys to select desired kernel or system

• To determine which kernel is running:–– uname -a


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Creating a Patch File

• In kernel tree, remove all object & config files by– make distclean

• One level above kernel tree, do– diff –urN original myKernel > patch1

• To recreate your directory from patch– cp –al original newKernel– cd newKernel– patch –p1 < patch1


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End of Part 1

Questions?


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Part 2: Adding a System Call

• See Silbershatz, pp 74-78• Similar problem statement

• Many details are different (due to version of Linux)

• Clone kernel tree from Part1• Use make oldconfig before building


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Linux Conventions (all versions)

• If your library routine is alarm, …• … then the corresponding system call is sys_alarm

• … and the corresponding function prototype for its implementation isasmlinkage unsigned long sys_alarm (unsigned int seconds)

• Note that asmlinkage is a compiler directive that tells how to compile the function call

• in gcc


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Linux Conventions (continued)

• To invoke alarm system call from a user or library routine, use macro_syscall1(unsigned long, alarm, unsigned int seconds)

• _syscalln has n+2 arguments• Return type

• Name of actual system call (in user space)

• Arguments to system call function

• This macro defines the functionunsigned long alarm(unsigned int seconds)


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helloworld System Call

• /* This is the text of the helloworld system call */

asmlinkage int sys_helloworld(void) {printk(KERN_EMERG “Hello,

world!”);return 0;

}

• Add to the file kernel/sys.c


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printk(), the Kernel Debug Print Tool

• Very robust• May be called from (almost) anywhere in kernel• Same calling convention as printf()

• Writes to system log

• Output survives crashes (almost all of the time)

• To read output, see• /var/log/messages

• Needs root privileges to read

• Circular log, newest messages at end

• See Linux Kernel Development, 2nd edition, by Robert Love, Chapter 18.


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Registering your System Call

• include/asm-i386/unistd.h– Add entry for your call number– Increment total number of calls

• arch/i386/kernel/entry.S– Lists entry points for system calls– Must be kept in numerical order!– Number must correspond to entry in unistd.h

• Rebuild and install your kernel


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Testing your System Call

• #include <linux/errno.h>#include <sys/syscall.h>#include <linux/unistd.h>#include <stdio.h>

#define __NR_helloworld 288 /* or whatever you set it in unistd.h */

_syscall0(int, helloworld);

main () {printf(“The return code from the

helloworld system call is %d\n”, helloworld());}

• Check log for the message!


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Create Patch File

• patch2 is difference between kernel tree for Part1 and kernel tree for Part2


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End of Part 2

Questions?


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Part 3: Get Process Information

• Modify you kernel of Part 2 to add system call to get information about process

• System call is– int getprinfo(struct prinfo *info)

– info is pointer to area to store results– Returns zero if successful, error code if not

• See handout for definition of struct prinfo


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Information needed for prinfo

• See task_struct in include/linux/sched.h• See getuid and getpid for examples of

simple system calls• See include/asm/current.h to find current

process information• Use copy_to_user to safely copy data from

kernel to user space• Return EFAULT error code if info argument

is not valid pointer in user space


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copy_to_user and copy_from_user

• Functions to safely copy data to/from user space

• Check validity of pointer arguments

• Return zero if successful, number of bytes that fail if there is a problem

• Immune to page faults, pre-emption, etc.


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Implementing getprinfo System Call

• Replace helloworld system call from Part 2• Implement in kernel/timer.c• Register in unistd.h and entry.S

• Use printk() to print debugging statements to system log


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Testing getprinfo

• Write test program in user space

• Run multiple times from same shell, different shell

• Note differences in results

• Compare with what you can find about processes from ps command


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Create patch3

• Patch3 is difference between Part 2 & Part 3.


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Submission

• Submit using web-based turnin program– http://turnin.cs.wpi.edu:8088/servlets/turnin/turnin.ss

• Include – patch1, patch2, and patch3– Write up explaining results of testing Part 3– Starting point for your kernel tree– Put your name on all documents and at top of

every edited file!

http://turnin.cs.wpi.edu:8088/servlets/turnin/turnin.ss


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Due Dates

• This project is due at start of class on Monday, October 16.

• Pace yourself:–– Part 1 should be complete by October 2– Part 2 should be complete by October 9– Part 3 should be complete by October 16

• Report to instructor any difficulties


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Questions?

Project #2, Linux Kernel Modifications CS-502 Fall 20061 Programming Project #2 Linux Kernel Hacking CS-502 Operating Systems Fall 2006.

Documents

linux kernel slide

linux kernel modifications

new linux kernel

linux kernel hacking

kernel mode system

kernel sources

suse linux

usrsrc linux