Week 3: Make Your Own System Call CS444/544 System Call

• A system call is used by application (user) programs to request service from the operating system. • Switch to kernel mode.(higher privilege) • Required to ensure security.

How to add a system call

• Register the system call. • Declare the system call in the system header file. • Create a source file and a make file for implementing your system call. • Compile the kernel. • Create a client to test it. Steps

• VM present in e:\systemcall\Lab1\ubuntu-server.vmx • For ubuntu go to /storage/systemcall/Lab1 • Log in the Virtual Machine • Username: root • Password: cspassword • Check the system kernel info (kernel version, 32/64bit) and note it down • uname -sr • Go to Linux kernel source code directory • cd /usr/src/linux

Before the Lab

● Be careful to modify the kernel file ● Be aware of the naming consistency ● Don't miss any steps WHY? ○ Because you are hacking the kernel ○ Because no debug environment is set up yet Assume: Print Working Directory(pwd) is /usr/src/linux

Edit arch/x86/syscalls/syscall_64.tbl to register your system call: Above x32-specific system call table around Line 323, register as 31x number, follow the same pattern 31? common yourname sys_yourname (should be 313)

Edit include/linux/syscalls.h to declare your system call function: Right at the very end before #endif, add the system call function asmlinkage int sys_yourname(int num1, int num2);

Create your directory(mkdir) kernel/yourname/ and create two files sys_yourname.c and Makefile inside to define your system call function sys_yourname.c in kernel/yourname/ should look like

#include #include asmlinkage int sys_yourname(int num1, int num2){ return num1+num2; }

Makefile in kernel/yourname/ should look like obj-y := sys_yourname.o

Edit kernel/Makefile to include kernel/yourname/ directory Right below obj-y +=power/, to add the line obj-y += yourname/ File Checklist

Assume: Your Print Working Directory(pwd) is /usr/src/linux ● arch/x86/syscalls/syscall_64.tbl ● include/linux/syscalls.h ● kernel/yourname/sys_yourname.c ● kernel/yourname/Makefile ● kernel/Makefile Compile the Kernel Assume: Print Working Directory(pwd) is /usr/src/linux Generate kernel configuration file .config using: make clean make mrproper make menuconfig make localmodconfig Compile the kernel using: make -j2 Copy all kernel modules into /lib/modules/3.7.2/kernel/folder using: make modules_install Copy vmlinuz, system.map, and initramfs into /boot using: make install Update bootloader using: update-grub Reboot the System command: reboot • Assume: Present Working Directory(pwd) is /root

• Check your system kernel version again

• If it is still the previous kernel version, you need to go back to check your kernel compilation steps. Test your System Call Assume: Present Working Directory(pwd) is /root/yourname

Create a directory yourname/ and create three files testyourname.h, testyourname.c, and Makefile in yourname/

Header file testanyname.h #include int yourname(int num1, int num2) { return syscall(syscallNO, num1, num2); }

User code file testanyname.c #include #include "testanyname.h" int main() { printf("the kernel returns %d\n", yourname(1, 25)); return 0; } Makefile will instruct the compiler and the linker how to compile and link all the separated files to be one program.

Makefile all: testanyname testanyname: testanyname.c testanyname.h gcc testanyname.c -o testanyname //should have one tab in front of gcc, not //whitespace clean: rm testanyname

Generate the executable file with command: make, testyourname should appear in yourname/ directory

Does it run?

Yes-> ask TA for credit. No-> ask for help.