how to add syscall on x86_64

There is little documentation about how to add a system call to Linux kernel on x86_64. This is how I do it.

Of course adding new system call is hardly a good idea. But my teacher simply gives us the homework ;-)

Things are different now in these aspects:

1. Entry.S no longer contains the syscall table; those tutorial mentioning entry.S don't apply to 2.6.

2. Now the i386 and x86_64 arch are merged into one: x86. (See arch and include directories.) However, some files, like syscall_table_32.S, don't have corresponding ones under x86_64.

For i386, here's a list of files to be edited, (see this tldp page):

1. arch/i386/kernel/syscall_table.S,
(now arch/x86/kernel/syscall_table_32.S)
2. include/asm-i386/unistd.h,
(now include/asm-x86/unistd_32.h)
3. include/linux/syscalls.h

For x86_64 arch, no corresponding syscall_table_64.S exists. To add new system calls to x86_64 kernel, edit these files:

1. include/asm-x86/unistd_64.h,

Add lines like:

#define __NR_my_call 288
__SYSCALL(__NR_my_call, sys_my_call)

2. include/linux/syscalls.h
Add a declaration here:

asmlinkage long sys_my_call(int n);

Of course, you can write the implementation in kernel/sys.c or another new foo.c file and then write Makefiles.

Thanks to this thread.