An example on how to hook a syscall.
Indexes in the sys_call_table can be found in /usr/include/x86_64-linux-gnu/asm/unistd{,_64}.h or /usr/include/asm/unistd_{32,64}.h depending on the distro.
A match between the indexes and the functions can be found in /usr/src/$(uname -r)/arch/x86/syscalls/syscall_{32,64}.tbl
The declaration of syscall functions can be found in /usr/src/$(uname -r)/include/linux/syscalls.h