Syscall

syscall.md

Syscall

Calling Convention

System call number and return value

Arch/ABI    Instruction           System  Ret  Ret  Error    Notes
                                    call #  val  val2
───────────────────────────────────────────────────────────────────
x86-64      syscall               rax     rax  rdx  -        5
x32         syscall               rax     rax  rdx  -        5

Arguments

Arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
──────────────────────────────────────────────────────────────
x86-64        rdi   rsi   rdx   r10   r8    r9    -
x32           rdi   rsi   rdx   r10   r8    r9    -

Code

    .text
    .global _start
_start:
    movq $1, %rax       # write() system call number
    movq $1, %rdi       # first parameter  -- standard output file descriptor
    movq $message, %rsi # second parameter -- buffer
    movq $12, %rdx      # third parameter  -- buffer length
    syscall             # invoke system call

    movq $60, %rax      # exit() system call number
    xor %rdi, %rdi      # first parameter  -- exit status
    syscall             # invoke system call; this will never return
message:
    .ascii "hello world\n"

$ gcc -c hello.s -o hello.o
$ ld hello.o

References

• https://man7.org/linux/man-pages/man2/syscall.2.html
• https://man7.org/linux/man-pages/man7/vdso.7.html
• https://unix.stackexchange.com/questions/461107/are-system-calls-stored-in-registers
• https://cs.lmu.edu/~ray/notes/gasexamples/
• https://www.linuxjournal.com/article/6100
• https://bbs.pediy.com/thread-265812.htm