Is syscall an instruction on x86_64?

Question

I wanted to check the code for performing system calls in glibc. I found something like this:

ENTRY (syscall)
    movq %rdi, %rax     /* Syscall number -> rax.  */
    movq %rsi, %rdi     /* shift arg1 - arg5.  */
    movq %rdx, %rsi
    movq %rcx, %rdx
    movq %r8, %r10
    movq %r9, %r8
    movq 8(%rsp),%r9    /* arg6 is on the stack.  */
    syscall         /* Do the system call.  */
    cmpq $-4095, %rax   /* Check %rax for error.  */
    jae SYSCALL_ERROR_LABEL /* Jump to error handler if error.  */
L(pseudo_end):
    ret         /* Return to caller.  */

Now my questions are:

Is syscall (before the cmpq instruction) an instruction?
If it is an instruction, what is the meaning of ENTRY (syscall)? The same name for an ENTRY (I don't know what an ENTRY is) and instruction?
What is L(pseudo_end)?

Note that normally specialized code is used for each system call, to set `%rax` from a constant (saving a register), and to skip setting up unused arguments. — o11c, Aug 31 '17 at 01:52

score 38 · Answer 1 · edited Aug 31 '17 at 01:25

38

syscall is an instruction in x86-64, and is used as part of the ABI for making system calls. (The 32-bit ABI uses int 80h or sysenter, and is also available in 64-bit mode, but using the 32-bit ABI from 64-bit code is a bad idea, especially for calls with pointer arguments.)

But there is also a C library function named syscall(2), a generic wrapper for the system-call ABI. Your code shows the dump of that function, including its decoding of the return value into errno-setting. ENTRY(syscall) just means that the function starts there.

L() and ENTRY() are CPP macros.

L(pseudo_end) is just a Label that can be a jump target. Maybe the code at SYSCALL_ERROR_LABEL jumps back to there, although it would be more efficient for that block of code to just ret, so maybe it's a relic from a former version, or used for something else.

edited Aug 31 '17 at 01:25

Peter Cordes

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answered May 14 '12 at 13:14

flolo

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If this is an actual instruction, what is its opcode? – SasQ Jun 07 '15 at 02:26
4

It is 0x0F 0x05 (just look at the software developer manual from Intel). – flolo Jul 30 '15 at 16:55
1

I wonder what would happen if a bare-metal program (i.e. without OS) call `syscall` instruction? – Shuai Jan 30 '23 at 04:44
@Shuai, take a look at https://en.wikipedia.org/wiki/Double_fault – Carlos Garcia Mar 24 '23 at 17:28

jpalecek · Answer 2 · 2012-05-14T14:25:41.557

6

Yes, syscall is an instruction on x86-64. There is a similar instruction sysenter on i686.

ENTRY(syscall) would be a macro. Probably expands to the symbol definition, you have to grep for that.

edited May 14 '12 at 14:25

answered May 14 '12 at 13:09

jpalecek

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Is syscall an instruction on x86_64?

2 Answers2