CODEWITHSUNDEEP
macosassembly
fairville
fairville

Reputation: 81

Segfault with pop/push in x86/OSX

I am struggling to understand why this x86 assembly code, compiles fine with gcc 4.2.1 (llvm) on OSX, but gives a segmentation fault when the executable is run: 

    .globl  _main
_main:
        push    %rbp
        mov     %rsp, %rbp
        mov     $1, %rbx
        push    %rbx
        lea     L_.str0(%rip), %rdi
        mov     %rbx, %rsi
        call    _printf
        pop     %rbx
        pop     %rbp
        ret

        .section        __TEXT,__cstring,cstring_literals
L_.str0:
        .asciz  "%d \000"

I observed that if the pop %rbx line is moved before call _printf, then the program works correctly. But why should it fail at all in its original form?

Upvotes: 3

Views: 1130

Answers (1)

fairville
fairville

Reputation: 81

This question is answered in detail by: How to print argv[0] in NASM? and also x86 Assembly on a Mac. It's essentially a gotcha when programming assembly on MacOSX.

To summarize:

  • This seg fault is due to a stack misalignment.
  • This occurs only on OSes that use the System V calling convention (including MacOSX, but excluding Linux), which insists that the stack pointer to be a multiple of 16 prior to making a function call (e.g. to printf).

A simple solution is to align the stack pointer (ie, align to a multiple of 16 bytes as per the Sys V requirement) prior to the call, and restore it after the call: 

.globl  _main
_main:
        push    %rbp
        mov     %rsp, %rbp
        mov     $1, %rbx
        lea     L_.str0(%rip), %rdi
        mov     %rbx, %rsi
        push    %rbx

    mov %rsp, %rax   ; Save copy of the stack pointer (SP)
    and $-16, %rsp   ; Align the SP to the nearest multiple of 16.
    sub $8, %rsp     ; Pad the SP by 8 bytes so that when we ...  
    push %rax        ; push the saved SP (=8 bytes on a 64-bit OS), 
                     ; we remain aligned to 16 bytes (8+8 = 16).

        call    _printf

    pop %rax         ; retrieve the saved SP
    mov %rax, %rsp   ; restore SP using saved value. 

        pop     %rbx
        pop     %rbp
        ret

        .section        __TEXT,__cstring,cstring_literals
L_.str0:
        .asciz  "%d \000"

Upvotes: 5

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