OS X - x64:堆栈不是16字节对齐错误

时间:2017-04-11 19:29:37

标签: macos assembly x86-64

我知道OS X是16字节堆栈对齐,但我不太明白为什么它会导致错误。

我在这里所做的就是将对象大小(即24)传递给%rdi,并调用malloc。这个错误是否意味着我要求32个字节?

错误信息是:

  

libdyld.dylib`stack_not_16_byte_aligned_error:    - > 0x7fffc12da2fa< + 0>:movdqa%xmm0,(%rsp)       0x7fffc12da2ff< + 5>:int3

     

libdyld.dylib`_dyld_func_lookup:       0x7fffc12da300< + 0>:pushq%rbp       0x7fffc12da301< + 1>:movq%rsp,%rbp

以下是代码:

Object_copy:
    pushq %rbp
    movq %rbp, %rsp

    subq $8, %rsp
    movq %rdi, 8(%rsp)          # save self address
    movq obj_size(%rdi), %rax   # get object size
    imul $8, %rax          
    movq %rax, %rdi 
    callq _malloc             <------------------- error in this call

    # rsi old object address
    # rax new object address
    # rdi object size, mutiple of 8

    # rcx temp reg

    # copy object tag
    movq 0(%rsi), %rcx
    movq %rcx, 0(%rax)

    # set rdx to counter, starting from 8
    movq $8, %rdx

    # add 8 to object size, since we are starting from 8
    addq $8, %rdi

    start_loop:
        cmpq %rdx, %rdi
        jle end_loop

        movq (%rdx, %rsi, 1), %rcx
        movq %rcx, (%rdx, %rax, 1)

        addq $8, %rdx
        jmp start_loop

    end_loop:
        leave 
        ret



Main_protoObj:
    .quad    5                          ; object tag
    .quad    3                          ; object size
    .quad    Main_dispatch_table        ; dispatch table

_main:
    leaq Main_protoObj(%rip), %rdi
    callq Object_copy                # copy main proto object
    subq $8, %rsp                    # save the main object on the stack
    movq %rax, 8(%rsp)
    movq %rax, %rdi                 # set rdi point to SELF
    callq Main_init
    callq Main_main

    addq $8, %rsp                    # restore stack

    leaq _term_msg(%rip), %rax
    callq _print_string

1 个答案:

答案 0 :(得分:3)

就像你说的那样,MacOS X有一个16字节的堆栈对齐,这意味着机器期望堆栈上的每个变量都从当前堆栈指针的16的倍数开始。

当堆栈未对齐时,意味着我们开始尝试从该16字节窗口的中间读取变量,并且通常最终会出现分段错误。

在代码中调用例程之前,需要确保堆栈正确对齐;在这种情况下,意味着基指针寄存器可以被16整除。

subq $8, %rsp               # stack is misaligned by 8 bytes
movq %rdi, 8(%rsp)          #
movq obj_size(%rdi), %rax   #
imul $8, %rax               #
movq %rax, %rdi             #
callq _malloc               # stack is still misaligned when this is called

要解决此问题,您可以subq %rsp使用16而不是8。

subq $16, %rsp               # stack is still aligned
movq %rdi, 16(%rsp)          #
...                          #
callq _malloc                # stack is still aligned when this is called, good