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系统write调用具有以下功能原型：

size_t write(int, const void *buf, size_t nbytes);

如何write在 MacOS X 下使用 GCC 中的内联汇编程序调用系统调用？

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2 回答 2

11

此类问题的通用解决方案：编写一个简短的测试程序，执行您感兴趣的 write() 调用，然后用于gcc -S生成汇编或用于otool反汇编二进制文件；找出 write() 调用是如何组装的，并将其转换为适当的内联程序集。

（编辑）要查看实际的系统调用，请按照汇编代码中的库调用。在 的示例中write()，遵循间接指令将引导您通过libSystem.B.dylibin _write，libsystem_kernel.dylib您可以使用 反汇编otool。

（edit2）下面的完整示例，使用此测试程序test.c：

#include <stdio.h>
int main(void)
{
    char buf[] = "test\n";
    ssize_t n;
    n = write(2, buf, sizeof(buf));
    return n;
}

编译测试，-O1 -fverbose-asm得到简洁易读的机器码：

% gcc -O1 -fverbose-asm -o test test.c
% ./test
test

用于在二进制文件otool中反汇编：main()test

% otool -p _main -tvV test
test:
(__TEXT,__text) section
_main:
0000000100000ef0    pushq   %rbp
0000000100000ef1    movq    %rsp,%rbp
0000000100000ef4    subq    $0x10,%rsp
0000000100000ef8    leaq    0xfa(%rbp),%rsi
0000000100000efc    movb    $0x74,0xfa(%rbp)
0000000100000f00    movb    $0x65,0xfb(%rbp)
0000000100000f04    movb    $0x73,0xfc(%rbp)
0000000100000f08    movb    $0x74,0xfd(%rbp)
0000000100000f0c    movb    $0x0a,0xfe(%rbp)
0000000100000f10    movb    $0x00,0xff(%rbp)
0000000100000f14    movl    $0x00000002,%edi
0000000100000f19    movl    $0x00000006,%edx
0000000100000f1e    xorb    %al,%al
0000000100000f20    callq   0x100000f32 ; symbol stub for: _write
0000000100000f25    addq    $0x10,%rsp
0000000100000f29    popq    %rbp
0000000100000f2a    ret

列出库test动态链接到 find _write：

% otool -L test
test:
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 159.1.0)

尝试_write定位libSystem.B.dylib：

% otool -p _write -tvV /usr/lib/libSystem.B.dylib
/usr/lib/libSystem.B.dylib:
(__TEXT,__text) section
Can't find -p symbol: _write

检查以下依赖项libSystem.B.dylib：

% otool -L /usr/lib/libSystem.B.dylib
/usr/lib/libSystem.B.dylib:
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 159.1.0)
    /usr/lib/system/libcache.dylib (compatibility version 1.0.0, current version 47.0.0)
    /usr/lib/system/libcommonCrypto.dylib (compatibility version 1.0.0, current version 55010.0.0)
    /usr/lib/system/libcompiler_rt.dylib (compatibility version 1.0.0, current version 6.0.0)
    /usr/lib/system/libcopyfile.dylib (compatibility version 1.0.0, current version 85.1.0)
    /usr/lib/system/libdispatch.dylib (compatibility version 1.0.0, current version 187.9.0)
    /usr/lib/system/libdnsinfo.dylib (compatibility version 1.0.0, current version 395.11.0)
    /usr/lib/system/libdyld.dylib (compatibility version 1.0.0, current version 195.6.0)
    /usr/lib/system/libkeymgr.dylib (compatibility version 1.0.0, current version 23.0.0)
    /usr/lib/system/liblaunch.dylib (compatibility version 1.0.0, current version 392.38.0)
    /usr/lib/system/libmacho.dylib (compatibility version 1.0.0, current version 800.0.0)
    /usr/lib/system/libmathCommon.A.dylib (compatibility version 1.0.0, current version 2026.0.0)
    /usr/lib/system/libquarantine.dylib (compatibility version 1.0.0, current version 36.6.0)
    /usr/lib/system/libremovefile.dylib (compatibility version 1.0.0, current version 21.1.0)
    /usr/lib/system/libsystem_blocks.dylib (compatibility version 1.0.0, current version 53.0.0)
    /usr/lib/system/libsystem_c.dylib (compatibility version 1.0.0, current version 763.13.0)
    /usr/lib/system/libsystem_dnssd.dylib (compatibility version 1.0.0, current version 1.0.0)
    /usr/lib/system/libsystem_info.dylib (compatibility version 1.0.0, current version 1.0.0)
    /usr/lib/system/libsystem_kernel.dylib (compatibility version 1.0.0, current version 1699.26.8)
    /usr/lib/system/libsystem_network.dylib (compatibility version 1.0.0, current version 1.0.0)
    /usr/lib/system/libsystem_notify.dylib (compatibility version 1.0.0, current version 80.1.0)
    /usr/lib/system/libsystem_sandbox.dylib (compatibility version 1.0.0, current version 1.0.0)
    /usr/lib/system/libunc.dylib (compatibility version 1.0.0, current version 24.0.0)
    /usr/lib/system/libunwind.dylib (compatibility version 1.0.0, current version 30.0.0)
    /usr/lib/system/libxpc.dylib (compatibility version 1.0.0, current version 77.19.0)

猜测_write可能包含在libsystem_kernel.dylib（或者尝试所有这些）：

% otool -p _write -tvV /usr/lib/system/libsystem_kernel.dylib | head -20
(__TEXT,__text) section
_write:
0000000000017fd4    movl    $0x02000004,%eax
0000000000017fd9    movq    %rcx,%r10
0000000000017fdc    syscall
0000000000017fde    jae 0x00017fe5
0000000000017fe0    jmp cerror
0000000000017fe5    ret
0000000000017fe6    nop
0000000000017fe7    nop
[...]

现在我们拥有了构建内联程序集版本所需的所有程序集test：

#include <stdio.h>
int main(void)
{
    char buf[] = "test\n";
    ssize_t n;
    asm volatile (
        "movl $0x00000002, %%edi\n"  /* first argument */
        "movl $0x00000006, %%edx\n"  /* third argument */
        "movl $0x02000004, %%eax\n"  /* syscall number */
        "syscall\n"
        : "=A"(n)         /* %rax: return value */
        : "S"(buf));      /* %rsi: second argument */
    return n;
}

编译和测试：

% gcc -O1 -fverbose-asm -o test-asm test-asm.c
% ./test-asm
test

这似乎行得通。上面的内联汇编不是很精炼；例如，您也可以动态传递第一个和第三个参数，而不是在汇编代码中硬编码它们。

于 2013-01-13T20:16:19.903 回答
2

Linux 解决方案是这样的：

.data mytext:
    .ascii "Hello World\n"  
.text 
  _mywrite:
    movl $0x04,         %eax     # Syscall No. 4 = write
    movl $0x01,         %ebx     # File. 1 = stdout 
    movl $mytext,       %ecx
    movl $0x0c,         %edx     # text length (hope I counted correctly)
    int  $0x80                   # Interrupt 0x80 -> make syscall

在 Mac 等其他 Unix 系统上应该类似，但最好检查文档中的系统调用号 (0x04)。

于 2013-01-13T20:41:35.707 回答