Question

write系统调用具有以下功能原型：

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

如何在MacOS X下的GCC中使用内联汇编程序调用{{1}}系统调用？

Answer 1

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，然后使用gcc -S生成程序集或使用otool来反汇编二进制文件;找出write（）调用是如何组装的，并将其转换为适当的内联汇编。

（编辑）要查看实际的系统调用，请在汇编代码中跟随库调用。在write()的示例中，根据间接情况，您将libSystem.B.dylib引导至_write中的libsystem_kernel.dylib，您可以使用otool对其进行反汇编。

（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;
}

使用-O1 -fverbose-asm进行编译和测试，以获得简洁易读的机器代码：

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

使用otool反汇编main()二进制文件中的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

列出动态链接的库test以查找_write：

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

尝试在_write中找到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libSystem.B.dylib：

的依赖关系

% 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)

猜猜_write可能包含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
[...]

现在我们拥有构建test的内联汇编版本所需的所有程序集：

#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

这似乎有效。上面的内联汇编不是很精致;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对它们进行硬编码。

Answer 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码（0x04）。

如何在MacOS X下使用GCC中的内联汇编程序调用write syscall？

2 个答案: