如何在MacOS X下使用GCC中的内联汇编程序调用写syscall ?

The write syscall has the following functional prototype:

write syscall具有以下功能原型:

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

How do I call the write syscall using inline assembler in GCC under MacOS X?

如何在MacOS X下使用GCC中的内联汇编程序调用写syscall ?

2 个解决方案

#1

A generic solution to this type of question: Write a short test program doing the write() call you are interested in, and then use gcc -S to produce assembly or use otool to disassemble the binary; find out how the write() call was assembled and transform that into the appropriate inline assembly.

这类问题的通用解决方案:编写一个简短的测试程序，编写您感兴趣的Write()调用，然后使用gcc -S来生成汇编或使用otool来分解二进制文件;了解write()调用是如何组装的，并将其转换为适当的内联程序集。

(edit) To see the actual syscall, follow the library call in the assembly code. In the example of write(), following the indirections will lead you via libSystem.B.dylib to _write in libsystem_kernel.dylib, which you can disassemble using otool.

(编辑)要查看实际的syscall，请遵循程序集代码中的库调用。在write()示例中，跟随indirections将通过libSystem.B引导您。在libsystem_kernel中写入_write的dylib。dylib，可以用otool拆卸。

(edit2) Full example below, using this test program test.c:

(edit2)完整示例，使用此测试程序测试。

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

Compile and test, using -O1 -fverbose-asm to get concise, readable machine code:

编译和测试，使用-O1 -fverbose-asm获得简洁、可读的机器代码:

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

Use otool to disassemble main() in the test binary:

在测试二进制文件中使用otool分解main():

% 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

List the libraries test is dynamically linked against to find _write:

列出库测试被动态链接到查找_write:

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

Try to locate _write in libSystem.B.dylib:

尝试在libSystem.B.dylib中找到_write:

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

Check the dependencies of libSystem.B.dylib:

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)

Guess that _write is probably contained in libsystem_kernel.dylib (or alternatively try all of them):

假设_write可能包含在libsystem_kernel中。dylib(或者尝试所有方法):

% 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
[...]

Now we have all the assembly we need to construct the inline assembly version of test:

现在我们已经有了构建内联测试程序集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;
}

Compile and test:

编译和测试:

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

That seems to work. The inline assembly above is not very refined; for example, you could pass in the first and third arguments dynamically as well, instead of hardcoding them in the assembly code.

这似乎工作。上面的内联程序集不是很完善;例如，您也可以动态地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。

#2

The Linux solution is something like this:

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

Should be similar on other Unix systems like Mac, but better check the syscall number (0x04) in the documentation.

应该类似于其他Unix系统，如Mac，但是最好检查文档中的syscall编号(0x04)。