vmonteco's questions

为什么 nm 不像目标文件那样显示调试符号？

我的目标文件中的所有符号地址都是 0，这也是正常的吗？

口粮：

• main.c：

#include "foo.h"

int     main()
{
    foo();
    return 0;
}

• foo.c：

void    foo()
{
}

• foo.h：

#ifndef FOO_H
# define FOO_H
void    foo();
#endif

• 制作和符号libfoo.a：

$ gcc -c -g -o foo.o foo.c
$ nm -a foo.o 
0000000000000000 N .debug_abbrev
0000000000000000 N .debug_info
0000000000000000 N .debug_line
0000000000000000 N .debug_line_str
0000000000000000 N .debug_str
0000000000000000 T foo
0000000000000000 a foo.c
0000000000000000 t .text
$ ar rc libfoo.a foo.o
$ nm -a libfoo.a

foo.o:
0000000000000000 N .debug_abbrev
0000000000000000 N .debug_info
0000000000000000 N .debug_line
0000000000000000 N .debug_line_str
0000000000000000 N .debug_str
0000000000000000 T foo
0000000000000000 a foo.c
0000000000000000 t .text
$

• 编译成可执行文件：

$ gcc -g main.c -L . -lfoo
$ nm -a a.out 
0000000000000000 a 
0000000000004010 B __bss_start
                 w __cxa_finalize@GLIBC_2.2.5
0000000000004000 D __data_start
0000000000004000 W data_start
0000000000004008 D __dso_handle
0000000000003e20 d _DYNAMIC
0000000000004010 D _edata
0000000000004018 B _end
0000000000001138 T _fini
000000000000112e T foo
0000000000000000 a foo.c
0000000000003fe8 d _GLOBAL_OFFSET_TABLE_
                 w __gmon_start__
0000000000002004 r __GNU_EH_FRAME_HDR
0000000000001000 T _init
0000000000002000 R _IO_stdin_used
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
                 U __libc_start_main@GLIBC_2.34
0000000000001119 T main
0000000000000000 a main.c
0000000000001020 T _start
0000000000004010 D __TMC_END__
$ 

• file输出：

$ file a.out
a.out: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=48cebaa6f43e93bb1805faa5b71d41f1d1e71501, for GNU/Linux 4.4.0, with debug_info, not stripped
$ 

• readelf -S输出（附录）：

$ readelf -S a.out
There are 34 section headers, starting at offset 0x37c0:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         0000000000000318  00000318
       000000000000001c  0000000000000000   A       0     0     1
  [ 2] .note.gnu.pr[...] NOTE             0000000000000338  00000338
       0000000000000040  0000000000000000   A       0     0     8
  [ 3] .note.gnu.bu[...] NOTE             0000000000000378  00000378
       0000000000000024  0000000000000000   A       0     0     4
  [ 4] .note.ABI-tag     NOTE             000000000000039c  0000039c
       0000000000000020  0000000000000000   A       0     0     4
  [ 5] .gnu.hash         GNU_HASH         00000000000003c0  000003c0
       000000000000001c  0000000000000000   A       6     0     8
  [ 6] .dynsym           DYNSYM           00000000000003e0  000003e0
       0000000000000090  0000000000000018   A       7     1     8
  [ 7] .dynstr           STRTAB           0000000000000470  00000470
       0000000000000088  0000000000000000   A       0     0     1
  [ 8] .gnu.version      VERSYM           00000000000004f8  000004f8
       000000000000000c  0000000000000002   A       6     0     2
  [ 9] .gnu.version_r    VERNEED          0000000000000508  00000508
       0000000000000030  0000000000000000   A       7     1     8
  [10] .rela.dyn         RELA             0000000000000538  00000538
       00000000000000c0  0000000000000018   A       6     0     8
  [11] .init             PROGBITS         0000000000001000  00001000
       000000000000001b  0000000000000000  AX       0     0     4
  [12] .text             PROGBITS         0000000000001020  00001020
       0000000000000115  0000000000000000  AX       0     0     16
  [13] .fini             PROGBITS         0000000000001138  00001138
       000000000000000d  0000000000000000  AX       0     0     4
  [14] .rodata           PROGBITS         0000000000002000  00002000
       0000000000000004  0000000000000004  AM       0     0     4
  [15] .eh_frame_hdr     PROGBITS         0000000000002004  00002004
       0000000000000024  0000000000000000   A       0     0     4
  [16] .eh_frame         PROGBITS         0000000000002028  00002028
       0000000000000074  0000000000000000   A       0     0     8
  [17] .init_array       INIT_ARRAY       0000000000003e10  00002e10
       0000000000000008  0000000000000008  WA       0     0     8
  [18] .fini_array       FINI_ARRAY       0000000000003e18  00002e18
       0000000000000008  0000000000000008  WA       0     0     8
  [19] .dynamic          DYNAMIC          0000000000003e20  00002e20
       00000000000001a0  0000000000000010  WA       7     0     8
  [20] .got              PROGBITS         0000000000003fc0  00002fc0
       0000000000000028  0000000000000008  WA       0     0     8
  [21] .got.plt          PROGBITS         0000000000003fe8  00002fe8
       0000000000000018  0000000000000008  WA       0     0     8
  [22] .data             PROGBITS         0000000000004000  00003000
       0000000000000010  0000000000000000  WA       0     0     8
  [23] .bss              NOBITS           0000000000004010  00003010
       0000000000000008  0000000000000000  WA       0     0     1
  [24] .comment          PROGBITS         0000000000000000  00003010
       0000000000000036  0000000000000001  MS       0     0     1
  [25] .debug_aranges    PROGBITS         0000000000000000  00003046
       0000000000000060  0000000000000000           0     0     1
  [26] .debug_info       PROGBITS         0000000000000000  000030a6
       00000000000000ab  0000000000000000           0     0     1
  [27] .debug_abbrev     PROGBITS         0000000000000000  00003151
       000000000000007b  0000000000000000           0     0     1
  [28] .debug_line       PROGBITS         0000000000000000  000031cc
       00000000000000a1  0000000000000000           0     0     1
  [29] .debug_str        PROGBITS         0000000000000000  0000326d
       000000000000003d  0000000000000001  MS       0     0     1
  [30] .debug_line_str   PROGBITS         0000000000000000  000032aa
       000000000000003e  0000000000000001  MS       0     0     1
  [31] .symtab           SYMTAB           0000000000000000  000032e8
       0000000000000258  0000000000000018          32     7     8
  [32] .strtab           STRTAB           0000000000000000  00003540
       000000000000011f  0000000000000000           0     0     1
  [33] .shstrtab         STRTAB           0000000000000000  0000365f
       000000000000015c  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), l (large), p (processor specific)

我的 python 代码可以访问通过调用的 C 函数接收和返回的实际指针值ctypes吗？

如果是，我该如何实现？

我想测试传递给共享库函数并从共享库函数返回的指针值，以使用 pytest 测试分配（这里，测试它strdup没有返回相同的指针而是返回指向不同地址的新指针）。

我把要实现的函数之一（strdup）包装在一个名为的文件中的一个新 C 函数中，wrapped_strdup.c以显示指针值和内存区域的内容：

/*
** I'm compiling this into a .so the following way:
**   - gcc -o wrapped_strdup.o -c wrapped_strdup.c
**   - ar rc wrapped_strdup.a wrapped_strdup.o
**   - ranlib wrapped_strdup.a
**   - gcc -shared -o wrapped_strdup.so -Wl,--whole-archive wrapped_strdup.a -Wl,--no-whole-archive
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *wrapped_strdup(char *src){
    char *dst;

    printf("From C:\n");
    printf("- src address: %X, src content: [%s].\n", src, src);
    dst = strdup(src);
    printf("- dst address: %X, dst content: [%s].\n", dst, dst);
    return dst;
}

我还在同一目录中创建了一个名为的 pytest 测试文件test_strdup.py：

#!/usr/bin/env python3

import ctypes
import pytest

# Setting wrapped_strdup:
lib_wrapped_strdup = ctypes.cdll.LoadLibrary("./wrapped_strdup.so")
wrapped_strdup = lib_wrapped_strdup.wrapped_strdup
wrapped_strdup.restype = ctypes.c_char_p
wrapped_strdup.argtypes = [ctypes.c_char_p]

@pytest.mark.parametrize("src", [b"", b"foo"])
def test_strdup(src: bytes):
    print("")
    dst = wrapped_strdup(src)

    print("From Python:")
    print(f"- src address: {hex(id(src))}, src content: [{src!r}].")
    print(f"- dst address: {hex(id(dst))}, dst content: [{dst!r}].")
    
    assert src == dst
    assert hex(id(src)) != hex(id(dst))

然后，运行测试得到以下输出：

$ pytest test_strdup.py --maxfail=2 -v -s
=================================== test session starts ====================================
platform linux -- Python 3.12.5, pytest-8.3.2, pluggy-1.5.0 -- /usr/bin/python
cachedir: .pytest_cache
rootdir: /home/vmonteco/code/MREs/MRe_strdup_test_with_ctypes
plugins: anyio-4.4.0, cov-5.0.0, typeguard-4.3.0
collected 2 items                                                                          

test_strdup.py::test_strdup[] 
From C:
- src address: C19BDBE8, src content: [].
- dst address: 5977DFA0, dst content: [].
From Python:
- src address: 0x75bcc19bdbc8, src content: [b''].
- dst address: 0x75bcc19bdbc8, dst content: [b''].
FAILED
test_strdup.py::test_strdup[foo] 
From C:
- src address: BF00A990, src content: [foo].
- dst address: 59791030, dst content: [foo].
From Python:
- src address: 0x75bcbf00a970, src content: [b'foo'].
- dst address: 0x75bcbefc18f0, dst content: [b'foo'].
PASSED

========================================= FAILURES =========================================
______________________________________ test_strdup[] _______________________________________

src = b''

    @pytest.mark.parametrize("src", [b"", b"foo"])
    def test_strdup(src: bytes):
        print("")
        dst = wrapped_strdup(src)
    
        print("From Python:")
        print(f"- src address: {hex(id(src))}, src content: [{src!r}].")
        print(f"- dst address: {hex(id(dst))}, dst content: [{dst!r}].")
    
        assert src == dst
>       assert hex(id(src)) != hex(id(dst))
E       AssertionError: assert '0x75bcc19bdbc8' != '0x75bcc19bdbc8'
E        +  where '0x75bcc19bdbc8' = hex(129453562518472)
E        +    where 129453562518472 = id(b'')
E        +  and   '0x75bcc19bdbc8' = hex(129453562518472)
E        +    where 129453562518472 = id(b'')

test_strdup.py:22: AssertionError
================================= short test summary info ==================================
FAILED test_strdup.py::test_strdup[] - AssertionError: assert '0x75bcc19bdbc8' != '0x75bcc19bdbc8'
=============================== 1 failed, 1 passed in 0.04s ================================

此输出显示了两件事：

• b''尽管从底层角度看地址不同，但 Python 中引用的变量的地址无论如何都是相同的（即同一个对象）。这与一些纯 Python 测试一致，我猜这可能是某种优化功能。
• C 和 Python 中的地址值dst和src变量实际上似乎没有关联。

因此，上述尝试实际上不可靠，无法检查函数是否返回指向不同区域的指针。

我还可以尝试检索指针值本身，并通过更改属性进行第二次测试运行以专门检查此部分restype：

#!/usr/bin/env python3                                                                      

import ctypes
import pytest

# Setting wrapped_strdup:                                                                   
lib_wrapped_strdup = ctypes.cdll.LoadLibrary("./wrapped_strdup.so")
wrapped_strdup = lib_wrapped_strdup.wrapped_strdup
wrapped_strdup.restype = ctypes.c_void_p   # Note that it's not a c_char_p anymore.
wrapped_strdup.argtypes = [ctypes.c_char_p]

@pytest.mark.parametrize("src", [b"", b"foo"])
def test_strdup_for_pointers(src: bytes):
    print("")
    dst = wrapped_strdup(src)

    print("From Python:")
    print(f"- retrieved dst address: {hex(dst)}.")

以上代码给出以下输出：

$ pytest test_strdup_for_pointers.py --maxfail=2 -v -s
=================================== test session starts ====================================
platform linux -- Python 3.12.5, pytest-8.3.2, pluggy-1.5.0 -- /usr/bin/python
cachedir: .pytest_cache
rootdir: /home/vmonteco/code/MREs/MRe_strdup_test_with_ctypes
plugins: anyio-4.4.0, cov-5.0.0, typeguard-4.3.0
collected 2 items                                                                          

test_strdup_for_pointers.py::test_strdup_for_pointers[] 
From C:
- src address: E15BDBE8, src content: [].
- dst address: 84D4D820, dst content: [].
From Python:
- retrieved dst address: 0x608984d4d820.
PASSED
test_strdup_for_pointers.py::test_strdup_for_pointers[foo] 
From C:
- src address: DEC7EA80, src content: [foo].
- dst address: 84EA7C40, dst content: [foo].
From Python:
- retrieved dst address: 0x608984ea7c40.
PASSED

==================================== 2 passed in 0.01s =====================================

这将提供实际的地址（或至少是看起来相关的信息）。

但如果不知道 C 函数接收的值，它就没有多大帮助。

附录：我从马克的回答中得出的结论（并且有效）：

这是一个测试，它实现了所接受的答案中建议的解决方案：

#!/usr/bin/env python3

import ctypes
import pytest

# Setting libc:
libc = ctypes.cdll.LoadLibrary("libc.so.6")
strlen = libc.strlen
strlen.restype = ctypes.c_size_t
strlen.argtypes = (ctypes.c_char_p,)

# Setting wrapped_strdup:
lib_wrapped_strdup = ctypes.cdll.LoadLibrary("./wrapped_strdup.so")
wrapped_strdup = lib_wrapped_strdup.wrapped_strdup
# Restype will be set directly in the tests.
wrapped_strdup.argtypes = (ctypes.c_char_p,)


@pytest.mark.parametrize("src", [b"", b"foo"])
def test_strdup(src: bytes):
    print("")  # Just to make pytest output more readable.

    # Set expected result type.
    wrapped_strdup.restype = ctypes.POINTER(ctypes.c_char)

    # Create the src buffer and retrieve its address.
    src_buffer = ctypes.create_string_buffer(src)
    src_addr = ctypes.addressof(src_buffer)
    src_content = src_buffer[:strlen(src_buffer)]

    # Run function to test.
    dst = wrapped_strdup(src_buffer)

    # Retrieve result address and content.
    dst_addr = ctypes.addressof(dst.contents)
    dst_content = dst[: strlen(dst)]

    # Assertions.
    assert src_content == dst_content
    assert src_addr != dst_addr

    # Output.
    print("From Python:")
    print(f"- Src content: {src_content!r}. Src address: {src_addr:X}.")
    print(f"- Dst content: {dst_content!r}. Dst address: {dst_addr:X}.")


@pytest.mark.parametrize("src", [b"", b"foo"])
def test_strdup_alternative(src: bytes):
    print("")  # Just to make pytest output more readable.

    # Set expected result type.
    wrapped_strdup.restype = ctypes.c_void_p

    # Create the src buffer and retrieve its address.
    src_buffer = ctypes.create_string_buffer(src)
    src_addr = ctypes.addressof(src_buffer)
    src_content = src_buffer[:strlen(src_buffer)]

    # Run function to test.
    dst = wrapped_strdup(src_buffer)

    # Retrieve result address and content.
    dst_addr = dst
    # cast dst:
    dst_pointer = ctypes.cast(dst, ctypes.POINTER(ctypes.c_char))
    dst_content = dst_pointer[:strlen(dst_pointer)]

    # Assertions.
    assert src_content == dst_content
    assert src_addr != dst_addr

    # Output.
    print("From Python:")
    print(f"- Src content: {src_content!r}. Src address: {src_addr:X}.")
    print(f"- Dst content: {dst_content!r}. Dst address: {dst_addr:X}.")

输出 ：

$ pytest test_strdup.py -v -s            
=============================== test session starts ===============================
platform linux -- Python 3.10.14, pytest-8.3.2, pluggy-1.5.0 -- /home/vmonteco/.pyenv/versions/3.10.14/envs/strduo_test/bin/python3.10
cachedir: .pytest_cache
rootdir: /home/vmonteco/code/MREs/MRe_strdup_test_with_ctypes
plugins: anyio-4.4.0, stub-1.1.0
collected 4 items                                                                 

test_strdup.py::test_strdup[] 
From C:
- src address: 661BBE90, src content: [].
- dst address: F5D8A7A0, dst content: [].
From Python:
- Src content: b''. Src address: 7C39661BBE90.
- Dst content: b''. Dst address: 57B4F5D8A7A0.
PASSED
test_strdup.py::test_strdup[foo] 
From C:
- src address: 661BBE90, src content: [foo].
- dst address: F5E03340, dst content: [foo].
From Python:
- Src content: b'foo'. Src address: 7C39661BBE90.
- Dst content: b'foo'. Dst address: 57B4F5E03340.
PASSED
test_strdup.py::test_strdup_alternative[] 
From C:
- src address: 661BBE90, src content: [].
- dst address: F5B0AC50, dst content: [].
From Python:
- Src content: b''. Src address: 7C39661BBE90.
- Dst content: b''. Dst address: 57B4F5B0AC50.
PASSED
test_strdup.py::test_strdup_alternative[foo] 
From C:
- src address: 661BBE90, src content: [foo].
- dst address: F5BF9C20, dst content: [foo].
From Python:
- Src content: b'foo'. Src address: 7C39661BBE90.
- Dst content: b'foo'. Dst address: 57B4F5BF9C20.
PASSED

================================ 4 passed in 0.01s ================================