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user3384486's questions

Martin Hope
user3384486
Asked: 2024-08-08 05:34:43 +0800 CST
  • 5
/* dlsym-main.c */

#include <stdio.h>

void doSomething(char const *msg)
{
    fprintf(stderr, "Main program is doing %s\n", msg);
}

void (*fnptr)(char const *) = doSomething;

int main() {

    fnptr("some printing");
    return 0;
}

/* dlsym-lib.c */

#define _GNU_SOURCE  1
#include <stdio.h>
#include <dlfcn.h>

void myLibraryFunc(char const *msg)
{
    fprintf(stdout, "My library is doing %s\n", msg);
}

void hook(void)
{
    // Load the current executable as a shared object
    void* handle = dlopen(NULL, RTLD_LAZY);
    if (!handle) {
        fprintf(stderr, "Error: %s\n", dlerror());
        return;
    }

   typedef void (*fnT)(char const *);

    // Dynamically resolve the global non-static variable
    fnT* fnPtr = (fnT*) dlsym(handle, "fnptr");
    if (!fnPtr)
    {
        fprintf(stderr, "Error locating fnptr: %s\n", dlerror());
        dlclose(handle);
        return;
    }
    *fnPtr = myLibraryFunc;

    // Close the handle
    dlclose(handle);
}

__attribute__((constructor))
void setup() { hook(); }

## Makefile

linky: dlsym-main.c
    gcc -o linky dlsym-main.c
libi.so: dlsym-lib.c
    gcc -shared -fPIC -o libi.so dlsym-lib.c -ldl

run: linky libi.so
    LD_PRELOAD=$$PWD/libi.so ./linky

输出make run：

Error locating fnptr: Symbol not found: fnptr
Main program is doing some printing

符号（fnptr）是一个全局数据对象：

> objdump -t linky | grep fnptr
0000000000020008 g     O .data  0000000000000008 fnptr

使用LD_DEBUG=symbols（Debian）表明它正在可执行文件中查找：

       318: symbol=fnptr;  lookup in file=./linky [0]
       318: symbol=fnptr;  lookup in file=/CDS/libi.so [0]
       318: symbol=fnptr;  lookup in file=/usr/lib/aarch64-linux-gnu/libstdc++.so.6 [0]
       318: symbol=fnptr;  lookup in file=/lib/aarch64-linux-gnu/libc.so.6 [0]
       318: symbol=fnptr;  lookup in file=/lib/aarch64-linux-gnu/libdl.so.2 [0]
       318: symbol=fnptr;  lookup in file=/lib/aarch64-linux-gnu/libm.so.6 [0]
       318: symbol=fnptr;  lookup in file=/lib/aarch64-linux-gnu/libgcc_s.so.1 [0]
       318: symbol=fnptr;  lookup in file=/lib/ld-linux-aarch64.so.1 [0]
       318: /CDS/libi.so: error: symbol lookup error: undefined symbol: fnptr (fatal)

我在 Debian、Alpine（musl libc）和其他变体（所有 GCC）上尝试过此操作，结果相同。我问过 AI 机器人如何做到这一点，它们吐出了这段代码。

相同的代码在 macOS 上运行良好

▶ make run_macOS
DYLD_INSERT_LIBRARIES=$PWD/libi.so ./linky
My library is doing some printing

我做错了什么，导致这在 GCC 上不起作用？

gcc