覆盖使用LD_ preLOAD机制'的malloc“ [英] Overriding 'malloc' using the LD_PRELOAD mechanism

问题描述

我想写日志会调用malloc的到stderr（一种'​​命令mtrace如果你愿意的）一个简单的共享库。

I'm trying to write a simple shared library that would log malloc calls to stderr (a sort of 'mtrace' if you will).

但是，这是行不通的。
这是我做的：

However, this is not working. Here's what I do:

/* mtrace.c */
#include <dlfcn.h>
#include <stdio.h>

static void* (*real_malloc)(size_t);

void *malloc(size_t size)
{
    void *p = NULL;
    fprintf(stderr, "malloc(%d) = ", size);
    p = real_malloc(size);
    fprintf(stderr, "%p\n", p);
    return p;
}

static void __mtrace_init(void) __attribute__((constructor));
static void __mtrace_init(void)
{
    void *handle = NULL;
    handle = dlopen("libc.so.6", RTLD_LAZY);
    if (NULL == handle) {
        fprintf(stderr, "Error in `dlopen`: %s\n", dlerror());
        return;
    }
    real_malloc = dlsym(handle, "malloc");
    if (NULL == real_malloc) {
        fprintf(stderr, "Error in `dlsym`: %s\n", dlerror());
        return;
    }
}

我编译这个用：

gcc -shared -fPIC -o mtrace.so mtrace.c

，然后当我尝试执行 LS ：

$ LD_PRELOAD=./mtrace.so ls
malloc(352) = Segmentation fault

现在，我怀疑的dlopen需求的malloc，和我共享库中重新定义它，它使用该版本的还是未分配 real_malloc 。

Now, I suspect that dlopen needs malloc, and as I am redefining it within the shared library, it uses that version with the still unassigned real_malloc.

现在的问题是...我怎么做工作？

The question is...how do I make it work?

P.S。对不起，在标签的缺乏，我找不到合适的标签，我仍然没有足够的信誉来塑造新的。

P.S. sorry for the paucity in tags, I couldn't find appropriate tags, and I still don't have enough reputation to create new ones.

推荐答案

我总是这样说：

#define _GNU_SOURCE

#include <stdio.h>
#include <dlfcn.h>

static void* (*real_malloc)(size_t)=NULL;

static void mtrace_init(void)
{
    real_malloc = dlsym(RTLD_NEXT, "malloc");
    if (NULL == real_malloc) {
        fprintf(stderr, "Error in `dlsym`: %s\n", dlerror());
    }
}

void *malloc(size_t size)
{
    if(real_malloc==NULL) {
        mtrace_init();
    }

    void *p = NULL;
    fprintf(stderr, "malloc(%d) = ", size);
    p = real_malloc(size);
    fprintf(stderr, "%p\n", p);
    return p;
}

不要使用构造，就在第一次调用初始化的malloc 。使用 RTLD_NEXT 来避免的dlopen 。您也可以尝试的malloc挂钩。请注意，所有这些都是GNU扩展，并可能不会在其他地方工作。

Don't use constructors, just initialize at first call to malloc. Use RTLD_NEXT to avoid dlopen. You can also try malloc hooks. Be aware that all those are GNU extensions, and probably wont work elsewhere.

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