在库中修改标准函数，然后调用本地库函数 [英] Hack the standard function in library and call the native library function afterwards

问题描述

我试图破解malloc函数以首先调用我的malloc函数，一旦在其中执行了我的malloc函数，我想调用标准的malloc.但是，我得到了递归，因为它只是加载了我定义的malloc.我该如何解决以下代码?

I am trying to hack the malloc function to call my malloc function first.Once my malloc function is executed within that, I want to invoke the standard malloc. But, I am getting a recursion, since it is just loading my defined malloc. How can i fix the below code?

#include <dlfcn.h>
#include "stdio.h"
//#include "stdlib.h"


void *handle;

void *handle_malloc;

int (*loadprg)(void);

void * (*malloc_sysm)(size_t);


void init()
{
    handle = dlopen ("/export/home/joshis1/Foxtel/temp/libloadmap.so", RTLD_LAZY);
    if( handle == NULL)
     {
       puts(dlerror());
     }


   handle_malloc = dlopen ("/lib/libc.so.6", RTLD_LAZY);
    if( handle_malloc == NULL)
     {
       puts("handle malloc error\r\n");
       puts(dlerror());
     }


}


#include "stdio.h"


void *malloc(int size)
{
   printf("called..my malloc\r\n");

   malloc_sysm = dlsym(handle_malloc,"malloc");

   if ( dlerror() != NULL)
    {
       puts("malloc symbol not found..");
       exit(1);
    }


    printf("This should call actual malloc now..\r\n");
    return  malloc_sysm(size);




}


int main()
{
  int *t;
  init();
  printf("call load program now\r\n");

  loadprg = dlsym(handle, "loadprg");

  if( dlerror()!= NULL)
   {
      puts("Symbol load errror");
   }

  (*loadprg)();  

  printf("Close the handle now..\r\n");

  dlclose(handle);


  t = (int *) malloc (100);

  dlclose(handle_malloc);



  return 0;

}

输出递归到我定义的malloc().该如何解决?

The output is recursion to my defined malloc(). how to fix this?

推荐答案

要覆盖共享函数，您需要编译自己的共享库并通过LD_PRELOAD环境变量对其进行预加载.

To override shared functions you need to compile your own shared library and preload it via the LD_PRELOAD environment variable.

#define _GNU_SOURCE

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

void *malloc(size_t size) {
    printf("called..my malloc\r\n");

    void *(*original_malloc)(size_t size);
    // Find original malloc function
    original_malloc = dlsym(RTLD_NEXT, "malloc");

    if ( dlerror() != NULL)
    {
        puts("malloc symbol not found..");
        exit(1);
    }

    printf("This should call actual malloc now..\r\n");
    return (*original_malloc)(size);
}

$ gcc -Wall -fPIC -shared -o mymalloc.so mymalloc.c -ldl
$ LD_PRELOAD=./mymalloc.so ./prog

现在您的程序将使用预加载库中的malloc.

Now your program will use malloc from preloaded library.

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