nonullfree.c -- throw errors when you call free(NULL)

nonullfree.c

/**
 * nonullfree.c -- throw errors when you call free(NULL)
 *
 * BUILDING:
 *
 *     $ gcc -shared -fPIC nonullfree.c -ldl -o nonullfree.so
 *
 * USAGE (Linux):
 *
 *     $ LD_PRELOAD=./nonullfree.so ./my-program
 *
 *
 * HOW TO USE WITH gdb: 
 *
 * First, compile YOUR program with -g:
 *
 *     $ gcc -g my-program.c -o my-program
 *
 * Then start gdb like normal:
 *
 *     $ gdb my-program
 *
 * And set the environment variable before running the program:
 *
 *     (gdb) set environment LD_PRELOAD ./nonullfree.so
 *     (gdb) run
 *
 * When a something calls free() with a NULL pointer, you will see this
 * message:
 *
 *     Called free() on a NULL pointer!
 *
 *     Program received signal SIGTRAP, Trace/breakpoint trap.
 *
 * In gdb, inspect the stack trace, and find where the actual call to free()
 * occured:
 *
 *     (gdb) backtrace
 *     #0  0x00007ffff7840428 in __GI_raise (sig=5) at ../sysdeps/unix/sysv/linux/raise.c:54
 *     #1  0x00007ffff7bd58c0 in free () from ./nonullfree.so
 *     #2  0x00000000004005f6 in main () at my-program.c:8
 *
 * In this example, frame 0 raised the error -- you may ignore this. Frame 1 is
 * the "fake" free(), defined in this file. Therefore, frame 2 is the culprit!
 * Switch to frame 2:
 * 
 *    (gdb) frame 2
 *    #2  0x00000000004005f6 in main () at my-program.c:8
 *    8	      free(p);
 *
 * Ah, what's the value of `p`? 
 *
 *    (gdb) print p
 *    $1 = (void *) 0x0
 *
 * Bingo!
 */

#define _GNU_SOURCE  // Required for RTLD_NEXT

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


/**
 * This is the TRUE free(void*).
 *
 * It is dynamically assigned in on_load().
 */
static void (*libc_free)(void*) = NULL;

#define BOLDRED "\033[1;31m"
#define RESET "\033[m"

/**
 * Our version of free(void*) that checks for NULL.
 */
void free(void *p) {
    if (p == NULL) {
        fprintf(stderr, BOLDRED "Called free() on a NULL pointer!" RESET "\n");
	raise(SIGTRAP);
    }

    libc_free(p);
}


////////////////////////////////// INTERNAL //////////////////////////////////

__attribute__((constructor)) static
void on_load() {
    // Get the NEXT definition of free().
    libc_free = dlsym(RTLD_NEXT, "free");
    if (libc_free == NULL) {
        perror(dlerror());
        abort();
    }
}