A wrapper around mprotect(2)

wrapper.c

/*
 * benjamin.poirier@gmail.com
 *
 * A wrapper around mprotect(2)
 * build with:
 * cc -Wall -g  -ldl  wrapper.c   -shared -fPIC -o wrapper.so
 *
 * run with:
 * LD_PRELOAD=./somepath/wrapper.so ./otherprogram
 *
 */
#define _GNU_SOURCE
#include <execinfo.h>
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))

int (*fp)(void *, size_t, int);

void __attribute__ ((constructor)) init(void)
{
	fp = dlsym(RTLD_NEXT, "mprotect");
}

/* Change the memory protection of the region starting at ADDR and
   extending LEN bytes to PROT.  Returns 0 if successful, -1 for errors
   (and sets errno).  */
int mprotect(void *__addr, size_t __len, int __prot)
{
	void *buffer[40];
	char **strings;
	int retval;
	int i;

	printf("Calling mprotect(%p, %zd, %d)\n", __addr, __len, __prot);

	retval = backtrace(buffer, ARRAY_SIZE(buffer));
	printf("    Backtrace (%d entries):\n", retval);
	strings = backtrace_symbols(buffer, retval);
	for (i = 0; i < retval; i++) {
		printf("        %s\n", strings[i]);
	}
	free(strings);

	return fp(__addr, __len, __prot);
}