1/22/2018 - 2:11 AM

dynamic interpose

#define _GNU_SOURCE

#include <dlfcn.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdarg.h>

// The following line declares a function pointer with the same prototype as the open function.  
int (*orig_open)(const char *pathname, int flags, ...);  // mode_t mode is needed when flags includes O_CREAT

// This is our replacement for the open function from libc.
int open(const char *pathname, int flags, ...) {
	mode_t m=0;
	if (flags & O_CREAT) {
		va_list a;
		va_start(a, flags);
		m = va_arg(a, mode_t);
	// we just print a message, then call through to the original open function (from libc)
	fprintf(stderr, "mylib: open called for path %s\n", pathname);
	return orig_open(pathname, flags, m);

// This function is automatically called when program is started
void _init(void) {
	// set function pointer orig_open to point to the original open function
	orig_open = dlsym(RTLD_NEXT, "open");
	fprintf(stderr, "Init mylib\n");