LD_PRELOAD Code Injection and Hooking

filesnooper.c

/* filesnooper.c - Fergus In London <fergus@fergus.london>
 *
 * Similar to snoopy.c, wrote to scratch an itch and demonstrate how LD_PRELOAD
 *  can be used for injecting code and/or hooking/wrapping around function calls.
 *
 * This is actually a good example, because if you try and use it with different
 *  ways of writing files, you'll see that to actually do this effectively you'd
 *  need to examine the target binary with strace, and potentially implement
 *  signatures from other libraries. (i.e a binary calling fprintf() may not be
 *  be effected by this.)
 */

#define _GNU_SOURCE
#include <dlfcn.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define MAX_FILE_PATH 255
#define FILE_TARGET "secret.txt"

static ssize_t (*real_write)(int fd, const void *buf, size_t count) = NULL;

ssize_t write(int fildes, const void *buf, size_t nbytes) {
// Ignore STDOUT and STDERR
    if (fildes < 3){
        goto passthrough;
    }

// Determine filepath via the file descriptor
    char linkpath[MAX_FILE_PATH];
    sprintf(linkpath, "/proc/self/fd/%d", fildes);
    char filepath[MAX_FILE_PATH];
    ssize_t pathLength = readlink(linkpath, filepath, MAX_FILE_PATH);

// Check that the path is valid and readlink worked, additionally check filename
    if ((pathLength < 0) || (strstr(filepath, FILE_TARGET) == NULL)){
        goto passthrough;
    }

// Filename matches, so log out buffer.
    printf("Looks like we got ourselves a secret here...\n");
    printf("[Message] %s\n", buf);

passthrough:
    if (real_write == NULL) real_write = dlsym(RTLD_NEXT, "write");
    real_write(fildes, buf, nbytes);
}

test.c

/* test.c - example target for filesnooper.c */

#include <unistd.h>
#include <fcntl.h>

int main(int argc, char **argv) {
    int sec_desc = open("/tmp/secret.txt", O_WRONLY | O_APPEND);
    int std_desc = open("/tmp/boring.txt", O_WRONLY | O_APPEND);

    write(sec_desc, "This is a secret.\n", 36);
    write(std_desc, "This is a NOT a secret.\n", 36);

    return 0;
}

$ gcc -fPIC -shared -o libwrite.so filesnooper.c -ldl
$ gcc -o t test.c
$ LD_PRELOAD=/home/fmorrow/libwrite.so ./t
  Looks like we got ourselves a secret here...
  [Message] This is a secret.

$