Execute embedded program with custom ld-linux

ld-wrapper.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <linux/limits.h>
#include <sys/mman.h>

extern const char _binary_executable_start, _binary_executable_end;

bool verbose = false;

int main(int argc, char **argv) {
  char *new_args[argc + 4], fd_path[PATH_MAX], *interp_path;

  // enable verbose logging if requested
  if (strcmp(getenv("LD_WRAPPER_VERBOSE") ?: "0", "1") == 0) verbose = true;

  // get interpreter path from environment
  interp_path = getenv("LD_WRAPPER_INTERPRETER_PATH") ?: "/usr/local/bin/ld.so";

  if (verbose) fprintf(stderr, "[+] ELF interpreter set to %s\n", interp_path);

  // create a memfd for the executable
  int memfd           = memfd_create("real-executable", 0),
      executable_size = &_binary_executable_end - &_binary_executable_start;

  if (memfd == -1) {
    fprintf(stderr, "memfd_create() failed: %s\n", strerror(errno));
    exit(errno);
  }

  // full path of memfd
  snprintf(fd_path, sizeof(fd_path), "/proc/self/fd/%i", memfd);

  // command line arguments for ld.so
  new_args[0] = "ld.so";
  new_args[1] = "--argv0";
  new_args[2] = argv[0];
  new_args[3] = fd_path;

  // copy all argument pointers to new array
  for (int i = 1; i < argc; i++) new_args[3 + i] = argv[i];
  new_args[argc + 3] = NULL;

  if (verbose) {
    fprintf(stderr, "[+] New command line:\n[+]\n[+]  ");
    for (int i = 0; i < argc + 3; i++) {
      fprintf(stderr, "%s ", new_args[i]);
    }
    fprintf(stderr, "\n[+]\n");
  }

  // load executable into memfd
  if (write(memfd, &_binary_executable_start, executable_size) == -1) {
    fprintf(stderr, "Failed to write into memfd: %s\n", strerror(errno));
    exit(errno);
  }

  if (verbose) fprintf(stderr, "[+] Now re-exec with %s...\n", interp_path);

  // execute the memfd with our dynamic linker
  if (execv(interp_path, new_args) == -1) {
    fprintf(stderr, "execv() failed: %s\n", strerror(errno));
    exit(errno);
  }

  return 0;
}