losetup-mini.c

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <fcntl.h>
#include <limits.h>
#include <linux/loop.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <unistd.h>

void fd_cleanup(int *fd) {
  if (fd != NULL && *fd >= 0) {
    close(*fd);
    *fd = -1;
  }
}

#define CLEANUP(__f) __attribute__((__cleanup__(__f)))

int main(int argc, char *argv[]) {
  const char *fname = NULL;
  if (argc < 2) {
    fprintf(stderr, "usage: %s <snap-file-name>\n", argv[0]);
    return 1;
  }
  fname = argv[1];

  int f CLEANUP(fd_cleanup) = open(fname, O_RDONLY | O_CLOEXEC | O_DIRECT);
  if (f < 0) {
    perror("open snap failed");
    return 1;
  }
  int lc CLEANUP(fd_cleanup) = open("/dev/loop-control", O_RDWR | O_CLOEXEC);
  if (lc < 0) {
    perror("opening loop-control failed");
    return 1;
  }

  int loop_free = ioctl(lc, LOOP_CTL_GET_FREE);
  if (loop_free < 0) {
    perror("get free failed");
    return 1;
  }
  fprintf(stderr, "got device %d\n", loop_free);

  char loop_dev_path[PATH_MAX] = {0};
  snprintf(loop_dev_path, sizeof(loop_dev_path), "/dev/loop%d", loop_free);

  int ldev CLEANUP(fd_cleanup) = open(loop_dev_path, O_RDWR | O_CLOEXEC);
  if (ldev < 0) {
    perror("cannot open loop device");
    return 1;
  }

  struct loop_config config = {
      .fd = f,
      .block_size = 0,
      .info =
          {
              .lo_flags = LO_FLAGS_DIRECT_IO | LO_FLAGS_READ_ONLY,
          },
  };
  strncpy(config.info.lo_file_name, fname, sizeof(config.info.lo_file_name));
  if (ioctl(ldev, LOOP_CONFIGURE, &config)) {
    perror("loop configure failed");
    return 1;
  }

  return 0;
}