Re: Problem with SYS_mount

On  6 May 2008 at 14:19, Walter Roberson wrote:
> ... and yes, people *do* make mistakes. For example, in the recent
> sys_MOUNT thread you were keen on, the answer you cited as "accurate"
> was *not* accurate.

There was indeed an inaccuracy in my answer, and it would have been a
courtesy to the OP and others reading this discussion if you'd pointed
it out instead of getting on your high-horse.

Specifically, I said:
> If you really want to work programatically, you can open one of the
> loop devices, perform an ioctl on it (request LOOP_SET_FD with extra
> argument "/home/sam/myfile.img"), then use mount(2).

This is wrong: the extra argument should be a file descriptor, not a
path name.

Here's a knock-up example of mounting a file programatically with a loop
device (of course it needs to be run with sufficient privileges to set up
the loop device and mount it).


#include <stdio.h>
#include <errno.h>

#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/mount.h>
#include <linux/loop.h>

#define FILE_TO_MOUNT "/tmp/foo.iso"
#define MOUNT_PATH "/mnt"

int main(int argc, char **argv)
{
  char dev[]="/dev/loop0";
  int i, fd, fd2;
  struct stat statbuf;
  struct loop_info loopinfo;

  /* find a free loop device */
  for(i=0; i<8; i++) {
    dev[9]='0'+i;
    if( stat (dev, &statbuf) == 0 && S_ISBLK(statbuf.st_mode) )
      if((fd = open (dev, O_RDONLY)) >= 0) {
        if(ioctl(fd, LOOP_GET_STATUS, &loopinfo) && (errno == ENXIO))
            break; /* got one! */
        close(fd);
      }
    fd=-1;
  }
  if(fd==-1) {
    fprintf(stderr, "%s: couldn't find free loop device\n", *argv);
    return 1;
  }

  /* set up the loop device */
  if((fd2=open(FILE_TO_MOUNT, O_RDONLY)) < 0) {
    fprintf(stderr, "%s: failed to open " FILE_TO_MOUNT "\n", *argv);
    return 2;
  }

  if(ioctl(fd, LOOP_SET_FD, (void *) fd2)) {
    fprintf(stderr, "%s: ioctl failed on %s\n", *argv, dev);
    return 3;
  }

  /* do the mount */
  if(mount(dev, MOUNT_PATH, "iso9660", MS_RDONLY, NULL)) {
    fprintf(stderr, "%s: failed to mount %s on " MOUNT_PATH "\n", *argv,
dev);
    if (ioctl (fd, LOOP_CLR_FD, 0))
      fprintf(stderr, "%s: failed to clear %s\n", *argv, dev);
    return 4;
  }

  /* pause */
  printf("Mounted " FILE_TO_MOUNT " at " MOUNT_PATH " using %s\n"
      "Press enter to umount...\n", dev);
  getchar();

  /* do the umount */
  if(umount(MOUNT_PATH)) {
    fprintf(stderr, "%s: failed to umount %s on " MOUNT_PATH "\n", *argv,
dev);
    if (ioctl (fd, LOOP_CLR_FD, 0))
      fprintf(stderr, "%s: failed to clear %s\n", *argv, dev);
    return 5;
  }

  /* clean up */
  close(fd2);
  if (ioctl (fd, LOOP_CLR_FD, 0)) {
    fprintf(stderr, "%s: failed to clear %s\n", *argv, dev);
    return 6;
  }
  close(fd);

  fprintf(stderr, "Umounted %s and cleared %s\n", MOUNT_PATH, dev);
  return 0;
}