[PATCH] mount -o loop race condition

[Matthias Koenig <mkoenig@xxxxxxx>]

Hi, 

in addition to the race described recently 
(http://article.gmane.org/gmane.linux.utilities.util-linux-ng/241), 
there is also a race if you have concurring processes running 
mount -o loop. Patch below.

Matthias

Fix race in mount -o loop

Retry acquiring a loop device if the setup failed with EBUSY.

Signed-Off-By: Matthias Koenig <mkoenig@xxxxxxx>

Index: util-linux-devel/mount/lomount.c
===================================================================
--- util-linux-devel.orig/mount/lomount.c
+++ util-linux-devel/mount/lomount.c
@@ -341,8 +341,15 @@ set_loop(const char *device, const char 
 	}
 
 	if (ioctl(fd, LOOP_SET_FD, ffd) < 0) {
-		perror("ioctl: LOOP_SET_FD");
-		return 1;
+ 		switch (errno) {
+ 		case EBUSY:
+ 			if (verbose)
+ 				perror("ioctl: LOOP_SET_FD");
+ 			return 2;
+ 		default:
+ 			perror("ioctl: LOOP_SET_FD");
+ 			return 1;
+ 		}
 	}
 	close (ffd);
 
Index: util-linux-devel/mount/mount.c
===================================================================
--- util-linux-devel.orig/mount/mount.c
+++ util-linux-devel/mount/mount.c
@@ -846,20 +846,34 @@ loop_check(const char **spec, const char
 	printf(_("mount: skipping the setup of a loop device\n"));
     } else {
       int loopro = (*flags & MS_RDONLY);
+      int res;
 
-      if (!*loopdev || !**loopdev)
-	*loopdev = find_unused_loop_device();
-      if (!*loopdev)
-	return EX_SYSERR;	/* no more loop devices */
-      if (verbose)
-	printf(_("mount: going to use the loop device %s\n"), *loopdev);
-      offset = opt_offset ? strtoull(opt_offset, NULL, 0) : 0;
-      if (set_loop(*loopdev, *loopfile, offset,
-		   opt_encryption, pfd, &loopro)) {
+      do {
+        if (!*loopdev || !**loopdev)
+	  *loopdev = find_unused_loop_device();
+	if (!*loopdev)
+	  return EX_SYSERR;	/* no more loop devices */
 	if (verbose)
-	  printf(_("mount: failed setting up loop device\n"));
-	return EX_FAIL;
-      }
+ 	  printf(_("mount: going to use the loop device %s\n"), *loopdev);
+        offset = opt_offset ? strtoull(opt_offset, NULL, 0) : 0;
+ 	if (res = set_loop (*loopdev, *loopfile, offset,
+			    opt_encryption, pfd, &loopro)) {
+ 	  switch(res) {
+ 	  case 2:
+ 		  /* loop dev has been grabbed by some other process,
+ 		     try again */
+ 		  if (verbose)
+ 		    printf("mount: stolen loop=%s ...trying again\n", *loopdev);
+ 		  *loopdev = NULL;
+ 		  continue;
+ 	  default:
+ 		  if (verbose)
+ 		    printf(_("mount: failed setting up loop device\n"));
+ 		  return EX_FAIL;
+ 	  }
+ 	}
+      } while (!*loopdev);
+
       if (verbose > 1)
 	printf(_("mount: setup loop device successfully\n"));
       *spec = *loopdev;