mount: create loop devices with LO_FLAGS_AUTOCLEAR flag

The "autolooped" mount (mount [-oloop] IMAGE /DIR/DIR)
always creates AUTOCLEARed loopdevs, so that umounting
drops them (and this does not require any code in the
umount userspace).
This happens since circa linux-2.6.25:
	commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
	Date:    Wed Feb 6 01:36:27 2008 -0800
	Subject: Allow auto-destruction of loop devices
IOW: in this case, umount does not have to use -d
to drop the loopdev.

The explicit loop mount (mount /dev/loopN /DIR/DIR)
does not do this. In this case, umount without -d
should not drop loopdev.
Unfortunately, bbox umount currently always implies -d,
this probably needs fixing.

function                                             old     new   delta
set_loop                                             537     597     +60
singlemount                                         1101    1138     +37
losetup_main                                         419     432     +13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 110/0)             Total: 110 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 46 insertions, 2 deletions

diff --git a/util-linux/losetup.c b/util-linux/losetup.c
index 4424d9cbb..d356f49c2 100644
--- a/util-linux/losetup.c
+++ b/util-linux/losetup.c
@@ -127,12 +127,37 @@ int losetup_main(int argc UNUSED_PARAM, char **argv)
 			d = *argv++;
 
 		if (argv[0]) {
-			if (set_loop(&d, argv[0], offset, (opt & OPT_r)) < 0)
+			if (set_loop(&d, argv[0], offset, (opt & OPT_r) ? BB_LO_FLAGS_READ_ONLY : 0) < 0)
 				bb_simple_perror_msg_and_die(argv[0]);
 			return EXIT_SUCCESS;
 		}
 	}
 
+	/* TODO: util-linux 2.28 shows this when run w/o params:
+	 * NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE     DIO
+	 * /dev/loop0         0      0         1  0 /PATH/TO/FILE   0
+	 *
+	 * implemented by reading /sys:
+	 *
+	 * open("/sys/block", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
+	 * newfstatat(3, "loop0/loop/backing_file", {st_mode=S_IFREG|0444, st_size=4096, ...}, 0) = 0
+	 * stat("/dev/loop0", {st_mode=S_IFBLK|0660, st_rdev=makedev(7, 0), ...}) = 0
+	 * open("/sys/dev/block/7:0/loop/offset", O_RDONLY|O_CLOEXEC) = 5
+	 * read(5, "0\n", 4096)                    = 2
+	 * open("/sys/dev/block/7:0/loop/sizelimit", O_RDONLY|O_CLOEXEC) = 5
+	 * read(5, "0\n", 4096)                    = 2
+	 * open("/sys/dev/block/7:0/loop/offset", O_RDONLY|O_CLOEXEC) = 5
+	 * read(5, "0\n", 4096)                    = 2
+	 * open("/sys/dev/block/7:0/loop/autoclear", O_RDONLY|O_CLOEXEC) = 5
+	 * read(5, "1\n", 4096)                    = 2
+	 * open("/sys/dev/block/7:0/ro", O_RDONLY|O_CLOEXEC)     = 5
+	 * read(5, "0\n", 4096)                    = 2
+	 * open("/sys/dev/block/7:0/loop/backing_file", O_RDONLY|O_CLOEXEC) = 5
+	 * read(5, "/PATH/TO/FILE", 4096) = 37
+	 * open("/sys/dev/block/7:0/loop/dio", O_RDONLY|O_CLOEXEC) = 5
+	 * read(5, "0\n", 4096)                    = 2
+	 */
+
 	bb_show_usage(); /* does not return */
 	/*return EXIT_FAILURE;*/
 }
diff --git a/util-linux/mount.c b/util-linux/mount.c
index f0245f714..6bb18524d 100644
--- a/util-linux/mount.c
+++ b/util-linux/mount.c
@@ -1887,6 +1887,7 @@ static int nfsmount(struct mntent *mp, unsigned long vfsflags, char *filteropts)
 // NB: mp->xxx fields may be trashed on exit
 static int singlemount(struct mntent *mp, int ignore_busy)
 {
+	int loopfd = -1;
 	int rc = -1;
 	unsigned long vfsflags;
 	char *loopFile = NULL, *filteropts = NULL;
@@ -2026,7 +2027,20 @@ static int singlemount(struct mntent *mp, int ignore_busy)
 		if (ENABLE_FEATURE_MOUNT_LOOP && S_ISREG(st.st_mode)) {
 			loopFile = bb_simplify_path(mp->mnt_fsname);
 			mp->mnt_fsname = NULL; // will receive malloced loop dev name
-			if (set_loop(&mp->mnt_fsname, loopFile, 0, /*ro:*/ (vfsflags & MS_RDONLY)) < 0) {
+
+			// mount always creates AUTOCLEARed loopdevs, so that umounting
+			// drops them without any code in the userspace.
+			// This happens since circa linux-2.6.25:
+			// commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
+			// Date:    Wed Feb 6 01:36:27 2008 -0800
+			// Subject: Allow auto-destruction of loop devices
+			loopfd = set_loop(&mp->mnt_fsname,
+					loopFile,
+					0,
+					((vfsflags & MS_RDONLY) ? BB_LO_FLAGS_READ_ONLY : 0)
+						| BB_LO_FLAGS_AUTOCLEAR
+			);
+			if (loopfd < 0) {
 				if (errno == EPERM || errno == EACCES)
 					bb_error_msg(bb_msg_perm_denied_are_you_root);
 				else
@@ -2074,6 +2088,8 @@ static int singlemount(struct mntent *mp, int ignore_busy)
 	}
 
 	// If mount failed, clean up loop file (if any).
+	// (Newer kernels which support LO_FLAGS_AUTOCLEAR should not need this,
+	// merely "close(loopfd)" should do it?)
 	if (ENABLE_FEATURE_MOUNT_LOOP && rc && loopFile) {
 		del_loop(mp->mnt_fsname);
 		if (ENABLE_FEATURE_CLEAN_UP) {
@@ -2086,6 +2102,9 @@ static int singlemount(struct mntent *mp, int ignore_busy)
 	if (ENABLE_FEATURE_CLEAN_UP)
 		free(filteropts);
 
+	if (loopfd >= 0)
+		close(loopfd);
+
 	if (errno == EBUSY && ignore_busy)
 		return 0;
 	if (errno == ENOENT && (vfsflags & MOUNT_NOFAIL))