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))