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>

4 files changed, 87 insertions, 21 deletions

diff --git a/include/libbb.h b/include/libbb.h
index b054e05..e97efcb 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1329,10 +1329,15 @@ extern int get_linux_version_code(void) FAST_FUNC;
 
 extern char *query_loop(const char *device) FAST_FUNC;
 extern int del_loop(const char *device) FAST_FUNC;
-/* If *devname is not NULL, use that name, otherwise try to find free one,
+/*
+ * If *devname is not NULL, use that name, otherwise try to find free one,
  * malloc and return it in *devname.
- * return value: 1: read-only loopdev was setup, 0: rw, < 0: error */
-extern int set_loop(char **devname, const char *file, unsigned long long offset, int ro) FAST_FUNC;
+ * return value is the opened fd to the loop device, or < on error
+ */
+extern int set_loop(char **devname, const char *file, unsigned long long offset, unsigned flags) FAST_FUNC;
+/* These constants match linux/loop.h (without BB_ prefix): */
+#define BB_LO_FLAGS_READ_ONLY 1
+#define BB_LO_FLAGS_AUTOCLEAR 4
 
 /* Like bb_ask below, but asks on stdin with no timeout.  */
 char *bb_ask_stdin(const char * prompt) FAST_FUNC;
diff --git a/libbb/loop.c b/libbb/loop.c
index d30b378..f0d4296 100644
--- a/libbb/loop.c
+++ b/libbb/loop.c
@@ -78,22 +78,24 @@ int FAST_FUNC del_loop(const char *device)
 	return rc;
 }
 
-/* Returns 0 if mounted RW, 1 if mounted read-only, <0 for error.
-   *device is loop device to use, or if *device==NULL finds a loop device to
-   mount it on and sets *device to a strdup of that loop device name.  This
-   search will re-use an existing loop device already bound to that
-   file/offset if it finds one.
+/* Returns opened fd to the loop device, <0 on error.
+ * *device is loop device to use, or if *device==NULL finds a loop device to
+ * mount it on and sets *device to a strdup of that loop device name.  This
+ * search will re-use an existing loop device already bound to that
+ * file/offset if it finds one.
  */
-int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offset, int ro)
+int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offset, unsigned flags)
 {
 	char dev[LOOP_NAMESIZE];
 	char *try;
 	bb_loop_info loopinfo;
 	struct stat statbuf;
-	int i, dfd, ffd, mode, rc = -1;
+	int i, dfd, ffd, mode, rc;
+
+	rc = dfd = -1;
 
 	/* Open the file.  Barf if this doesn't work.  */
-	mode = ro ? O_RDONLY : O_RDWR;
+	mode = (flags & BB_LO_FLAGS_READ_ONLY) ? O_RDONLY : O_RDWR;
  open_ffd:
 	ffd = open(file, mode);
 	if (ffd < 0) {
@@ -144,20 +146,35 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
 
 		/* If device is free, claim it.  */
 		if (rc && errno == ENXIO) {
-			memset(&loopinfo, 0, sizeof(loopinfo));
-			safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
-			loopinfo.lo_offset = offset;
 			/* Associate free loop device with file.  */
 			if (ioctl(dfd, LOOP_SET_FD, ffd) == 0) {
-				if (ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo) == 0)
-					rc = 0;
-				else
+				memset(&loopinfo, 0, sizeof(loopinfo));
+				safe_strncpy((char *)loopinfo.lo_file_name, file, LO_NAME_SIZE);
+				loopinfo.lo_offset = offset;
+				/*
+				 * Used by mount to set LO_FLAGS_AUTOCLEAR.
+				 * LO_FLAGS_READ_ONLY is not set because RO is controlled by open type of the file.
+				 * Note that closing LO_FLAGS_AUTOCLEARed dfd before mount
+				 * is wrong (would free the loop device!)
+				 */
+				loopinfo.lo_flags = (flags & ~BB_LO_FLAGS_READ_ONLY);
+				rc = ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo);
+				if (rc != 0 && (loopinfo.lo_flags & BB_LO_FLAGS_AUTOCLEAR)) {
+					/* Old kernel, does not support LO_FLAGS_AUTOCLEAR? */
+					/* (this code path is not tested) */
+					loopinfo.lo_flags -= BB_LO_FLAGS_AUTOCLEAR;
+					rc = ioctl(dfd, BB_LOOP_SET_STATUS, &loopinfo);
+				}
+				if (rc != 0) {
 					ioctl(dfd, LOOP_CLR_FD, 0);
+				}
 			}
 		} else {
 			rc = -1;
 		}
-		close(dfd);
+		if (rc != 0) {
+			close(dfd);
+		}
  try_again:
 		if (*device) break;
 	}
@@ -165,7 +182,7 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
 	if (rc == 0) {
 		if (!*device)
 			*device = xstrdup(dev);
-		return (mode == O_RDONLY); /* 1:ro, 0:rw */
+		return dfd;
 	}
 	return rc;
 }
diff --git a/util-linux/losetup.c b/util-linux/losetup.c
index 4424d9c..d356f49 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 f0245f7..6bb1852 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))