lklfuse: support exclusive locks to avoid duplicate mounts

Documentation/lkl/lklfuse.rst

@@ -39,7 +39,9 @@ OPTIONS
 
 -o part=parition        mount <partition>.
 
--o ro                   open file read-only.
+-o ro                   open block-device read-only.
+
+-o lock=<file>          only mount after taking an exclusive lock on <file>.
 
 -o opts=options         Linux kernel mount <options> (use \\ to escape , and =).
 

tools/lkl/lklfuse.c

@@ -6,6 +6,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <sys/file.h>
 #include <sys/stat.h>
 #include <string.h>
 #include <errno.h>
@@ -24,6 +25,7 @@ struct lklfuse {
 	const char *log;
 	const char *type;
 	const char *opts;
+	const char *lock;
 	struct lkl_disk disk;
 	int disk_id;
 	int part;
@@ -46,6 +48,7 @@ static struct fuse_opt lklfuse_opts[] = {
 	LKLFUSE_OPT("mb=%d", mb, 0),
 	LKLFUSE_OPT("opts=%s", opts, 0),
 	LKLFUSE_OPT("part=%d", part, 0),
+	LKLFUSE_OPT("lock=%s", lock, 0),
 	FUSE_OPT_KEY("-h", KEY_HELP),
 	FUSE_OPT_KEY("--help", KEY_HELP),
 	FUSE_OPT_KEY("-V",             KEY_VERSION),
@@ -58,7 +61,7 @@ static struct fuse_opt lklfuse_opts[] = {
 static void usage(void)
 {
 	printf(
-"usage: lklfuse file mountpoint [options]\n"
+"usage: lklfuse block-device mountpoint [options]\n"
 "\n"
 "general options:\n"
 "    -o opt,[opt...]        mount options\n"
@@ -70,7 +73,8 @@ static void usage(void)
 "    -o type=fstype         filesystem type\n"
 "    -o mb=memory           amount of memory to allocate in MB (default: 64)\n"
 "    -o part=parition       partition to mount\n"
-"    -o ro                  open file read-only\n"
+"    -o ro                  open block-device read-only\n"
+"    -o lock=FILE           only mount after taking an exclusive lock on FILE\n"
 "    -o opts=options        mount options (use \\ to escape , and =)\n"
 );
 }
@@ -791,7 +795,7 @@ int main(int argc, char **argv)
 	struct fuse_cmdline_opts cli_opts;
 	struct fuse *fuse;
 	struct stat st;
-	int ret;
+	int ret, lockfd = -1;
 
 	if (fuse_opt_parse(&args, &lklfuse, lklfuse_opts, lklfuse_opt_proc))
 		return 1;
@@ -801,6 +805,23 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
+	if (lklfuse.lock) {
+		lockfd = open(lklfuse.lock, O_RDWR | O_CREAT, 0644);
+		if (lockfd < 0) {
+			fprintf(stderr, "failed to open %s: %s\n",
+				lklfuse.lock, strerror(errno));
+			return 1;
+		}
+
+		ret = flock(lockfd, LOCK_EX | LOCK_NB);
+		if (ret < 0) {
+			fprintf(stderr, "unable to exclusively lock %s: %s\n",
+				lklfuse.lock, strerror(errno));
+			return 2;
+		}
+		/* lock dropped when lockfd is closed on program exit */
+	}
+
 	if (fuse_parse_cmdline(&args, &cli_opts))
 		return 1;
 

tools/lkl/systemd/lklfuse-mount@.service

@@ -6,6 +6,7 @@ Requires=modprobe@fuse.service
 
 [Service]
 RuntimeDirectory=lklfuse-%i
+StateDirectory=lklfuse/fsid-mutex
 # The "allow_other" mount option permits fuse mount access by users other than
 # the lklfuse user, and requires a "user_allow_other" setting in fuse3.conf
 Environment=LKLFUSE_ARGS="-s -oallow_other"
@@ -18,8 +19,11 @@ EnvironmentFile=-/etc/lklfuse.conf
 # run as unprivileged user
 User=lklfuse
 Group=lklfuse
-ExecCondition=/bin/bash -c "udevadm info -q env -x --property=ID_FS_TYPE -n \"%I\" > ${RUNTIME_DIRECTORY}/udev.env"
-ExecStart=/bin/bash -c ". ${RUNTIME_DIRECTORY}/udev.env; rm ${RUNTIME_DIRECTORY}/udev.env; /usr/bin/lklfuse -f -ofsname=\"/dev/%I\",subtype=\"lkl.$ID_FS_TYPE\",type=\"$ID_FS_TYPE\" $LKLFUSE_ARGS \"/dev/%I\" $RUNTIME_DIRECTORY"
+ExecCondition=/bin/bash -xc "udevadm info -q env -x --property=ID_FS_TYPE,ID_FS_UUID -n \"%I\" > ${RUNTIME_DIRECTORY}/udev.env"
+# Use an ID_FS_UUID based lock file to avoid duplicate mounts.
+# If udev doesn't provide an id then use a static noid path, ensuring lock
+# conflict with any other id-less mount.
+ExecStart=/bin/bash -xc ". ${RUNTIME_DIRECTORY}/udev.env; rm ${RUNTIME_DIRECTORY}/udev.env; /usr/bin/lklfuse -f -ofsname=\"/dev/%I\",subtype=\"lkl.$ID_FS_TYPE\",type=\"$ID_FS_TYPE\",lock=\"${STATE_DIRECTORY}/${ID_FS_UUID:-noid}\" $LKLFUSE_ARGS \"/dev/%I\" ${RUNTIME_DIRECTORY}"
 
 [Install]
 WantedBy=default.target

tools/lkl/tests/lklfuse.sh

@@ -8,7 +8,6 @@ cleanup()
 {
     set -e
 
-    sleep 1
     if type -P fusermount3 > /dev/null; then
         fusermount3 -u $dir
     else
@@ -18,7 +17,6 @@ cleanup()
     rmdir $dir
 }
 
-
 # $1 - disk image
 # $2 - fstype
 function prepfs()
@@ -33,9 +31,10 @@ function prepfs()
 # $1 - disk image
 # $2 - mount point
 # $3 - filesystem type
+# $4 - lock file
 lklfuse_mount()
 {
-    ${script_dir}/../lklfuse $1 $2 -o type=$3
+    ${script_dir}/../lklfuse $1 $2 -o type=$3,lock=$4
 }
 
 # $1 - mount point
@@ -74,6 +73,21 @@ lklfuse_stressng()
 	      --sync-file-bytes 10m
 }
 
+# $1 - disk image
+# $2 - filesystem type
+# $3 - lock file
+lklfuse_lock_conflict()
+{
+    local ret=$TEST_FAILURE unused_mnt=`mktemp -d`
+
+    set +e
+    # assume lklfuse already running with same lock file, causing lock conflict
+    ${script_dir}/../lklfuse -f $1 $unused_mnt -o type=$2,lock=$3
+    [ $? -eq 2 ] && ret=$TEST_SUCCESS
+    rmdir "$unused_mnt"
+    return $ret
+}
+
 if [ "$1" = "-t" ]; then
     shift
     fstype=$1
@@ -102,18 +116,19 @@ if [ -z $(which mkfs.$fstype) ]; then
     exit 0
 fi
 
-
 file=`mktemp`
 dir=`mktemp -d`
+lock_file="$file"
 
 trap cleanup EXIT
 
-lkl_test_plan 4 "lklfuse $fstype"
+lkl_test_plan 5 "lklfuse $fstype"
 
 lkl_test_run 1 prepfs $file $fstype
-lkl_test_run 2 lklfuse_mount $file $dir $fstype
+lkl_test_run 2 lklfuse_mount $file $dir $fstype $lock_file
 lkl_test_run 3 lklfuse_basic $dir
 # stress-ng returns 2 with no apparent failures so skip it for now
 #lkl_test_run 4 lklfuse_stressng $dir $fstype
+lkl_test_run 4 lklfuse_lock_conflict $file $fstype $lock_file
 trap : EXIT
-lkl_test_run 4 cleanup
+lkl_test_run 5 cleanup

tools/lkl/tests/net.sh

@@ -127,7 +127,7 @@ setup_backend()
         ;;
     *)
         echo "don't know how to setup backend $1"
-        return $TEST_FAILED
+        return $TEST_FAILURE
         ;;
     esac
 }