lib/mount.c: Restore "subtype#fsname" format for legacy kernel fallback

Commit 50fa7f0bc2 ("Move fuse_mnt_build_{source,type} to mount_util.c"
changed the ENODEV fallback behavior from using
"subtype#fsname" format to just "fsname" for the mount source.

This adds a use_subtype_prefix parameter to fuse_mnt_build_source() to
control whether the legacy "subtype#fsname" format should be used. This
centralizes the source string building logic and avoids code duplication
in the ENODEV fallback paths.

Original behavior (before 50fa7f0bc2):
    if (mo->fsname) {
        if (!mo->blkdev)
            sprintf(source, "%s#%s", mo->subtype, mo->fsname);
    } else {
        strcpy(source, type);
    }

Behavior after 50fa7f0bc2 (unintended change):
    source = fuse_mnt_build_source(mo->fsname, NULL, devname);
    // Returns just fsname, not "subtype#fsname"

Behavior before 50fa7f0bc2 is restored again.

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>

lib/mount.c

@@ -659,7 +659,7 @@ int fuse_kern_do_mount(const char *mnt, struct mount_opts *mo,
 	int res;
 	const char *devname = fuse_mnt_get_devname();
 	res = -ENOMEM;
-	source = fuse_mnt_build_source(mo->fsname, mo->subtype, devname);
+	source = fuse_mnt_build_source(mo->fsname, mo->subtype, devname, 0);
 	type = fuse_mnt_build_type(mo->blkdev, mo->subtype);
 	if (!type || !source) {
 		fuse_log(FUSE_LOG_ERR, "%s: failed to allocate memory\n",
@@ -670,16 +670,16 @@ int fuse_kern_do_mount(const char *mnt, struct mount_opts *mo,
 	res = mount(source, mnt, type, mo->flags, mo->kernel_opts);
 	if (res == -1 && errno == ENODEV && mo->subtype) {
 		/* Probably missing subtype support */
-
-		/*
-		 * The allocated space by fuse_mnt_build_{source,type}
-		 * might be too small.
-		 */
 		free(source);
 		free(type);
 
 		type = fuse_mnt_build_type(mo->blkdev, NULL);
-		source = fuse_mnt_build_source(mo->fsname, NULL, devname);
+		if (mo->fsname && !mo->blkdev) {
+			source = fuse_mnt_build_source(mo->fsname, mo->subtype,
+						       devname, 1);
+		} else {
+			source = strdup(type);
+		}
 
 		if (!type || !source) {
 			fuse_log(FUSE_LOG_ERR,

lib/mount_fsmount.c

@@ -353,7 +353,7 @@ int fuse_kern_fsmount(const char *mnt, unsigned long flags, int blkdev,
 
 	/* Build type and source strings */
 	type = fuse_mnt_build_type(blkdev, subtype);
-	source = fuse_mnt_build_source(fsname, subtype, source_dev);
+	source = fuse_mnt_build_source(fsname, subtype, source_dev, 0);
 	err = -ENOMEM;
 	if (!type || !source) {
 		fprintf(stderr, "fuse: failed to allocate memory\n");

lib/mount_util.c

@@ -536,13 +536,25 @@ const char *fuse_mnt_get_devname(void)
 }
 
 char *fuse_mnt_build_source(const char *fsname, const char *subtype,
-			     const char *devname)
+			     const char *devname, int use_subtype_prefix)
 {
 	char *source;
 	int ret;
 
-	ret = asprintf(&source, "%s",
-		       fsname ? fsname : (subtype ? subtype : devname));
+	if (use_subtype_prefix && fsname && subtype) {
+		ret = asprintf(&source, "%s#%s", subtype, fsname);
+	} else {
+		const char *src_str;
+
+		if (fsname)
+			src_str = fsname;
+		else if (subtype)
+			src_str = subtype;
+		else
+			src_str = devname;
+		ret = asprintf(&source, "%s", src_str);
+	}
+
 	if (ret == -1)
 		return NULL;
 

lib/mount_util.h

@@ -39,7 +39,7 @@ int fuse_mnt_add_mount_helper(const char *mnt, const char *source,
 
 /* Build source and type strings for mounting */
 char *fuse_mnt_build_source(const char *fsname, const char *subtype,
-			     const char *devname);
+			     const char *devname, int use_subtype_prefix);
 char *fuse_mnt_build_type(int blkdev, const char *subtype);
 
 #endif /* FUSE_MOUNT_UTIL_H_ */