ovl: relax requirement for uuid=off,index=on

uuid=off,index=on required that all upper/lower directories are on the
same filesystem.

Relax the requirement so that only all the lower directories need to be
on the same filesystem.

Reported-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20260114-tonyk-get_disk_uuid-v1-3-e6a319e25d57@igalia.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

Author: amir73il

Documentation/filesystems/overlayfs.rst

@@ -753,9 +753,9 @@ Note: the mount options index=off,nfs_export=on are conflicting for a
 read-write mount and will result in an error.
 
 Note: the mount option uuid=off can be used to replace UUID of the underlying
-filesystem in file handles with null, and effectively disable UUID checks. This
+filesystem in file handles with null, in order to relax the UUID checks. This
 can be useful in case the underlying disk is copied and the UUID of this copy
-is changed. This is only applicable if all lower/upper/work directories are on
+is changed. This is only applicable if all lower directories are on
 the same filesystem, otherwise it will fallback to normal behaviour.
 
 
@@ -769,7 +769,7 @@ controlled by the "uuid" mount option, which supports these values:
     UUID of overlayfs is null. fsid is taken from upper most filesystem.
 - "off":
     UUID of overlayfs is null. fsid is taken from upper most filesystem.
-    UUID of underlying layers is ignored.
+    UUID of underlying layers is ignored and null used instead.
 - "on":
     UUID of overlayfs is generated and used to report a unique fsid.
     UUID is stored in xattr "trusted.overlay.uuid", making overlayfs fsid

fs/overlayfs/namei.c

@@ -158,6 +158,18 @@ static struct ovl_fh *ovl_get_fh(struct ovl_fs *ofs, struct dentry *upperdentry,
 	goto out;
 }
 
+bool ovl_uuid_match(struct ovl_fs *ofs, const struct super_block *sb,
+		    const uuid_t *uuid)
+{
+	/*
+	 * Make sure that the stored uuid matches the uuid of the lower
+	 * layer where file handle will be decoded.
+	 * In case of uuid=off option just make sure that stored uuid is null.
+	 */
+	return ovl_origin_uuid(ofs) ? uuid_equal(uuid, &sb->s_uuid) :
+				      uuid_is_null(uuid);
+}
+
 struct dentry *ovl_decode_real_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
 				  struct vfsmount *mnt, bool connected)
 {
@@ -167,14 +179,7 @@ struct dentry *ovl_decode_real_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
 	if (!capable(CAP_DAC_READ_SEARCH))
 		return NULL;
 
-	/*
-	 * Make sure that the stored uuid matches the uuid of the lower
-	 * layer where file handle will be decoded.
-	 * In case of uuid=off option just make sure that stored uuid is null.
-	 */
-	if (ovl_origin_uuid(ofs) ?
-	    !uuid_equal(&fh->fb.uuid, &mnt->mnt_sb->s_uuid) :
-	    !uuid_is_null(&fh->fb.uuid))
+	if (!ovl_uuid_match(ofs, mnt->mnt_sb, &fh->fb.uuid))
 		return NULL;
 
 	bytes = (fh->fb.len - offsetof(struct ovl_fb, fid));

fs/overlayfs/overlayfs.h

@@ -710,6 +710,8 @@ static inline int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
 	return ovl_check_fb_len(&fh->fb, fh_len - OVL_FH_WIRE_OFFSET);
 }
 
+bool ovl_uuid_match(struct ovl_fs *ofs, const struct super_block *sb,
+		    const uuid_t *uuid);
 struct dentry *ovl_decode_real_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
 				  struct vfsmount *mnt, bool connected);
 int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,

fs/overlayfs/super.c

@@ -940,7 +940,7 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
 		 * disable lower file handle decoding on all of them.
 		 */
 		if (ofs->fs[i].is_lower &&
-		    uuid_equal(&ofs->fs[i].sb->s_uuid, uuid)) {
+		    ovl_uuid_match(ofs, ofs->fs[i].sb, uuid)) {
 			ofs->fs[i].bad_uuid = true;
 			return false;
 		}
@@ -952,6 +952,7 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
 static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
 {
 	struct super_block *sb = path->mnt->mnt_sb;
+	const uuid_t *uuid = ovl_origin_uuid(ofs) ? &sb->s_uuid : &uuid_null;
 	unsigned int i;
 	dev_t dev;
 	int err;
@@ -963,7 +964,7 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
 			return i;
 	}
 
-	if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
+	if (!ovl_lower_uuid_ok(ofs, uuid)) {
 		bad_uuid = true;
 		if (ofs->config.xino == OVL_XINO_AUTO) {
 			ofs->config.xino = OVL_XINO_OFF;
@@ -975,9 +976,8 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
 			warn = true;
 		}
 		if (warn) {
-			pr_warn("%s uuid detected in lower fs '%pd2', falling back to xino=%s,index=off,nfs_export=off.\n",
-				uuid_is_null(&sb->s_uuid) ? "null" :
-							    "conflicting",
+			pr_warn("%s uuid in non-single lower fs '%pd2', falling back to xino=%s,index=off,nfs_export=off.\n",
+				uuid_is_null(uuid) ? "null" : "conflicting",
 				path->dentry, ovl_xino_mode(&ofs->config));
 		}
 	}
@@ -1469,10 +1469,7 @@ static int ovl_fill_super_creds(struct fs_context *fc, struct super_block *sb)
 	if (!ovl_upper_mnt(ofs))
 		sb->s_flags |= SB_RDONLY;
 
-	if (!ovl_origin_uuid(ofs) && ofs->numfs > 1) {
-		pr_warn("The uuid=off requires a single fs for lower and upper, falling back to uuid=null.\n");
-		ofs->config.uuid = OVL_UUID_NULL;
-	} else if (ovl_has_fsid(ofs) && ovl_upper_mnt(ofs)) {
+	if (ovl_has_fsid(ofs) && ovl_upper_mnt(ofs)) {
 		/* Use per instance persistent uuid/fsid */
 		ovl_init_uuid_xattr(sb, ofs, &ctx->upper);
 	}