ovl: auto generate uuid for new overlay filesystems

Add a new mount option uuid=auto, which is the default.

If a persistent UUID xattr is found it is used.

Otherwise, an existing ovelrayfs with copied up subdirs in upper dir
that was never mounted with uuid=on retains the null UUID.

A new overlayfs with no copied up subdirs, generates the persistent UUID
on first mount.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

Documentation/filesystems/overlayfs.rst

@@ -664,7 +664,7 @@ UUID and fsid
 The UUID of overlayfs instance itself and the fsid reported by statfs(2) are
 controlled by the "uuid" mount option, which supports these values:
 
-- "null": (default)
+- "null":
     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.
@@ -674,6 +674,12 @@ controlled by the "uuid" mount option, which supports these values:
     UUID is stored in xattr "trusted.overlay.uuid", making overlayfs fsid
     unique and persistent.  This option requires an overlayfs with upper
     filesystem that supports xattrs.
+- "auto": (default)
+    UUID is taken from xattr "trusted.overlay.uuid" if it exists.
+    Upgrade to "uuid=on" on first time mount of new overlay filesystem that
+    meets the prerequites.
+    Downgrade to "uuid=null" for existing overlay filesystems that were never
+    mounted with "uuid=on".
 
 
 Volatile mount

fs/overlayfs/overlayfs.h

@@ -71,6 +71,7 @@ enum {
 enum {
 	OVL_UUID_OFF,
 	OVL_UUID_NULL,
+	OVL_UUID_AUTO,
 	OVL_UUID_ON,
 };
 
@@ -550,7 +551,8 @@ static inline bool ovl_origin_uuid(struct ovl_fs *ofs)
 
 static inline bool ovl_has_fsid(struct ovl_fs *ofs)
 {
-	return ofs->config.uuid == OVL_UUID_ON;
+	return ofs->config.uuid == OVL_UUID_ON ||
+	       ofs->config.uuid == OVL_UUID_AUTO;
 }
 
 /*

fs/overlayfs/params.c

@@ -68,6 +68,7 @@ static const struct constant_table ovl_parameter_bool[] = {
 static const struct constant_table ovl_parameter_uuid[] = {
 	{ "off",	OVL_UUID_OFF  },
 	{ "null",	OVL_UUID_NULL },
+	{ "auto",	OVL_UUID_AUTO },
 	{ "on",		OVL_UUID_ON   },
 	{}
 };
@@ -79,7 +80,7 @@ static const char *ovl_uuid_mode(struct ovl_config *config)
 
 static int ovl_uuid_def(void)
 {
-	return OVL_UUID_NULL;
+	return OVL_UUID_AUTO;
 }
 
 static const struct constant_table ovl_parameter_xino[] = {

fs/overlayfs/util.c

@@ -695,6 +695,28 @@ bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
 	if (res != -ENODATA)
 		goto fail;
 
+	/*
+	 * With uuid=auto, if uuid xattr is found, it will be used.
+	 * If uuid xattrs is not found, generate a persistent uuid only on mount
+	 * of new overlays where upper root dir is not yet marked as impure.
+	 * An upper dir is marked as impure on copy up or lookup of its subdirs.
+	 */
+	if (ofs->config.uuid == OVL_UUID_AUTO) {
+		res = ovl_path_getxattr(ofs, upperpath, OVL_XATTR_IMPURE, NULL,
+					0);
+		if (res > 0) {
+			/* Any mount of old overlay - downgrade to uuid=null */
+			ofs->config.uuid = OVL_UUID_NULL;
+			return true;
+		} else if (res == -ENODATA) {
+			/* First mount of new overlay - upgrade to uuid=on */
+			ofs->config.uuid = OVL_UUID_ON;
+		} else if (res < 0) {
+			goto fail;
+		}
+
+	}
+
 	/* Generate overlay instance uuid */
 	uuid_gen(&sb->s_uuid);