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>

1 files changed, 22 insertions, 0 deletions

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 8602982ae579..9ebb9598e7ec 100644
--- a/fs/overlayfs/util.c
+++ b/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);