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>

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 8275ed7..3585390 100644 (file)
--- a/Documentation/filesystems/overlayfs.rst
+++ b/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

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 8b026d7..72f57d9 100644 (file)
--- a/fs/overlayfs/overlayfs.h
+++ b/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;
 }
 
 /*

diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index 5a59c87..3fc01fe 100644 (file)
--- a/fs/overlayfs/params.c
+++ b/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[] = {

diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 8602982..9ebb959 100644 (file)
--- 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);