[RFC][PATCH 3/3] ovl: use persistent s_uuid with index=on

[Amir Goldstein <amir73il@gmail.com>]

With index=on, overlayfs instances are non-migratable, meaning that
the layers cannot be copied without breaking the index.

So when indexdir exists, store a persistent uuid in xattr on the
indexdir to give the overlayfs instance a persistent identifier.

This also makes f_fsid persistent and more reliable for reporting
fid info in fanotify events.

With mount option uuid=nogen, a persistent uuid is not be initialized
on indexdir, but if a persistent uuid already exists, it will be used.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/overlayfs/overlayfs.h |  3 +++
 fs/overlayfs/super.c     |  7 +++++++
 fs/overlayfs/util.c      | 41 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 51 insertions(+)

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index dcdb02d0ddf8..9927472a3aaa 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -36,6 +36,7 @@ enum ovl_xattr {
 	OVL_XATTR_IMPURE,
 	OVL_XATTR_NLINK,
 	OVL_XATTR_UPPER,
+	OVL_XATTR_UUID,
 	OVL_XATTR_METACOPY,
 	OVL_XATTR_PROTATTR,
 };
@@ -431,6 +432,8 @@ bool ovl_already_copied_up(struct dentry *dentry, int flags);
 bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
 			      enum ovl_xattr ox);
 bool ovl_path_check_origin_xattr(struct ovl_fs *ofs, const struct path *path);
+bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
+			 struct dentry *upperdentry, bool set);
 
 static inline bool ovl_check_origin_xattr(struct ovl_fs *ofs,
 					  struct dentry *upperdentry)
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index ad2250f98b38..8364620e8722 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1535,6 +1535,9 @@ static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
 		if (err)
 			pr_err("failed to verify index dir 'upper' xattr\n");
 
+		/* Best effort get or set persistent uuid */
+		ovl_init_uuid_xattr(sb, ofs, ofs->indexdir, true);
+
 		/* Cleanup bad/stale/orphan index entries */
 		if (!err)
 			err = ovl_indexdir_cleanup(ofs);
@@ -2052,6 +2055,10 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 			ovl_uuid_str[ofs->config.uuid]);
 	}
 
+	/*
+	 * This uuid may be overridden by a persistent uuid stored in xattr on
+	 * index dir and it may be persisted in xattr on first index=on mount.
+	 */
 	if (ovl_want_uuid_gen(ofs))
 		uuid_gen(&sb->s_uuid);
 
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 923d66d131c1..8902db4b2975 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -589,6 +589,45 @@ bool ovl_path_check_origin_xattr(struct ovl_fs *ofs, const struct path *path)
 	return false;
 }
 
+/*
+ * Load persistent uuid from xattr into s_uuid if found, possibly overriding
+ * the random generated value in s_uuid.
+ * Otherwise, if @set is true and s_uuid contains a valid value, store this
+ * value in xattr.
+ */
+bool ovl_init_uuid_xattr(struct super_block *sb, struct ovl_fs *ofs,
+			 struct dentry *upperdentry, bool set)
+{
+	struct path path = {
+		.dentry = upperdentry,
+		.mnt = ovl_upper_mnt(ofs),
+	};
+	uuid_t uuid;
+	int res;
+
+	res = ovl_path_getxattr(ofs, &path, OVL_XATTR_UUID, uuid.b, UUID_SIZE);
+	if (res == UUID_SIZE) {
+		uuid_copy(&sb->s_uuid, &uuid);
+		return true;
+	}
+
+	if (res == -ENODATA) {
+		if (!set || uuid_is_null(&sb->s_uuid))
+			return false;
+
+		res = ovl_setxattr(ofs, upperdentry, OVL_XATTR_UUID,
+				   sb->s_uuid.b, UUID_SIZE);
+		if (res == 0)
+			return true;
+	} else {
+		set = false;
+	}
+
+	pr_warn("failed to %s uuid (%pd2, err=%i)\n",
+		set ? "set" : "get", upperdentry, res);
+	return false;
+}
+
 bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
 			       enum ovl_xattr ox)
 {
@@ -611,6 +650,7 @@ bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
 #define OVL_XATTR_IMPURE_POSTFIX	"impure"
 #define OVL_XATTR_NLINK_POSTFIX		"nlink"
 #define OVL_XATTR_UPPER_POSTFIX		"upper"
+#define OVL_XATTR_UUID_POSTFIX		"uuid"
 #define OVL_XATTR_METACOPY_POSTFIX	"metacopy"
 #define OVL_XATTR_PROTATTR_POSTFIX	"protattr"
 
@@ -625,6 +665,7 @@ const char *const ovl_xattr_table[][2] = {
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_IMPURE),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_NLINK),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_UPPER),
+	OVL_XATTR_TAB_ENTRY(OVL_XATTR_UUID),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_METACOPY),
 	OVL_XATTR_TAB_ENTRY(OVL_XATTR_PROTATTR),
 };
-- 
2.34.1