ovl: add support for unique fsid per instance

The legacy behavior of ovl_statfs() reports the f_fsid filled by
underlying upper fs. This fsid is not unique among overlayfs instances
on the same upper fs.

With mount option uuid=on, generate a non-persistent uuid per overlayfs
instance and use it as the seed for f_fsid, similar to tmpfs.

This is useful for reporting fanotify events with fid info from different
instances of overlayfs over the same upper fs.

The old behavior of null uuid and upper fs fsid is retained with the
mount option uuid=null, which is the default.

The mount option uuid=off that disables uuid checks in underlying layers
also retains the legacy behavior.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 453610fb9bf9..000dd89fe319 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -68,6 +68,12 @@ enum {
 };
 
 enum {
+	OVL_UUID_OFF,
+	OVL_UUID_NULL,
+	OVL_UUID_ON,
+};
+
+enum {
 	OVL_XINO_OFF,
 	OVL_XINO_AUTO,
 	OVL_XINO_ON,
@@ -534,6 +540,16 @@ static inline bool ovl_redirect_dir(struct ovl_fs *ofs)
 	return ofs->config.redirect_mode == OVL_REDIRECT_ON;
 }
 
+static inline bool ovl_origin_uuid(struct ovl_fs *ofs)
+{
+	return ofs->config.uuid != OVL_UUID_OFF;
+}
+
+static inline bool ovl_has_fsid(struct ovl_fs *ofs)
+{
+	return ofs->config.uuid == OVL_UUID_ON;
+}
+
 /*
  * With xino=auto, we do best effort to keep all inodes on same st_dev and
  * d_ino consistent with st_ino.