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/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index b63e0db03631..d55381d3fa0f 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -657,6 +657,22 @@ can be useful in case the underlying disk is copied and the UUID of this copy
 is changed. This is only applicable if all lower/upper/work directories are on
 the same filesystem, otherwise it will fallback to normal behaviour.
 
+
+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)
+    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.
+    UUID of underlying layers is ignored.
+- "on":
+    UUID of overlayfs is generated and used to report a unique fsid.
+
+
 Volatile mount
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