Instead of failing, encode an invalid file handle in fanotify_encode_fh
if no inode is provided.  This bogus file handle will be reported by
FAN_FS_ERROR for non-inode errors.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Gabriel Krisman Bertazi <redacted>

---
Changes since v6:
  - Use FILEID_ROOT as the internal value (jan)
  - Create an empty FH (jan)

Changes since v5:
  - Preserve flags initialization (jan)
  - Add BUILD_BUG_ON (amir)
  - Require minimum of FANOTIFY_NULL_FH_LEN for fh_len(amir)
  - Improve comment to explain the null FH length (jan)
  - Simplify logic
---
 fs/notify/fanotify/fanotify.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index ec84fee7ad01..c64d61b673ca 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c

@@ -370,8 +370,14 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
 	fh->type = FILEID_ROOT;
 	fh->len = 0;
 	fh->flags = 0;
+
+	/*
+	 * Invalid FHs are used by FAN_FS_ERROR for errors not
+	 * linked to any inode. The f_handle won't be reported
+	 * back to userspace.
+	 */
 	if (!inode)
-		return 0;
+		goto out;
 
 	/*
 	 * !gpf means preallocated variable size fh, but fh_len could

@@ -403,6 +409,7 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
 	fh->type = type;
 	fh->len = fh_len;
 
+out:
 	/*
 	 * Mix fh into event merge key.  Hash might be NULL in case of
 	 * unhashed FID events (i.e. FAN_FS_ERROR).

-- 
2.33.0