fanotify: report FAN_ONDIR to listener with FAN_REPORT_FID

dirent modification events (create/delete/move) do not carry the
child entry name/inode information. Instead, we report FAN_ONDIR
for mkdir/rmdir so user can differentiate them from creat/unlink.

This is consistent with inotify reporting IN_ISDIR with dirent events
and is useful for implementing recursive directory tree watcher.

We avoid merging dirent events referring to subdirs with dirent events
referring to non subdirs, otherwise, user won't be able to tell from a
mask FAN_CREATE|FAN_DELETE|FAN_ONDIR if it describes mkdir+unlink pair
or rmdir+create pair of events.

For backward compatibility and consistency, do not report FAN_ONDIR
to user in legacy fanotify mode (reporting fd) and report FAN_ONDIR
to user in FAN_REPORT_FID mode for all event types.

Cc: <linux-api@vger.kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

2 files changed, 32 insertions, 4 deletions

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 158c69acb04d..4ff84bc5772e 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -34,7 +34,16 @@ static bool should_merge(struct fsnotify_event *old_fsn,
 		return old->path.mnt == new->path.mnt &&
 			old->path.dentry == new->path.dentry;
 	} else if (fanotify_event_has_fid(old)) {
-		return fanotify_fid_equal(&old->fid, &new->fid, old->fh_len);
+		/*
+		 * We want to merge many dirent events in the same dir (i.e.
+		 * creates/unlinks/renames), but we do not want to merge dirent
+		 * events referring to subdirs with dirent events referring to
+		 * non subdirs, otherwise, user won't be able to tell from a
+		 * mask FAN_CREATE|FAN_DELETE|FAN_ONDIR if it describes mkdir+
+		 * unlink pair or rmdir+create pair of events.
+		 */
+		return (old->mask & FS_ISDIR) == (new->mask & FS_ISDIR) &&
+			fanotify_fid_equal(&old->fid, &new->fid, old->fh_len);
 	}
 
 	/* Do not merge events if we failed to encode fid */
@@ -112,6 +121,7 @@ static u32 fanotify_group_event_mask(struct fsnotify_group *group,
 				     int data_type)
 {
 	__u32 marks_mask = 0, marks_ignored_mask = 0;
+	__u32 test_mask, user_mask = FANOTIFY_OUTGOING_EVENTS;
 	const struct path *path = data;
 	struct fsnotify_mark *mark;
 	int type;
@@ -145,12 +155,30 @@ static u32 fanotify_group_event_mask(struct fsnotify_group *group,
 		marks_ignored_mask |= mark->ignored_mask;
 	}
 
+	test_mask = event_mask & marks_mask & ~marks_ignored_mask;
+
+	/*
+	 * dirent modification events (create/delete/move) do not carry the
+	 * child entry name/inode information. Instead, we report FAN_ONDIR
+	 * for mkdir/rmdir so user can differentiate them from creat/unlink.
+	 *
+	 * For backward compatibility and consistency, do not report FAN_ONDIR
+	 * to user in legacy fanotify mode (reporting fd) and report FAN_ONDIR
+	 * to user in FAN_REPORT_FID mode for all event types.
+	 */
+	if (FAN_GROUP_FLAG(group, FAN_REPORT_FID)) {
+		/* Do not report FAN_ONDIR without any event */
+		if (!(test_mask & ~FAN_ONDIR))
+			return 0;
+	} else {
+		user_mask &= ~FAN_ONDIR;
+	}
+
 	if (event_mask & FS_ISDIR &&
 	    !(marks_mask & FS_ISDIR & ~marks_ignored_mask))
 		return 0;
 
-	return event_mask & FANOTIFY_OUTGOING_EVENTS & marks_mask &
-		~marks_ignored_mask;
+	return test_mask & user_mask;
 }
 
 static int fanotify_encode_fid(struct fanotify_event *event,
diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
index e9d45387089f..b79fa9bb7359 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -67,7 +67,7 @@
 /* Events that may be reported to user */
 #define FANOTIFY_OUTGOING_EVENTS	(FANOTIFY_EVENTS | \
 					 FANOTIFY_PERM_EVENTS | \
-					 FAN_Q_OVERFLOW)
+					 FAN_Q_OVERFLOW | FAN_ONDIR)
 
 #define ALL_FANOTIFY_EVENT_BITS		(FANOTIFY_OUTGOING_EVENTS | \
 					 FANOTIFY_EVENT_FLAGS)