fsnotify: consistent behavior for parent not watching children

The logic for handling events on child in groups that have a mark on
the parent inode, but without FS_EVENT_ON_CHILD flag in the mask is
duplicated in several places and inconsistent.

Move the logic into the preparation of mark type iterator, so that the
parent mark type will be excluded from all mark type iterations in that
case.

This results in several subtle changes of behavior, hopefully all
desired changes of behavior, for example:

- Group A has a mount mark with FS_MODIFY in mask
- Group A has a mark with ignore mask that does not survive FS_MODIFY
  and does not watch children on directory D.
- Group B has a mark with FS_MODIFY in mask that does watch children
  on directory D.
- FS_MODIFY event on file D/foo should not clear the ignore mask of
  group A, but before this change it does

And if group A ignore mask was set to survive FS_MODIFY:
- FS_MODIFY event on file D/foo should be reported to group A on account
  of the mount mark, but before this change it is wrongly ignored

Fixes: 2f02fd3f ("fanotify: fix ignore mask logic for events on child and on dir")
Reported-by: Jan Kara <jack@suse.com>
Link: https://lore.kernel.org/linux-fsdevel/20220314113337.j7slrb5srxukztje@quack3.lan/Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220511190213.831646-3-amir73il@gmail.com

fs/notify/fanotify/fanotify.c

@@ -320,7 +320,7 @@ static u32 fanotify_group_event_mask(struct fsnotify_group *group,
 	}
 
 	fsnotify_foreach_iter_mark_type(iter_info, mark, type) {
-		/* Apply ignore mask regardless of ISDIR and ON_CHILD flags */
+		/* Apply ignore mask regardless of mark's ISDIR flag */
 		marks_ignored_mask |= mark->ignored_mask;
 
 		/*
@@ -330,14 +330,6 @@ static u32 fanotify_group_event_mask(struct fsnotify_group *group,
 		if (event_mask & FS_ISDIR && !(mark->mask & FS_ISDIR))
 			continue;
 
-		/*
-		 * If the event is on a child and this mark is on a parent not
-		 * watching children, don't send it!
-		 */
-		if (type == FSNOTIFY_ITER_TYPE_PARENT &&
-		    !(mark->mask & FS_EVENT_ON_CHILD))
-			continue;
-
 		marks_mask |= mark->mask;
 
 		/* Record the mark types of this group that matched the event */

fs/notify/fsnotify.c

@@ -290,22 +290,15 @@ static int fsnotify_handle_event(struct fsnotify_group *group, __u32 mask,
 	}
 
 	if (parent_mark) {
-		/*
-		 * parent_mark indicates that the parent inode is watching
-		 * children and interested in this event, which is an event
-		 * possible on child. But is *this mark* watching children and
-		 * interested in this event?
-		 */
-		if (parent_mark->mask & FS_EVENT_ON_CHILD) {
-			ret = fsnotify_handle_inode_event(group, parent_mark, mask,
-							  data, data_type, dir, name, 0);
-			if (ret)
-				return ret;
-		}
-		if (!inode_mark)
-			return 0;
+		ret = fsnotify_handle_inode_event(group, parent_mark, mask,
+						  data, data_type, dir, name, 0);
+		if (ret)
+			return ret;
 	}
 
+	if (!inode_mark)
+		return 0;
+
 	if (mask & FS_EVENT_ON_CHILD) {
 		/*
 		 * Some events can be sent on both parent dir and child marks
@@ -422,8 +415,19 @@ static bool fsnotify_iter_select_report_types(
 	iter_info->report_mask = 0;
 	fsnotify_foreach_iter_type(type) {
 		mark = iter_info->marks[type];
-		if (mark && mark->group == iter_info->current_group)
+		if (mark && mark->group == iter_info->current_group) {
+			/*
+			 * FSNOTIFY_ITER_TYPE_PARENT indicates that this inode
+			 * is watching children and interested in this event,
+			 * which is an event possible on child.
+			 * But is *this mark* watching children?
+			 */
+			if (type == FSNOTIFY_ITER_TYPE_PARENT &&
+			    !(mark->mask & FS_EVENT_ON_CHILD))
+				continue;
+
 			fsnotify_iter_set_report_type(iter_info, type);
+		}
 	}
 
 	return true;