fanotify: use FAN_EVENT_ON_CHILD as implicit flag on sb/mount/non-dir marks

Up to now, fanotify allowed to set the FAN_EVENT_ON_CHILD flag on
sb/mount marks and non-directory inode mask, but the flag was ignored.

Mask out the flag if it is provided by user on sb/mount/non-dir marks
and define it as an implicit flag that cannot be removed by user.

This flag is going to be used internally to request for events with
parent and name info.

Link: https://lore.kernel.org/r/20200716084230.30611-8-amir73il@gmail.comSigned-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify_user.c

@@ -1050,6 +1050,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 	unsigned int mark_type = flags & FANOTIFY_MARK_TYPE_BITS;
 	bool ignored = flags & FAN_MARK_IGNORED_MASK;
 	unsigned int obj_type, fid_mode;
+	u32 umask = 0;
 	int ret;
 
 	pr_debug("%s: fanotify_fd=%d flags=%x dfd=%d pathname=%p mask=%llx\n",
@@ -1167,6 +1168,12 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 	else
 		mnt = path.mnt;
 
+	/* Mask out FAN_EVENT_ON_CHILD flag for sb/mount/non-dir marks */
+	if (mnt || !S_ISDIR(inode->i_mode)) {
+		mask &= ~FAN_EVENT_ON_CHILD;
+		umask = FAN_EVENT_ON_CHILD;
+	}
+
 	/* create/update an inode mark */
 	switch (flags & (FAN_MARK_ADD | FAN_MARK_REMOVE)) {
 	case FAN_MARK_ADD:
@@ -1183,13 +1190,13 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 	case FAN_MARK_REMOVE:
 		if (mark_type == FAN_MARK_MOUNT)
 			ret = fanotify_remove_vfsmount_mark(group, mnt, mask,
-							    flags, 0);
+							    flags, umask);
 		else if (mark_type == FAN_MARK_FILESYSTEM)
 			ret = fanotify_remove_sb_mark(group, mnt->mnt_sb, mask,
-						      flags, 0);
+						      flags, umask);
 		else
 			ret = fanotify_remove_inode_mark(group, inode, mask,
-							 flags, 0);
+							 flags, umask);
 		break;
 	default:
 		ret = -EINVAL;