ANDROID: fsnotify: Notify lower fs of open

If the filesystem being watched supports d_canonical_path,
notify the lower filesystem of the open as well.

Note: I've changed the Change-Id since this was previously present, but
      disappeared at some point.

Fixes: f37e05049b ("ANDROID: vfs: d_canonical_path for stacked FS")
Test: atest CtsOsTestCases:android.os.cts.FileObserverTest
Bug: 70706497
Change-Id: I7bf5c2bb2d6da18c831a8ece9ad9cf9ef3661b6b
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
(cherry picked from commit b289d1706b672f064bbe04c3a8bfb1a5e8ac0923)

include/linux/fsnotify.h

@@ -128,6 +128,25 @@ static inline int fsnotify_file(struct file *file, __u32 mask)
 					       FSNOTIFY_PRIO_CONTENT))
 		return 0;
 
+	/*
+	 * Open calls notify early on, so lower file system must be notified
+	 */
+	if (mask & FS_OPEN) {
+		if (path->dentry->d_op &&
+		    path->dentry->d_op->d_canonical_path) {
+			struct path lower_path;
+			int ret;
+
+			path->dentry->d_op->d_canonical_path(path, &lower_path);
+			ret = fsnotify_parent(lower_path.dentry, mask,
+					      &lower_path, FSNOTIFY_EVENT_PATH);
+			path_put(&lower_path);
+
+			if (ret)
+				return ret;
+		}
+	}
+
 	return fsnotify_parent(path->dentry, mask, path, FSNOTIFY_EVENT_PATH);
 }