ANDROID: vfs: add d_canonical_path for stacked filesystem support

Inotify does not currently know when a filesystem
is acting as a wrapper around another fs. This means
that inotify watchers will miss any modifications to
the base file, as well as any made in a separate
stacked fs that points to the same file.
d_canonical_path solves this problem by allowing the fs
to map a dentry to a path in the lower fs. Inotify
can use it to find the appropriate place to watch to
be informed of all changes to a file.

Bug: 70706497
Bug: 120446149
Change-Id: I09563baffad1711a045e45c1bd0bd8713c2cc0b6
Signed-off-by: Daniel Rosenberg <drosen@google.com>
[astrachan: Folded 34df4102216e ("ANDROID: fsnotify: Notify lower fs of
            open") into this patch]
Signed-off-by: Alistair Strachan <astrachan@google.com>
[AmitP: TODO]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

3 files changed, 21 insertions, 2 deletions

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 593c8ab225de..44e84da735b6 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -699,6 +699,8 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname,
 	struct fsnotify_group *group;
 	struct inode *inode;
 	struct path path;
+	struct path alteredpath;
+	struct path *canonical_path = &path;
 	struct fd f;
 	int ret;
 	unsigned flags = 0;
@@ -744,13 +746,22 @@ SYSCALL_DEFINE3(inotify_add_watch, int, fd, const char __user *, pathname,
 	if (ret)
 		goto fput_and_out;
 
+	/* support stacked filesystems */
+	if(path.dentry && path.dentry->d_op) {
+		if (path.dentry->d_op->d_canonical_path) {
+			path.dentry->d_op->d_canonical_path(&path, &alteredpath);
+			canonical_path = &alteredpath;
+			path_put(&path);
+		}
+	}
+
 	/* inode held in place by reference to path; group by fget on fd */
-	inode = path.dentry->d_inode;
+	inode = canonical_path->dentry->d_inode;
 	group = f.file->private_data;
 
 	/* create/update an inode mark */
 	ret = inotify_update_watch(group, inode, mask);
-	path_put(&path);
+	path_put(canonical_path);
 fput_and_out:
 	fdput(f);
 	return ret;
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 60996e64c579..1ff446bd1527 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -147,6 +147,7 @@ struct dentry_operations {
 	struct vfsmount *(*d_automount)(struct path *);
 	int (*d_manage)(const struct path *, bool);
 	struct dentry *(*d_real)(struct dentry *, const struct inode *);
+	void (*d_canonical_path)(const struct path *, struct path *);
 } ____cacheline_aligned;
 
 /*
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 2ccb08cb5d6a..b0508592d51d 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -222,6 +222,7 @@ static inline void fsnotify_modify(struct file *file)
 static inline void fsnotify_open(struct file *file)
 {
 	const struct path *path = &file->f_path;
+	struct path lower_path;
 	struct inode *inode = file_inode(file);
 	__u32 mask = FS_OPEN;
 
@@ -230,6 +231,12 @@ static inline void fsnotify_open(struct file *file)
 	if (file->f_flags & __FMODE_EXEC)
 		mask |= FS_OPEN_EXEC;
 
+	if (path->dentry->d_op && path->dentry->d_op->d_canonical_path) {
+		path->dentry->d_op->d_canonical_path(path, &lower_path);
+		fsnotify_parent(&lower_path, NULL, mask);
+		fsnotify(lower_path.dentry->d_inode, mask, &lower_path, FSNOTIFY_EVENT_PATH, NULL, 0);
+		path_put(&lower_path);
+	}
 	fsnotify_path(inode, path, mask);
 }