fsnotify: check if file is actually being watched for pre-content events on open

So far, we set FMODE_NONOTIFY_ flags at open time if we know that there
are no permission event watchers at all on the filesystem, but lack of
FMODE_NONOTIFY_ flags does not mean that the file is actually watched.

For pre-content events, it is possible to optimize things so that we
don't bother trying to send pre-content events if file was not watched
(through sb, mnt, parent or inode itself) on open. Set FMODE_NONOTIFY_
flags according to that.

Signed-off-by: Amir Goldstein <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Link: https://patch.msgid.link/2ddcc9f8d1fde48d085318a6b5a889289d8871d8.1731684329.git.josef@toxicpanda.com

Author: amir73il

fs/notify/fsnotify.c

@@ -193,7 +193,7 @@ static bool fsnotify_event_needs_parent(struct inode *inode, __u32 mnt_mask,
 	return mask & marks_mask;
 }
 
-/* Are there any inode/mount/sb objects that are interested in this event? */
+/* Are there any inode/mount/sb objects that watch for these events? */
 static inline bool fsnotify_object_watched(struct inode *inode, __u32 mnt_mask,
 					   __u32 mask)
 {
@@ -632,7 +632,9 @@ EXPORT_SYMBOL_GPL(fsnotify);
  */
 void file_set_fsnotify_mode(struct file *file)
 {
-	struct super_block *sb = file->f_path.dentry->d_sb;
+	struct dentry *dentry = file->f_path.dentry, *parent;
+	struct super_block *sb = dentry->d_sb;
+	__u32 mnt_mask, p_mask;
 
 	/* Is it a file opened by fanotify? */
 	if (FMODE_FSNOTIFY_NONE(file->f_mode))
@@ -653,11 +655,32 @@ void file_set_fsnotify_mode(struct file *file)
 	 * If there are permission event watchers but no pre-content event
 	 * watchers, set FMODE_NONOTIFY | FMODE_NONOTIFY_PERM to indicate that.
 	 */
-	if (likely(!fsnotify_sb_has_priority_watchers(sb,
+	if ((!d_is_dir(dentry) && !d_is_reg(dentry)) ||
+	    likely(!fsnotify_sb_has_priority_watchers(sb,
 						FSNOTIFY_PRIO_PRE_CONTENT))) {
 		file->f_mode |= FMODE_NONOTIFY | FMODE_NONOTIFY_PERM;
 		return;
 	}
+
+	/*
+	 * OK, there are some pre-content watchers. Check if anybody is
+	 * watching for pre-content events on *this* file.
+	 */
+	mnt_mask = READ_ONCE(real_mount(file->f_path.mnt)->mnt_fsnotify_mask);
+	if (unlikely(fsnotify_object_watched(d_inode(dentry), mnt_mask,
+				     FSNOTIFY_PRE_CONTENT_EVENTS)))
+		return;
+
+	/* Is parent watching for pre-content events on this file? */
+	if (dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED) {
+		parent = dget_parent(dentry);
+		p_mask = fsnotify_inode_watches_children(d_inode(parent));
+		dput(parent);
+		if (p_mask & FSNOTIFY_PRE_CONTENT_EVENTS)
+			return;
+	}
+	/* Nobody watching for pre-content events from this file */
+	file->f_mode |= FMODE_NONOTIFY | FMODE_NONOTIFY_PERM;
 }
 #endif
 

include/linux/fsnotify_backend.h

@@ -77,6 +77,9 @@
  */
 #define ALL_FSNOTIFY_DIRENT_EVENTS (FS_CREATE | FS_DELETE | FS_MOVE | FS_RENAME)
 
+/* Pre-content events can be used to fill file content */
+#define FSNOTIFY_PRE_CONTENT_EVENTS 0
+
 #define ALL_FSNOTIFY_PERM_EVENTS (FS_OPEN_PERM | FS_ACCESS_PERM | \
 				  FS_OPEN_EXEC_PERM)