[PATCH 03/19] fsnotify: check if file is actually being watched for pre-content events on open

[Jan Kara <jack@suse.cz>]

From: Amir Goldstein <amir73il@gmail.com>

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 <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/2ddcc9f8d1fde48d085318a6b5a889289d8871d8.1731684329.git.josef@toxicpanda.com
---
 fs/notify/fsnotify.c             | 27 +++++++++++++++++++++++++--
 include/linux/fsnotify_backend.h |  3 +++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 569ec356e4ce..dd1dffd89fd6 100644
--- a/fs/notify/fsnotify.c
+++ b/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))
@@ -658,6 +660,27 @@ void file_set_fsnotify_mode(struct file *file)
 		file->f_mode |= FMODE_NONOTIFY | FMODE_NONOTIFY_PERM;
 		return;
 	}
+
+	/*
+	 * OK, there are some pre-content watchers. Check if anybody can be
+	 * watching for pre-content events on *this* file.
+	 */
+	mnt_mask = READ_ONCE(real_mount(file->f_path.mnt)->mnt_fsnotify_mask);
+	if (likely(!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED) &&
+	    !fsnotify_object_watched(d_inode(dentry), mnt_mask,
+				     FSNOTIFY_PRE_CONTENT_EVENTS))) {
+		file->f_mode |= FMODE_NONOTIFY | FMODE_NONOTIFY_PERM;
+		return;
+	}
+
+	/* Even parent is not watching for pre-content events on this file? */
+	parent = dget_parent(dentry);
+	p_mask = fsnotify_inode_watches_children(d_inode(parent));
+	dput(parent);
+	if (!(p_mask & FSNOTIFY_PRE_CONTENT_EVENTS)) {
+		file->f_mode |= FMODE_NONOTIFY | FMODE_NONOTIFY_PERM;
+		return;
+	}
 }
 #endif
 
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 3ecf7768e577..9c105244815d 100644
--- a/include/linux/fsnotify_backend.h
+++ b/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)
 
-- 
2.35.3