fsnotify: opt-in for permission events at file open time

Legacy inotify/fanotify listeners can add watches for events on inode,
parent or mount and expect to get events (e.g. FS_MODIFY) on files that
were already open at the time of setting up the watches.

fanotify permission events are typically used by Anti-malware sofware,
that is watching the entire mount and it is not common to have more that
one Anti-malware engine installed on a system.

To reduce the overhead of the fsnotify_file_perm() hooks on every file
access, relax the semantics of the legacy FAN_ACCESS_PERM event to generate
events only if there were *any* permission event listeners on the
filesystem at the time that the file was opened.

The new semantic is implemented by extending the FMODE_NONOTIFY bit into
two FMODE_NONOTIFY_* bits, that are used to store a mode for which of the
events types to report.

This is going to apply to the new fanotify pre-content events in order
to reduce the cost of the new pre-content event vfs hooks.

[Thanks to Bert Karwatzki <spasswolf@web.de> for reporting a bug in this
code with CONFIG_FANOTIFY_ACCESS_PERMISSIONS disabled]

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wj8L=mtcRTi=NECHMGfZQgXOp_uix1YVh04fEmrKaMnXA@mail.gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/5ea5f8e283d1edb55aa79c35187bfe344056af14.1731684329.git.josef@toxicpanda.com

1 files changed, 38 insertions, 0 deletions

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index f976949d2634..569ec356e4ce 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -623,6 +623,44 @@ out:
 }
 EXPORT_SYMBOL_GPL(fsnotify);
 
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+/*
+ * At open time we check fsnotify_sb_has_priority_watchers() and set the
+ * FMODE_NONOTIFY_ mode bits accordignly.
+ * Later, fsnotify permission hooks do not check if there are permission event
+ * watches, but that there were permission event watches at open time.
+ */
+void file_set_fsnotify_mode(struct file *file)
+{
+	struct super_block *sb = file->f_path.dentry->d_sb;
+
+	/* Is it a file opened by fanotify? */
+	if (FMODE_FSNOTIFY_NONE(file->f_mode))
+		return;
+
+	/*
+	 * Permission events is a super set of pre-content events, so if there
+	 * are no permission event watchers, there are also no pre-content event
+	 * watchers and this is implied from the single FMODE_NONOTIFY_PERM bit.
+	 */
+	if (likely(!fsnotify_sb_has_priority_watchers(sb,
+						FSNOTIFY_PRIO_CONTENT))) {
+		file->f_mode |= FMODE_NONOTIFY_PERM;
+		return;
+	}
+
+	/*
+	 * 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,
+						FSNOTIFY_PRIO_PRE_CONTENT))) {
+		file->f_mode |= FMODE_NONOTIFY | FMODE_NONOTIFY_PERM;
+		return;
+	}
+}
+#endif
+
 static __init int fsnotify_init(void)
 {
 	int ret;