[ 3.5.y.z extended stable ] Patch "fanotify: dont merge permission events" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    fanotify: dont merge permission events

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From c71a3a1ee282b68e2566f4d3bd2c70485538c142 Mon Sep 17 00:00:00 2001
From: Lino Sanfilippo <LinoSanfilippo at gmx.de>
Date: Fri, 23 Mar 2012 02:42:23 +0100
Subject: [PATCH] fanotify: dont merge permission events

commit 03a1cec1f17ac1a6041996b3e40f96b5a2f90e1b upstream.

Boyd Yang reported a problem for the case that multiple threads of the same
thread group are waiting for a reponse for a permission event.
In this case it is possible that some of the threads are never woken up, even
if the response for the event has been received
(see http://marc.info/?l=linux-kernel&m=131822913806350&w=2).

The reason is that we are currently merging permission events if they belong to
the same thread group. But we are not prepared to wake up more than one waiter
for each event. We do

wait_event(group->fanotify_data.access_waitq, event->response ||
			atomic_read(&group->fanotify_data.bypass_perm));
and after that
  event->response = 0;

which is the reason that even if we woke up all waiters for the same event
some of them may see event->response being already set 0 again, then go back to
sleep and block forever.

With this patch we avoid that more than one thread is waiting for a response
by not merging permission events for the same thread group any more.

Reported-by: Boyd Yang <boyd.yang at gmail.com>
Signed-off-by: Lino Sanfilippo <LinoSanfilipp at gmx.de>
Signed-off-by: Eric Paris <eparis at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/notify/fanotify/fanotify.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index a506360..0c2f912 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -18,6 +18,12 @@ static bool should_merge(struct fsnotify_event *old, struct fsnotify_event *new)
 	    old->tgid == new->tgid) {
 		switch (old->data_type) {
 		case (FSNOTIFY_EVENT_PATH):
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+			/* dont merge two permission events */
+			if ((old->mask & FAN_ALL_PERM_EVENTS) &&
+			    (new->mask & FAN_ALL_PERM_EVENTS))
+				return false;
+#endif
 			if ((old->path.mnt == new->path.mnt) &&
 			    (old->path.dentry == new->path.dentry))
 				return true;
--
1.8.3.2