Re: [PATCH] fanotify: use killable wait for waiting response for permission events

[Konstantin Khlebnikov]

On 20.08.2018 13:53, Jan Kara wrote:
> Hi!
>
> On Mon 20-08-18 10:09:42, Konstantin Khlebnikov wrote:
> > Waiting in uninterruptible state for response from userspace
> > easily produces deadlocks and hordes of unkillable tasks.
> >
> > This patch makes this wait killable.
> >
> > At receiving fatal signal task will remove queued event and die.
> > If event is already handled then response will be received as usual.
> >
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>
> Thanks for the patch. I like the idea. Some comments inline.
>
> > ---
> >   fs/notify/fanotify/fanotify.c |   22 +++++++++++++++++++++-
> >   1 file changed, 21 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> > index eb4e75175cfb..7a0c37790c89 100644
> > --- a/fs/notify/fanotify/fanotify.c
> > +++ b/fs/notify/fanotify/fanotify.c
> > @@ -64,7 +64,27 @@ static int fanotify_get_response(struct fsnotify_group *group,
> >   
> >   	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> >   
> > -	wait_event(group->fanotify_data.access_waitq, event->response);
> > +	ret = wait_event_killable(group->fanotify_data.access_waitq,
> > +				  event->response);
> > +	if (ret) {
> > +		/* Try to remove pending event from the queue */
> > +		spin_lock(&group->notification_lock);
> > +		if (!list_empty(&event->fae.fse.list))
> > +			list_del_init(&event->fae.fse.list);
>
> Here you forget to decrement group->q_len like
> fsnotify_remove_first_event() does.

Yep

> > +		else
> > +			ret = 0;
> > +		spin_unlock(&group->notification_lock);
>
> So the above check for list_empty can hit either when response is just
> being processed (and then we'll be woken up very soon) or when the event is
> just in the process of being copied from event queue to userspace (in which
> case we are in the same situation as in the old code). So it would be
> weird that in rare cases wait would not be really killable. I think we
> could detect this situation in fanotify_read() before adding event to
> access_list and just wakeup waiter in fanotify_get_response() again and
> avoid reporting the event to userspace. Hmm?

I've missed that move from list to list in fanotify_read().

So, fanotify_read needs event alive for a long time - copy_to_user might block forever.

We have to transfer ownership and destroy event in fanotify_read.
I'll try this approach.

> 								Honza
>
> > +
> > +		if (ret)
> > +			return ret;
> > +
> > +		/*
> > +		 * We cannot return, this will destroy event while
> > +		 * process_access_response() fills response.
> > +		 * Just wait for wakeup and continue normal flow.
> > +		 */
> > +		wait_event(group->fanotify_data.access_waitq, event->response);
> > +	}
> >   
> >   	/* userspace responded, convert to something usable */
> >   	switch (event->response & ~FAN_AUDIT) {