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SubjectRe: [PATCH] fanotify: use killable wait for waiting response for permission events
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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@yandex-team.ru>
>
> 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) {
>>

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