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Date:   Fri, 27 Aug 2021 14:18:12 -0400
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Jan Kara <jack@...e.cz>
Cc:     amir73il@...il.com, jack@...e.com, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        khazhy@...gle.com, dhowells@...hat.com, david@...morbit.com,
        tytso@....edu, djwong@...nel.org, repnop@...gle.com,
        kernel@...labora.com
Subject: Re: [PATCH v6 15/21] fanotify: Preallocate per superblock mark
 error event

Jan Kara <jack@...e.cz> writes:

> On Thu 12-08-21 17:40:04, Gabriel Krisman Bertazi wrote:
>> Error reporting needs to be done in an atomic context.  This patch
>> introduces a single error slot for superblock marks that report the
>> FAN_FS_ERROR event, to be used during event submission.
>> 
>> Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
>> 
>> ---
>> Changes v5:
>>   - Restore mark references. (jan)
>>   - Tie fee slot to the mark lifetime.(jan)
>>   - Don't reallocate event(jan)
>> ---
>>  fs/notify/fanotify/fanotify.c      | 12 ++++++++++++
>>  fs/notify/fanotify/fanotify.h      | 13 +++++++++++++
>>  fs/notify/fanotify/fanotify_user.c | 31 ++++++++++++++++++++++++++++--
>>  3 files changed, 54 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
>> index ebb6c557cea1..3bf6fd85c634 100644
>> --- a/fs/notify/fanotify/fanotify.c
>> +++ b/fs/notify/fanotify/fanotify.c
>> @@ -855,6 +855,14 @@ static void fanotify_free_name_event(struct fanotify_event *event)
>>  	kfree(FANOTIFY_NE(event));
>>  }
>>  
>> +static void fanotify_free_error_event(struct fanotify_event *event)
>> +{
>> +	/*
>> +	 * The actual event is tied to a mark, and is released on mark
>> +	 * removal
>> +	 */
>> +}
>> +
>
> I was pondering about the lifetime rules some more. This is also related to
> patch 16/21 but I'll comment here. When we hold mark ref from queued event,
> we introduce a subtle race into group destruction logic. There we first
> evict all marks, wait for them to be destroyed by worker thread after SRCU
> period expires, and then we remove queued events. When we hold mark
> reference from an event we break this as mark will exist until the event is
> dequeued and then group can get freed before we actually free the mark and
> so mark freeing can hit use-after-free issues.
>
> So we'll have to do this a bit differently. I have two options:
>
> 1) Instead of preallocating events explicitely like this, we could setup a
> mempool to allocate error events from for each notification group. We would
> resize the mempool when adding error mark so that it has as many reserved
> events as error marks. Upside is error events will be much less special -
> no special lifetime rules. We'd just need to setup & resize the mempool. We
> would also have to provide proper merge function for error events (to merge
> events from the same sb). Also there will be limitation of number of error
> marks per group because mempools use kmalloc() for an array tracking
> reserved events. But we could certainly manage 512, likely 1024 error marks
> per notification group.
>
> 2) We would keep attaching event to mark as currently. As far as I have
> checked the event doesn't actually need a back-ref to sb_mark. It is
> really only used for mark reference taking (and then to get to sb from
> fanotify_handle_error_event() but we can certainly get to sb by easier
> means there). So I would just remove that. What we still need to know in
> fanotify_free_error_event() though is whether the sb_mark is still alive or
> not. If it is alive, we leave the event alone, otherwise we need to free it.
> So we need a mark_alive flag in the error event and then do in ->freeing_mark
> callback something like:
>
> 	if (mark->flags & FANOTIFY_MARK_FLAG_SB_MARK) {
> 		struct fanotify_sb_mark *fa_mark = FANOTIFY_SB_MARK(mark);
>
> ###		/* Maybe we could use mark->lock for this? */
> 		spin_lock(&group->notification_lock);
> 		if (fa_mark->fee_slot) {
> 			if (list_empty(&fa_mark->fee_slot->fae.fse.list)) {
> 				kfree(fa_mark->fee_slot);
> 				fa_mark->fee_slot = NULL;
> 			} else {
> 				fa_mark->fee_slot->mark_alive = 0;
> 			}
> 		}
> 		spin_unlock(&group->notification_lock);
> 	}
>
> And then when queueing and dequeueing event we would have to carefully
> check what is the mark & event state under appropriate lock (because
> ->handle_event() callbacks can see marks on the way to be destroyed as they
> are protected just by SRCU).

Thanks for the review.  That is indeed a subtle race that I hadn't
noticed.

Option 2 is much more straightforward.  And considering the uABI won't
be changed if we decide to change to option 1 later, I gave that a try
and should be able to prepare a new version that leaves the error event
with a weak association to the mark, without the back reference, and
allowing it to be deleted by the latest between dequeue and
->freeing_mark, as you suggested.

-- 
Gabriel Krisman Bertazi

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