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Date:   Thu, 02 Sep 2021 17:24:44 -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

Gabriel Krisman Bertazi <krisman@...labora.com> writes:

> 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.

Actually, I don't think this will work for insertion unless we keep a
bounce buffer for the file_handle, because we need to keep the
group->notification_lock to ensure the fee doesn't go away with the mark
(since it is not yet enqueued) but, as discussed before, we don't want
to hold that lock when generating the FH.

I think the correct way is to have some sort of refcount of the error
event slot.  We could use err_count for that and change the suggestion
above to:

if (mark->flags & FANOTIFY_MARK_FLAG_SB_MARK) {
	struct fanotify_sb_mark *fa_mark = FANOTIFY_SB_MARK(mark);

	spin_lock(&group->notification_lock);
	if (fa_mark->fee_slot) {
		if (!fee->err_count) {
			kfree(fa_mark->fee_slot);
			fa_mark->fee_slot = NULL;
		} else {
			fa_mark->fee_slot->mark_alive = 0;
		}
	}
	spin_unlock(&group->notification_lock);
}

And insertion would look like this:

static int fanotify_handle_error_event(....) {

	spin_lock(&group->notification_lock);

	if (!mark->fee || (mark->fee->err_count++) {
		spin_unlock(&group->notification_lock);
		return 0;
	}

	spin_unlock(&group->notification_lock);

	mark->fee->fae.type = FANOTIFY_EVENT_TYPE_FS_ERROR;

	/* ... Write report data to error event ... */

	fanotify_encode_fh(&fee->object_fh, fanotify_encode_fh_len(inode),
 			   NULL, 0);

	fsnotify_add_event(group, &fee->fae.fse, NULL);
   }

Unless you think this is too hack-ish.

To be fair, I think it is hack-ish.  I would add a proper refcount_t
to the error event, and let the mark own a reference to it, which is
dropped when the mark goes away.  Enqueue and Dequeue will acquire and
drop references, respectively. In this case, err_count is not
overloaded.

Will it work?

-- 
Gabriel Krisman Bertazi

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