From: Gabriel Krisman Bertazi <krisman@collabora.com>
To: Jan Kara <jack@suse.cz>
Cc: amir73il@gmail.com, jack@suse.com, linux-api@vger.kernel.org,
linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
khazhy@google.com, dhowells@redhat.com, david@fromorbit.com,
tytso@mit.edu, djwong@kernel.org, repnop@google.com,
kernel@collabora.com
Subject: Re: [PATCH v6 15/21] fanotify: Preallocate per superblock mark error event
Date: Fri, 27 Aug 2021 14:18:12 -0400 [thread overview]
Message-ID: <877dg6rbtn.fsf@collabora.com> (raw)
In-Reply-To: <20210816155758.GF30215@quack2.suse.cz> (Jan Kara's message of "Mon, 16 Aug 2021 17:57:58 +0200")
Jan Kara <jack@suse.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@collabora.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
next prev parent reply other threads:[~2021-08-27 18:18 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-12 21:39 [PATCH v6 00/21] File system wide monitoring Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 01/21] fsnotify: Don't insert unmergeable events in hashtable Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 02/21] fanotify: Fold event size calculation to its own function Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 03/21] fanotify: Split fsid check from other fid mode checks Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 04/21] fsnotify: Reserve mark flag bits for backends Gabriel Krisman Bertazi
2021-08-13 7:28 ` Amir Goldstein
2021-08-16 13:15 ` Jan Kara
2021-08-23 14:36 ` Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 05/21] fanotify: Split superblock marks out to a new cache Gabriel Krisman Bertazi
2021-08-16 13:18 ` Jan Kara
2021-08-12 21:39 ` [PATCH v6 06/21] inotify: Don't force FS_IN_IGNORED Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 07/21] fsnotify: Add helper to detect overflow_event Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 08/21] fsnotify: Add wrapper around fsnotify_add_event Gabriel Krisman Bertazi
2021-08-12 21:39 ` [PATCH v6 09/21] fsnotify: Allow events reported with an empty inode Gabriel Krisman Bertazi
2021-08-13 7:58 ` Amir Goldstein
2021-08-25 18:40 ` Gabriel Krisman Bertazi
2021-08-25 19:45 ` Amir Goldstein
2021-08-25 21:50 ` Gabriel Krisman Bertazi
2021-08-26 10:44 ` Amir Goldstein
2021-08-27 2:26 ` Paul Moore
2021-08-12 21:39 ` [PATCH v6 10/21] fsnotify: Support FS_ERROR event type Gabriel Krisman Bertazi
2021-08-13 7:48 ` Amir Goldstein
2021-08-16 13:23 ` Jan Kara
2021-08-12 21:40 ` [PATCH v6 11/21] fanotify: Allow file handle encoding for unhashed events Gabriel Krisman Bertazi
2021-08-13 7:59 ` Amir Goldstein
2021-08-12 21:40 ` [PATCH v6 12/21] fanotify: Encode invalid file handle when no inode is provided Gabriel Krisman Bertazi
2021-08-13 8:27 ` Amir Goldstein
2021-08-16 14:06 ` Jan Kara
2021-08-16 15:54 ` Amir Goldstein
2021-08-16 16:11 ` Jan Kara
2021-08-12 21:40 ` [PATCH v6 13/21] fanotify: Require fid_mode for any non-fd event Gabriel Krisman Bertazi
2021-08-13 8:28 ` Amir Goldstein
2021-08-12 21:40 ` [PATCH v6 14/21] fanotify: Reserve UAPI bits for FAN_FS_ERROR Gabriel Krisman Bertazi
2021-08-13 8:29 ` Amir Goldstein
2021-08-12 21:40 ` [PATCH v6 15/21] fanotify: Preallocate per superblock mark error event Gabriel Krisman Bertazi
2021-08-13 8:40 ` Amir Goldstein
2021-08-16 15:57 ` Jan Kara
2021-08-27 18:18 ` Gabriel Krisman Bertazi [this message]
2021-09-02 21:24 ` Gabriel Krisman Bertazi
2021-09-03 4:16 ` Amir Goldstein
2021-09-15 10:31 ` Jan Kara
2021-08-12 21:40 ` [PATCH v6 16/21] fanotify: Handle FAN_FS_ERROR events Gabriel Krisman Bertazi
2021-08-13 9:35 ` Amir Goldstein
2021-08-12 21:40 ` [PATCH v6 17/21] fanotify: Report fid info for file related file system errors Gabriel Krisman Bertazi
2021-08-13 9:00 ` Amir Goldstein
2021-08-13 9:03 ` Amir Goldstein
2021-08-16 16:18 ` Jan Kara
2021-08-12 21:40 ` [PATCH v6 18/21] fanotify: Emit generic error info type for error event Gabriel Krisman Bertazi
2021-08-13 8:47 ` Amir Goldstein
2021-08-16 16:23 ` Jan Kara
2021-08-16 21:41 ` Darrick J. Wong
2021-08-17 9:05 ` Jan Kara
2021-08-17 10:08 ` Amir Goldstein
2021-08-18 0:16 ` Darrick J. Wong
2021-08-18 3:24 ` Amir Goldstein
2021-08-18 9:58 ` Jan Kara
2021-08-19 3:58 ` Darrick J. Wong
2021-08-18 0:10 ` Darrick J. Wong
2021-08-24 16:53 ` Gabriel Krisman Bertazi
2021-08-25 4:09 ` Darrick J. Wong
2021-08-12 21:40 ` [PATCH v6 19/21] ext4: Send notifications on error Gabriel Krisman Bertazi
2021-08-16 16:26 ` Jan Kara
2021-08-12 21:40 ` [PATCH v6 20/21] samples: Add fs error monitoring example Gabriel Krisman Bertazi
2021-08-18 13:02 ` Jan Kara
2021-08-23 14:49 ` Gabriel Krisman Bertazi
2021-08-12 21:40 ` [PATCH v6 21/21] docs: Document the FAN_FS_ERROR event Gabriel Krisman Bertazi
2021-08-16 16:40 ` Jan Kara
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