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From: Amir Goldstein <amir73il@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: Ignore mask handling in fanotify_group_event_mask()
Date: Sat, 23 May 2020 20:14:58 +0300
Message-ID: <CAOQ4uxgBGTAnZUedY3dEwR9V=hdrr_4PH_snj9E=sz-_UuVzTg@mail.gmail.com> (raw)
In-Reply-To: <CAOQ4uxirUfcpOdxFG9TAHUFSz+A5FMJdT=y4UKwpFUVov43nSA@mail.gmail.com>

On Thu, May 21, 2020 at 9:10 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Thu, May 21, 2020 at 7:24 PM Jan Kara <jack@suse.cz> wrote:
> >
> > Hello Amir!
> >
> > I was looking into backporting of commit 55bf882c7f13dd "fanotify: fix
> > merging marks masks with FAN_ONDIR" and realized one oddity in
> > fanotify_group_event_mask(). The thing is: Even if the mark mask is such
> > that current event shouldn't trigger on the mark, we still have to take
> > mark's ignore mask into account.
> >
> > The most realistic example that would demonstrate the issue that comes to my
> > mind is:
> >
> > mount mark watching for FAN_OPEN | FAN_ONDIR.
> > inode mark on a directory with mask == 0 and ignore_mask == FAN_OPEN.
> >
> > I'd expect the group will not get any event for opening the dir but the
> > code in fanotify_group_event_mask() would not prevent event generation. Now
> > as I've tested the event currently actually does not get generated because
> > there is a rough test in send_to_group() that actually finds out that there
> > shouldn't be anything to report and so fanotify handler is actually never
> > called in such case. But I don't think it's good to have an inconsistent
> > test in fanotify_group_event_mask(). What do you think?
> >
>
> I agree this is not perfect.
> I think that moving the marks_ignored_mask line
> To the top of the foreach loop should fix the broken logic.
> It will not make the code any less complicated to follow though.
> Perhaps with a comment along the lines of:
>
>              /* Ignore mask is applied regardless of ISDIR and ON_CHILD flags */
>              marks_ignored_mask |= mark->ignored_mask;
>
> Now is there a real bug here?
> Probably not because send_to_group() always applied an ignore mask
> that is greater or equal to that of fanotify_group_event_mask().
>

That's a wrong statement of course.
We do need to re-apply the ignore mask when narrowing the event mask.

Exposing the bug requires a "compound" event.

The only case of compound event I could think of is this:

mount mark with mask == 0 and ignore_mask == FAN_OPEN. inode mark
on a directory with mask == FAN_EXEC | FAN_EVENT_ON_CHILD.

The event: FAN_OPEN | FAN_EXEC | FAN_EVENT_ON_CHILD
would be reported to group with the FAN_OPEN flag despite the
fact that FAN_OPEN is in ignore mask of mount mark because
the mark iteration loop skips over non-inode marks for events
on child.

I'll try to work that case into the relevant LTP test to prove it and
post a fix.

Thanks,
Amir.

  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-21 16:24 Jan Kara
2020-05-21 18:10 ` Amir Goldstein
2020-05-23 17:14   ` Amir Goldstein [this message]
2020-05-25  7:23     ` Jan Kara
2020-05-25  8:52       ` Amir Goldstein
2020-05-25 12:42         ` Jan Kara

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