Linux-Fsdevel Archive on
 help / color / Atom feed
From: Amir Goldstein <>
To: Jan Kara <>
Cc: linux-fsdevel <>
Subject: Re: Ignore mask handling in fanotify_group_event_mask()
Date: Sat, 23 May 2020 20:14:58 +0300
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, May 21, 2020 at 9:10 PM Amir Goldstein <> wrote:
> On Thu, May 21, 2020 at 7:24 PM Jan Kara <> 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.

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.


  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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='' \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Fsdevel Archive on

Archives are clonable:
	git clone --mirror linux-fsdevel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fsdevel linux-fsdevel/ \
	public-inbox-index linux-fsdevel

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone