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SubjectRe: [PATCH] fanotify: on group destroy allow all waiters to bypass permission check
On Tue, Nov 23, 2010 at 05:13:44PM -0500, Eric Paris wrote:
>
> I'm not sure what you mean by 'processes for which no event has been
> queued.' You must mean a process that is about to send a notify event
> and is about to put itself on the wait queue...
>

Hm, i admit i did not explain very well what i meant.

> In any case I think I described all of the possibilities here:
>
> Lets think about the 4 relevant code paths from the PoV of the
> 'operator' 'listener' 'responder' and 'closer'. Where operator is the
> process doing an action (like open/read) which could require permission.
> Listener is the task (or in this case thread) slated with reading from
> the fanotify file descriptor. The 'responder' is the thread responsible
> for responding to access requests. 'Closer' is the thread attempting to
> close the fanotify file descriptor.
>
> The 'operator' is going to end up in:
> fanotify_handle_event()
> get_response_from_access()
> (THIS BLOCKS WAITING ON USERSPACE)
>
> The 'listener' interesting code path
> fanotify_read()
> copy_event_to_user()
> prepare_for_access_response()
> (THIS CREATES AN fanotify_response_event)
>
> The 'responder' code path:
> fanotify_write()
> process_access_response()
> (REMOVE A fanotify_response_event, SET RESPONSE, WAKE UP 'operator')
>
> The 'closer':
> fanotify_release()
> (SUPPOSED TO CLEAN UP THE REST OF THIS MESS)
>
> What we have today is that in the closer we remove all of the
> fanotify_response_events and set a bit so no more response events are
> ever created in prepare_for_access_response().
>
> The bug is that we never wake all of the operators up and tell them to
> move along.

Right, we did not wake up the operators that generated events which have not
been moved to the access_list yet, but are still on the access_waitq (because
the listener never read these events).

>
> > Beside this it removes the unnecessary check for the bypass_perm flag in
> > prepare_for_access_response(), since this function cant be called any more at
> > the time release() is called and the flag is set.
>
> Which I guess is also correct but I don't like it in the same patch.
> It's dropping dead code rather than fixing this bug. So it's
> distracting to review the patch.

Yes right, i should have split that.

>
> I'm going to split this into two patches, include my analysis in your
> changelog and apply them separately. I hope you don't mind.

Absolutely ok :)



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