linux-kernel - Re: [PATCH] epoll: Fix user space breakage related to EPOLLWAKEUP (was: Re: [-next regression] TCP window full with EPOLLWAKEUP)

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Date:	Tue, 22 May 2012 17:56:34 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.cz>, NeilBrown <neilb@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: [PATCH] epoll: Fix user space breakage related to EPOLLWAKEUP (was: Re: [-next regression] TCP window full with EPOLLWAKEUP)

On Tuesday, May 22, 2012, Arve Hjønnevåg wrote:
> On Mon, May 21, 2012 at 12:28 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > From: Rafael J. Wysocki <rjw@...k.pl>
> >
> > Commit 4d7e30d (epoll: Add a flag, EPOLLWAKEUP, to prevent
> > suspend while epoll events are ready) caused some applications to
> > malfunction, because they set the bit corresponding to the new
> > EPOLLWAKEUP flag in their eventpoll flags and they don't have the
> > new CAP_EPOLLWAKEUP capability.
> >
> > To prevent that from happening, change epoll_ctl() to clear
> > EPOLLWAKEUP in epds.events if the caller doesn't have the
> > CAP_EPOLLWAKEUP capability instead of failing and returning an
> > error code, which allows the affected applications to function
> > normally.
> >
> > Reported-and-tested-by: Jiri Slaby <jslaby@...e.cz>
> > Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> > ---
> >  fs/eventpoll.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > Index: linux/fs/eventpoll.c
> > ===================================================================
> > --- linux.orig/fs/eventpoll.c
> > +++ linux/fs/eventpoll.c
> > @@ -1711,7 +1711,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in
> >
> >        /* Check if EPOLLWAKEUP is allowed */
> >        if ((epds.events & EPOLLWAKEUP) && !capable(CAP_EPOLLWAKEUP))
> > -               goto error_tgt_fput;
> > +               epds.events &= ~EPOLLWAKEUP;
> >
> >        /*
> >         * We have to check that the file structure underneath the file descriptor
> 
> Is there any way for the application to detect that it did not get the
> EPOLLWAKEUP feature?

Well, it should check its capabilities beforehand if it really cares ...

Moreover, if the creation of the wakeup source in ep_modify() fails, the
application won't be notified either.

Thanks,
Rafael

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