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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Message-ID: <CAMP5XgcOgpRmbVoWvZ0ZqVCKXuyOCrB1yrTUv=qhY1tsg+SUGA@mail.gmail.com>
Date:	Mon, 21 May 2012 15:11:46 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 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?

-- 
Arve Hjønnevåg
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