linux-kernel - Re: Change in functionality of futex() system call.

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Message-ID: <BANLkTin-_0CgcykGiQq81fmH_ipa0WNtKA@mail.gmail.com>
Date:	Wed, 8 Jun 2011 11:21:58 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	David Oliver <david@...advisors.com>
Cc:	Kyle Moffett <kyle@...fetthome.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Shawn Bohrer <sbohrer@...advisors.com>,
	Zachary Vonler <zvonler@...advisors.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Change in functionality of futex() system call.

On Wed, Jun 8, 2011 at 11:20 AM, David Oliver <david@...advisors.com> wrote:
>
> Having a new call is inelegant from a futex(2) user perspective, as
> the need for a change is due to the kernel implementation and/or mutex
> requirements. The futex() system call, as documented, is ideal for a
> single producer to signal multiple receivers of state updates.
>
> If it is truly necessary to add new variants to futex() to protect
> applications that allow untrusted applications read access to their
> mutexes, I would avoid both the names suggested, as consumption of
> wakeups is not an obvious issue to users, and POLL suggests waiting
> for multiple entities as in poll(2) (which is not provided), or
> returning immediately (which is orthogonally provided by the timeout
> parameter). What is being provided from the user point of view is a
> FUTEX_WAIT per the man page, which doesn't require write access. How
> about FUTEX_WAIT_RDONLY?

That name sounds good.

>
> Alternatively, use the current call and document that when process
> performing a FUTEX_WAIT on read-only memory are woken, they do not
> count towards the number reported as being woken.

I don't see anything wrong with that, either.

>
> Best, IMHO, would be to document that providing read access to mutexes
> to untrusted software is unsafe behavior, and restore read only access
> to readers of futexes.

I think it should be safe, and it would be easy to make it be safe
(i.e. make no changes at all).

--Andy
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