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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1307418562.2642.51.camel@edumazet-laptop>
Date:	Tue, 07 Jun 2011 05:49:22 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	David Oliver <david@...advisors.com>,
	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.

Le lundi 06 juin 2011 à 20:13 -0700, Darren Hart a écrit :
> 
> On 06/06/2011 11:11 AM, Eric Dumazet wrote:
> > Le lundi 06 juin 2011 à 10:53 -0700, Darren Hart a écrit :
> >>
> > 
> >> If I understand the problem correctly, RO private mapping really doesn't
> >> make any sense and we should probably explicitly not support it, while
> >> special casing the RO shared mapping in support of David's scenario.
> >>
> > 
> > We supported them in 2.6.18 kernels, apparently. This might sounds
> > stupid but who knows ?
> 
> 
> I guess this is actually the key point we need to agree on to provide a
> solution. This particular case "worked" in 2.6.18 kernels, but that
> doesn't necessarily mean it was supported, or even intentional.
> 
> It sounds to me that we agree that we should support RO shared mappings.
> The question remains about whether we should introduce deliberate
> support of RO private mappings, and if so, if the forced COW approach is
> appropriate or not.
> 
> Does anyone with a longer history working with futexes than I have an
> opinion on this? Is support for RO private mappings part of our futex
> API, or was it an unintentional side effect of the futex simply being a
> userspace address.
> 

I personnally dont care as I dont use ro mappings for my futexes land,
but I can feel the pain of people discovering yet another
incompatibility in their user apps after a kernel upgrade, spending so
much time to find the root of the problem (hey, not everybody is a
kernel hacker)

If we think about it, futex_wait() should not touch memory, only read
it. Some smart layer could be upset by this (valgrind ?)

Its like saying write(int fd, const void *buffer, size_t count) could
try to do a COW on buffer, because it makes kernel programmer life more
comfortable, this makes litle sense to me IMHO.

Part of the problem comes from futex() syscall being a multiplexor.
What a mess.

If we had a clean API at the beginning, then we would have :

int sys_futex_wait(const void *futex, int val, const struct timespec *t);

And really, doing COW in futex_wait() would clearly be wrong.



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/