linux-kernel - Re: [RFC] ipc: introduce shm_rmid

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Message-ID: <20110702173136.GF26232@openwall.com>
Date:	Sat, 2 Jul 2011 21:31:36 +0400
From:	Solar Designer <solar@...nwall.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vasiliy Kulikov <segoon@...nwall.com>,
	kernel-hardening@...ts.openwall.com,
	Randy Dunlap <rdunlap@...otime.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge E. Hallyn" <serge.hallyn@...onical.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...e.hu>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [RFC] ipc: introduce shm_rmid_forced sysctl

On Wed, Jun 29, 2011 at 03:14:36PM -0700, Andrew Morton wrote:
> What a horrid patch.  But given the POSIX (mis?)feature I don't see a
> better way, and the feature seems desirable.  Sigh.
> 
> What sort of users would want to turn this on, and why?

Originally, I introduced it into Linux 2.0.x-ow to allow for resource
limits to be enforced on shared servers, such as with shared web
hosting.  A user is supposed to be limited by RLIMIT_AS * RLIMIT_NPROC.
(This is awfully inflexible, lacking a separate per-user memory limit,
but at least it's something.)  However, with shared memory segments a
user could bypass that limit, because those segments don't have to be
tied to a process.  So the patch changed that, requiring that any shm
segment be tied to a process, or be destroyed.

Alexander
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