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Date: Wed, 29 Jun 2011 15:14:36 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Vasiliy Kulikov <segoon@...nwall.com>
Cc: 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, 22 Jun 2011 19:25:14 +0400
Vasiliy Kulikov <segoon@...nwall.com> wrote:
> This patch adds support for shm_rmid_forced sysctl. If set to 1, all
> shared memory objects in current ipc namespace will be automatically
> forced to use IPC_RMID. POSIX way of handling shmem allows to create
> shm objects and call shmdt() leaving shm object associated with no
> process, thus consuming memory not counted via rlimits. With
> shm_rmid_forced=1 the shared memory object is counted at least for one
> process, so OOM killer may effectively kill the fat process holding
> the shared memory.
>
> It obviously breaks POSIX, some programs relying on the feature would
> stop working. So, set shm_rmid_forced=1 only if you're sure nobody uses
> "orphaned" memory. shm_rmid_forced=0 by default for compatability
> reasons.
>
> The feature was previously impemented in -ow as a configure option.
>
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?
> --- a/include/linux/ipc_namespace.h
> +++ b/include/linux/ipc_namespace.h
> @@ -44,6 +44,7 @@ struct ipc_namespace {
> size_t shm_ctlall;
> int shm_ctlmni;
> int shm_tot;
> + int shm_rmid_forced;
>
> struct notifier_block ipcns_nb;
Please send a patch which adds a nice comment to this field.
Perhaps shm_rmid_forced should have had bool type.
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -74,6 +74,7 @@ void shm_init_ns(struct ipc_namespace *ns)
> ns->shm_ctlmax = SHMMAX;
> ns->shm_ctlall = SHMALL;
> ns->shm_ctlmni = SHMMNI;
> + ns->shm_rmid_forced = 0;
> ns->shm_tot = 0;
> ipc_init_ids(&shm_ids(ns));
> }
The problem is that nobody will test your feature. So for testing
purposes, let's enable the feature by default. I assume this:
--- a/ipc/shm.c~ipc-introduce-shm_rmid_forced-sysctl-ipc-introduce-shm_rmid_forced-sysctl-testing
+++ a/ipc/shm.c
@@ -74,7 +74,7 @@ void shm_init_ns(struct ipc_namespace *n
ns->shm_ctlmax = SHMMAX;
ns->shm_ctlall = SHMALL;
ns->shm_ctlmni = SHMMNI;
- ns->shm_rmid_forced = 0;
+ ns->shm_rmid_forced = 1;
ns->shm_tot = 0;
ipc_init_ids(&shm_ids(ns));
}
will do that?
> +static bool shm_may_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)
> +{
> + return (shp->shm_nattch == 0) &&
> + (ns->shm_rmid_forced ||
> + (shp->shm_perm.mode & SHM_DEST));
> +}
Just because the existing code is crappily documented doesn't mean that
we have to copy that ;)
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