Re: POSIX shared memory, jails, and (lack of) limits

From: Konstantin Belousov <>
Date: Mon, 02 Aug 2021 13:55:48 UTC
On Mon, Aug 02, 2021 at 02:19:00PM +0200, Michael Gmelin wrote:
> Hi,
> I've been playing a bit with POSIX shared memory and, unlike for SysV
> shared memory, I couldn't find any way to limit its use by jails.
> First, I looked at racct/rctl, but there is no resource for POSIX shared
> memory and memoryuse/vmemoryuse don't seem to have an effect (which
> makes sense).
> Then I checked if there are jail parameters that could help, but there
> doesn't seem to be anything like "allow.sysvshm" for POSIX shared
> memory to limit access to the feature.
> So, unless I'm missing something, it seems like all jails on a system
> have unlimited access to POSIX shared memory and therefore any single
> jail can use up the jailhost's virtual memory until the jailhost comes
> to a grinding halt.
> I wrote a little test program that keeps allocating POSIX shared memory
> inside of a jail and it can easily bring the host down to its knees:
>   login: Aug  2 12:12:09 test kernel: pid 11825 (getty), jid 0, uid 0,
>   was killed: out of swap space
>   Aug  2 12:12:10 test init[11827]: getty repeating too quickly on port
>   /dev/ttyu0, sleeping 30 secs
>   Aug  2 12:12:10 test kernel: pid 11826 (getty), jid 0, uid 0, was
>   killed: out of swap space

Posix shm is limited by the swap accounting.  For non-jail consumers,
it is per-uid RLIMIT_SWAP.  I do not know if other mechanisms make
RLIMIT_SWAP per-jail per-uid.