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

From: Michael Gmelin <>
Date: Mon, 02 Aug 2021 15:06:43 UTC

> On 2. Aug 2021, at 15:56, Konstantin Belousov <> wrote:
> 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.

Unfortunately it seems like POSIX shared memory is not linked to the jail it was created in (we discussed this on this list in June and I created a few PRs about that), so per jail rctl rules don’t apply (and limiting uid 0 won’t have the desired effect ^_^).