linux-kernel - Re: [PATCH v2] Document /proc/pid PID reuse behavior

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Date:   Mon, 19 Nov 2018 22:16:09 +1100
From:   Aleksa Sarai <asarai@...e.de>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, rppt@...ux.ibm.com,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior

On 2018-11-07, Daniel Colascione <dancol@...gle.com> wrote:
> On Wed, Nov 7, 2018 at 4:00 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Wed 07-11-18 15:48:20, Daniel Colascione wrote:
> >> On Tue, Nov 6, 2018 at 1:05 PM, Michal Hocko <mhocko@...nel.org> wrote:
> >> > otherwise anybody could simply DoS the system
> >> > by consuming all available pids.
> >>
> >> People can do that today using the instrument of terror widely known
> >> as fork(2). The only thing standing between fork(2) and a full process
> >> table is RLIMIT_NPROC.
> >
> > not really.
> 
> What else, besides memory consumption and (as you mention below)
> cgroups? In practice, nobody uses RLIMIT_NPROC, so outside of various
> container-y namespaced setups, avoidance of
> system-DoS-through-PID-exhaustion isn't a pressing problem.

systemd has had a default pid cgroup controller policy (for both user
and system slices) for a quite long time. I believe that the most recent
version of most enterprise and community distributions support it by
default (and probably even some older versions -- commit 49b786ea146f
was merged in 2015 and I think systemd grew support for it in 2016).

I agree with your overall point, but it should be noted that the vast
majority of Linux systems these days have protections against this (by
default) that use the pids cgroup controller.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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