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

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Message-ID: <20181107160015.GI27423@dhcp22.suse.cz>
Date:   Wed, 7 Nov 2018 17:00:15 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     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 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:
> > On Mon 05-11-18 13:22:05, Daniel Colascione wrote:
> >> State explicitly that holding a /proc/pid file descriptor open does
> >> not reserve the PID. Also note that in the event of PID reuse, these
> >> open file descriptors refer to the old, now-dead process, and not the
> >> new one that happens to be named the same numeric PID.
> >
> > This sounds quite obvious
> 
> Many people *on* *LKML* were wrong about this behavior. If it's not
> obvious to experienced kernel developers, it's certainly not obvious
> to the public.

Fair enough

> > 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. If you really do care about pid space depletion then you
should use pid cgroup controller.
-- 
Michal Hocko
SUSE Labs