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Date:   Wed, 21 Nov 2018 18:06:33 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Daniel Colascione <dancol@google.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Primiano Tucci <primiano@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>,
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        David Howells <dhowells@redhat.com>, ktsanaktsidis@zendesk.com,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH] Add /proc/pid_generation
Message-ID: <20181122020633.GN3065@bombadil.infradead.org>
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On Wed, Nov 21, 2018 at 12:38:20PM -0800, Daniel Colascione wrote:
> On Wed, Nov 21, 2018 at 12:31 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote:
> > > This change adds a per-pid-namespace 64-bit generation number,
> > > incremented on PID rollover, and exposes it via a new proc file
> > > /proc/pid_generation. By examining this file before and after /proc
> > > enumeration, user code can detect the potential reuse of a PID and
> > > restart the task enumeration process, repeating until it gets a
> > > coherent snapshot.
> > >
> > > PID rollover ought to be rare, so in practice, scan repetitions will
> > > be rare.
> >
> > Then why does it need to be 64-bit?
> 
> [Resending because of accidental HTML. I really need to switch to a
> better email client.]
> 
> Because 64 bits is enough for anyone. :-) A u64 is big enough that
> we'll never observe an overflow on a running system, and PID
> namespaces are rare enough that we won't miss the four extra bytes we
> use by upgrading from a u32.  And after reading about some security
> problems caused by too-clever handling of 32-bit rollover, I'd rather
> the code be obviously correct than save a trivial amount of space.

I don't think you understand how big 4 billion is.  If it happens once a
second, it will take 136 years for a 2^32 count to roll over.  How often
does a PID roll over happen?