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>,
"Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
Prashant Dhamdhere <pdhamdhe@redhat.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
rostedt@goodmis.org, tglx@linutronix.de, mingo@kernel.org,
linux@dominikbrodowski.net, pasha.tatashin@oracle.com,
jpoimboe@redhat.com, ard.biesheuvel@linaro.org,
Michal Hocko <mhocko@suse.com>,
David Howells <dhowells@redhat.com>,
ktsanaktsidis@zendesk.com,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH] Add /proc/pid_generation
Date: Wed, 21 Nov 2018 18:06:33 -0800 [thread overview]
Message-ID: <20181122020633.GN3065@bombadil.infradead.org> (raw)
In-Reply-To: <CAKOZueuCYOsaHZzoz5nNPr8Jb-fEiHTR4MektaJx8vuUXcrKCQ@mail.gmail.com>
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?
next prev parent reply other threads:[~2018-11-22 2:06 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-21 20:14 [PATCH] Add /proc/pid_generation Daniel Colascione
2018-11-21 20:31 ` Matthew Wilcox
2018-11-21 20:38 ` Daniel Colascione
2018-11-22 2:06 ` Matthew Wilcox [this message]
2018-11-25 22:55 ` Pavel Machek
2018-11-21 20:54 ` [PATCH v2] Add /proc/pid_gen Daniel Colascione
2018-11-21 22:12 ` Andrew Morton
2018-11-21 22:40 ` Daniel Colascione
2018-11-21 22:48 ` Jann Horn
2018-11-21 22:52 ` Daniel Colascione
2018-11-21 22:50 ` Andrew Morton
2018-11-21 23:21 ` Daniel Colascione
2018-11-21 23:35 ` Andy Lutomirski
2018-11-22 0:21 ` Daniel Colascione
2018-11-22 13:58 ` Cyrill Gorcunov
2018-11-22 0:22 ` Andrew Morton
2018-11-22 0:28 ` Daniel Colascione
2018-11-22 0:30 ` Daniel Colascione
2018-11-22 15:27 ` Mathieu Desnoyers
2018-11-22 0:57 ` Andrew Morton
2018-11-22 1:08 ` Daniel Colascione
2018-11-22 1:29 ` Andrew Morton
2018-11-22 2:35 ` Tim Murray
2018-11-22 5:30 ` Daniel Colascione
2018-11-22 11:19 ` [PATCH] Add /proc/pid_generation Kevin Easton
2018-11-23 11:14 ` David Laight
2018-11-25 23:00 ` Pavel Machek
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