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From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 21 Nov 2018 12:38:20 -0800
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Subject: Re: [PATCH] Add /proc/pid_generation
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
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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.