From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Subject: Re: [PATCH] Add /proc/pid_generation Date: Wed, 21 Nov 2018 18:06:33 -0800 Message-ID: <20181122020633.GN3065@bombadil.infradead.org> References: <20181121201452.77173-1-dancol@google.com> <20181121203150.GK3065@bombadil.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Daniel Colascione Cc: linux-kernel , Linux API , Tim Murray , Primiano Tucci , Joel Fernandes , Jonathan Corbet , Andrew Morton , Mike Rapoport , Roman Gushchin , Vlastimil Babka , "Dennis Zhou (Facebook)" , Prashant Dhamdhere , "Eric W. Biederman" , 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 , David Howells , ktsanaktsidis@zendesk.com List-Id: linux-api@vger.kernel.org On Wed, Nov 21, 2018 at 12:38:20PM -0800, Daniel Colascione wrote: > On Wed, Nov 21, 2018 at 12:31 PM Matthew Wilcox 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?