Date: Thu, 2 Apr 2015 15:51:57 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Gleb Smirnoff <glebius@FreeBSD.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Ian Lepore <ian@freebsd.org> Subject: Re: svn commit: r280971 - in head: contrib/ipfilter/tools share/man/man4 sys/contrib/ipfilter/netinet sys/netinet sys/netipsec sys/netpfil/pf Message-ID: <20150402135157.GB549@dft-labs.eu> In-Reply-To: <20150402134217.GG64665@FreeBSD.org> References: <201504012226.t31MQedN044443@svn.freebsd.org> <1427929676.82583.103.camel@freebsd.org> <20150402123522.GC64665@FreeBSD.org> <20150402133751.GA549@dft-labs.eu> <20150402134217.GG64665@FreeBSD.org>
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On Thu, Apr 02, 2015 at 04:42:17PM +0300, Gleb Smirnoff wrote: > On Thu, Apr 02, 2015 at 03:37:51PM +0200, Mateusz Guzik wrote: > M> On Thu, Apr 02, 2015 at 03:35:22PM +0300, Gleb Smirnoff wrote: > M> > On Wed, Apr 01, 2015 at 05:07:56PM -0600, Ian Lepore wrote: > M> > I> > Author: glebius > M> > I> > Date: Wed Apr 1 22:26:39 2015 > M> > I> > New Revision: 280971 > M> > I> > URL: https://svnweb.freebsd.org/changeset/base/280971 > M> > I> > > M> > I> > Log: > M> > I> > o Use new function ip_fillid() in all places throughout the kernel, > M> > I> > where we want to create a new IP datagram. > M> > I> > o Add support for RFC6864, which allows to set IP ID for atomic IP > M> > I> > datagrams to any value, to improve performance. The behaviour is > M> > I> > controlled by net.inet.ip.rfc6864 sysctl knob, which is enabled by > M> > I> > default. > M> > I> > o In case if we generate IP ID, use counter(9) to improve performance. > M> > I> > o Gather all code related to IP ID into ip_id.c. > M> > I> > > M> > I> > Differential Revision: https://reviews.freebsd.org/D2177 > M> > I> > Reviewed by: adrian, cy, rpaulo > M> > I> > Tested by: Emeric POUPON <emeric.poupon stormshield.eu> > M> > I> > Sponsored by: Netflix > M> > I> > Sponsored by: Nginx, Inc. > M> > I> > Relnotes: yes > M> > I> > > M> > I> [...] > M> > I> > +void > M> > I> > +ip_fillid(struct ip *ip) > M> > I> > +{ > M> > I> > + > M> > I> > + /* > M> > I> > + * Per RFC6864 Section 4 > M> > I> > + * > M> > I> > + * o Atomic datagrams: (DF==1) && (MF==0) && (frag_offset==0) > M> > I> > + * o Non-atomic datagrams: (DF==0) || (MF==1) || (frag_offset>0) > M> > I> > + */ > M> > I> > + if (V_ip_rfc6864 && (ip->ip_off & htons(IP_DF)) == htons(IP_DF)) > M> > I> > + ip->ip_id = 0; > M> > I> > + else if (V_ip_do_randomid) > M> > I> > + ip->ip_id = ip_randomid(); > M> > I> > + else { > M> > I> > + counter_u64_add(V_ip_id, 1); > M> > I> > + ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff); > M> > I> > + } > M> > I> > +} > M> > I> > + > M> > I> > M> > I> This is completely bogus. It's a big opacity violation (it relies on > M> > I> what should be opaque private internal implementation details of > M> > I> counter(9)). The fact that the counter api doesn't provide a function > M> > I> for retrieving one cpu's counter value should be a big clue there -- the > M> > I> fact that you know the internals doesn't make it okay to reach behind > M> > I> the counter and grab a value like that. It may not even be safe to do > M> > I> so on any given architecture; it certainly isn't safe on arm, and that > M> > I> line of code above will work only by accident because you're throwing > M> > I> way all but 16 bits. > M> > > M> > I though about providing that API, but since it isn't safe in general, > M> > I decided to not do that. > M> > > M> > I> But even more importantly, this WILL result in multiple threads using > M> > I> the same value at the same time... > M> > I> > M> > I> - Thread A on CPU 1 and thread B on CPU 2 both begin executing here at > M> > I> the same time, and both get through counter_u64_add(). > M> > I> - Thread A keeps running and uses CPU 1's new value, call it 27. > M> > I> - Thread B gets prempted between counter_u64_add() and zpcpu_get(). > M> > I> When it resumes it's now on CPU 1, so it retrieves value 27 as well. > M> > > M> > This was already discussed in this thread: > M> > > M> > https://lists.freebsd.org/pipermail/svn-src-head/2015-March/069864.html > M> > > M> > M> For this particular use-case you never care what CPU you are executing > M> on, you only want to obtain a unique number. > M> > M> per-cpu counters can serve this purpose no problem, just provide an > M> operation which guarantees to return the new value of the counter it > M> incremented. Should be easily achieved with e.g. just pinning curthread > M> to the cpu it executes on for the duration of inc + fetch. > > I'd ask to pay attention to this particular email: > > https://lists.freebsd.org/pipermail/svn-src-head/2015-March/069966.html > > Just to justify probabilities, risks and countermeasures. > > For those, who don't believe in theory and prefers practice: > > https://lists.freebsd.org/pipermail/svn-src-head/2015-March/070091.html > > Note that Emeric was the one who observed collisions for the ip_id++ > code, that we used before. > Well in that case this at least deserves a comment in the code. Everyone spotting that counter_u64_add + zpcpu_get will think it's a bug. -- Mateusz Guzik <mjguzik gmail.com>
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