Date: Thu, 2 Apr 2015 15:35:22 +0300 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Ian Lepore <ian@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@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: <20150402123522.GC64665@FreeBSD.org> In-Reply-To: <1427929676.82583.103.camel@freebsd.org> References: <201504012226.t31MQedN044443@svn.freebsd.org> <1427929676.82583.103.camel@freebsd.org>
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On Wed, Apr 01, 2015 at 05:07:56PM -0600, Ian Lepore wrote: I> > Author: glebius I> > Date: Wed Apr 1 22:26:39 2015 I> > New Revision: 280971 I> > URL: https://svnweb.freebsd.org/changeset/base/280971 I> > I> > Log: I> > o Use new function ip_fillid() in all places throughout the kernel, I> > where we want to create a new IP datagram. I> > o Add support for RFC6864, which allows to set IP ID for atomic IP I> > datagrams to any value, to improve performance. The behaviour is I> > controlled by net.inet.ip.rfc6864 sysctl knob, which is enabled by I> > default. I> > o In case if we generate IP ID, use counter(9) to improve performance. I> > o Gather all code related to IP ID into ip_id.c. I> > I> > Differential Revision: https://reviews.freebsd.org/D2177 I> > Reviewed by: adrian, cy, rpaulo I> > Tested by: Emeric POUPON <emeric.poupon stormshield.eu> I> > Sponsored by: Netflix I> > Sponsored by: Nginx, Inc. I> > Relnotes: yes I> > I> [...] I> > +void I> > +ip_fillid(struct ip *ip) I> > +{ I> > + I> > + /* I> > + * Per RFC6864 Section 4 I> > + * I> > + * o Atomic datagrams: (DF==1) && (MF==0) && (frag_offset==0) I> > + * o Non-atomic datagrams: (DF==0) || (MF==1) || (frag_offset>0) I> > + */ I> > + if (V_ip_rfc6864 && (ip->ip_off & htons(IP_DF)) == htons(IP_DF)) I> > + ip->ip_id = 0; I> > + else if (V_ip_do_randomid) I> > + ip->ip_id = ip_randomid(); I> > + else { I> > + counter_u64_add(V_ip_id, 1); I> > + ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff); I> > + } I> > +} I> > + I> I> This is completely bogus. It's a big opacity violation (it relies on I> what should be opaque private internal implementation details of I> counter(9)). The fact that the counter api doesn't provide a function I> for retrieving one cpu's counter value should be a big clue there -- the I> fact that you know the internals doesn't make it okay to reach behind I> the counter and grab a value like that. It may not even be safe to do I> so on any given architecture; it certainly isn't safe on arm, and that I> line of code above will work only by accident because you're throwing I> way all but 16 bits. I though about providing that API, but since it isn't safe in general, I decided to not do that. I> But even more importantly, this WILL result in multiple threads using I> the same value at the same time... I> I> - Thread A on CPU 1 and thread B on CPU 2 both begin executing here at I> the same time, and both get through counter_u64_add(). I> - Thread A keeps running and uses CPU 1's new value, call it 27. I> - Thread B gets prempted between counter_u64_add() and zpcpu_get(). I> When it resumes it's now on CPU 1, so it retrieves value 27 as well. This was already discussed in this thread: https://lists.freebsd.org/pipermail/svn-src-head/2015-March/069864.html -- Totus tuus, Glebius.
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