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Date:      Thu, 02 Apr 2015 15:04:29 +0200
From:      Hans Petter Selasky <hps@selasky.org>
To:        Gleb Smirnoff <glebius@FreeBSD.org>, 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:  <551D3E5D.6060606@selasky.org>
In-Reply-To: <20150402123522.GC64665@FreeBSD.org>
References:  <201504012226.t31MQedN044443@svn.freebsd.org> <1427929676.82583.103.camel@freebsd.org> <20150402123522.GC64665@FreeBSD.org>

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On 04/02/15 14:35, Gleb Smirnoff wrote:
> 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
>

Hi,

At least the generation of IP IDs is now in a common place. I think the 
random likelyhood argument is not acceptable. Also there should be a 
critical section around the incrementing of the counter and fetching it. 
What if a different CPU is handling the same IP connection, then ID 
ranges are are likely to repeat.

What I don't understand is in the following link:

https://tools.ietf.org/html/rfc6864

It says:

"is required to be unique within the maximum lifetime for all datagrams 
with a given source address/destination address/protocol tuple."

When you have a IP based socket, why is this counter not part of the IP 
based socket? Why do we have a global counter?

--HPS



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