netdev - Re: Linux 2.6.35/TIPC 2.0 ABI breaking changes

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Message-ID: <20101020175732.GB14407@hmsreliant.think-freely.org>
Date:	Wed, 20 Oct 2010 13:57:32 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Leandro Lucarella <luca@...cax.com.ar>
Cc:	David Miller <davem@...emloft.net>, paul.gortmaker@...driver.com,
	jon.maloy@...csson.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, tipc-discussion@...ts.sourceforge.net
Subject: Re: Linux 2.6.35/TIPC 2.0 ABI breaking changes

On Wed, Oct 20, 2010 at 02:20:18PM -0300, Leandro Lucarella wrote:
> Neil Horman, el 19 de octubre a las 16:18 me escribiste:
> > 	Heres what I have so far.  Dave as a heads up please don't apply this
> > yet.  I'd like to go over it a bit more and be sure of the implications here
> > before I post it for inclusion officially.  I wanted Leandro to have a copy
> > though so he could confirm functionality for us.  Leandro, This patch lets me
> > pass the tipc test code for TIPC 1.6 that you posted earlier this morning.  If
> > you could confirm that it works for you that would be grand.  While your doing
> > that, I want to read over the spec for TIPC and make sure that I'm not breaking
> > anything new with this patch.
> 
> I tried the patch (swapping the values of TIPC_SUB_SERVICE and
> TIPC_SUB_PORTS) based on 2.6.35.4 and it didn't worked. dmesg sais:
> NOT Swapping endianess in subscr_subscribe
> NOT Swapping endianess in subscr_subscribe
> TIPC: Subscription rejected, illegal request
thats odd, it worked fine for me.  I wonder If I had an older tipc.h header that
set the flags properly in user space.

> 
> I tried with a binary compiled with an older tipc.h header, I didn't
> tried to recompile it using the new tipc.h (on purpose as the thing that
> should be fixed is backwards compatibility).
> 
> I've read the TIPC 2.0 specification[1] a little more, and as I see, the
> subscription messages are not supposed to go through the wire[2].
> 
> 	8.  Topology Service
> 
> 	TIPC provides a message-based mechanism for an application to
> 	learn about the port names that are visible to its node. This is
> 	achieved by communicating with a Topology Service that has
> 	knowledge of the contents of the node's name table.
> 
> So, if the idea is to comply with TIPC 2.0, the topology service should
> accept the new TIPC_SUB_SERVICE and TIPC_SUB_PORTS values (0 and
> 1 in NBO respectively), and all the fields in the subscr struct should
> be filled in NBO too.
> 
> However, if the idea is to keep backwards compatibility too, HBO should
> be accepted as well as the old TIPC_SUB_SERVICE and TIPC_SUB_PORTS
> values (2 and 1 in HBO respectively).
> 
> The real problem is, we can't figure out the endianess of the subscr
> struct because 0x0 is a valid filter in TIPC 2.0.
> 
> The only solution I see is to change the TIPC 2.0 specification (which
> is a "work-in-progress") to make the topology service use the port name
> {2,2}, leaving {1,1} for backwards compatibility. Then add the constants
> TIPC_SUB_SERVICE2 (0) and TIPC_TOP_SRV2 (2), or similar, to use the TIPC
> 2.0 interface and leave TIPC_SUB_SERVICE and TIPC_TOP_SRV for the TIPC
> 1.x interface.
> 
> Another option is to change the TIPC 2.0 specification to use the old
> format (use HBO in subscriptions and keep TIPC_SUB_SERVICE as a separate
> flag with value 2) and forget about all this. After all, I can't see
> what advantages gives having to change the BO for internal messages
> between the applications and the stack.
> 
> [1] http://tipc.sourceforge.net/doc/draft-spec-tipc-06.html
> [2] http://tipc.sourceforge.net/doc/draft-spec-tipc-06.html#anchor92
> 
Ugh, the tipc 'spec' is just a mess (note section 2.4.2, 8.2.1, etc  also indicates all
mesages should be in network byte order)

What we should probably do is, for the time being, just revert my endian swap
commit, plus pauls bit field change to get us back to a point where we're no
longer breaking user space.  Then we can take our time to find a way to conform
to the spec in a backwards compatible manner.  I'll send patches to do that
shortly.

Neil

> -- 
> Leandro Lucarella (AKA luca)                     http://llucax.com.ar/
> ----------------------------------------------------------------------
> GPG Key: 5F5A8D05 (F8CD F9A7 BF00 5431 4145  104C 949E BFB6 5F5A 8D05)
> ----------------------------------------------------------------------
> CARANCHO OBNUBILADO APARECE EN PARQUE CHACABUCO!
> 	-- Crónica TV
> 
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