linux-kernel - Re: Linux 2.6.35/TIPC 2.0 ABI breaking changes

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Date:	Mon, 18 Oct 2010 19:11:37 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	David Miller <davem@...emloft.net>
CC:	luca@...cax.com.ar, jon.maloy@...csson.com,
	tipc-discussion@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Neil Horman <nhorman@...driver.com>
Subject: Re: Linux 2.6.35/TIPC 2.0 ABI breaking changes

On 10-10-18 06:17 PM, David Miller wrote:
> From: Paul Gortmaker<paul.gortmaker@...driver.com>
> Date: Mon, 18 Oct 2010 16:42:33 -0400
> 
>> If you have access to the user space code in question, you can just
>> switch behaviour semantics based on the results of a uname call, knowing
>> that this change was included in versions since approx last Feb.  There
>> is also /proc/version which can be parsed manually if you prefer.
> 
> Requiring userspace to check kernel versioning information in order
> to user an exported userspace API correctly is _ALWAYS_ _WRONG_.
> 
> You cannot and must not make backwards incompatible changes to
> userspace interfaces.

What I think has happened here (and I'll double check this
tomorrow, since it is before I started assisting with tipc)
is that a backwards incompatible change *did* inadvertently
creep in via these two (related) commits:

--------------
commit d88dca79d3852a3623f606f781e013d61486828a
Author: Neil Horman <nhorman@...driver.com>
Date:   Mon Mar 8 12:20:58 2010 -0800

    tipc: fix endianness on tipc subscriber messages
--------------

and

---------------
commit c6537d6742985da1fbf12ae26cde6a096fd35b5c
Author: Jon Paul Maloy <jon.maloy@...csson.com>
Date:   Tue Apr 6 11:40:52 2010 +0000

    TIPC: Updated topology subscription protocol according to latest spec
---------------

Based on Leandro's info, I think it comes down to userspace
not knowing exactly where to find these bits anymore:

#define TIPC_SUB_SERVICE       0x00    /* Filter for service availability    */
#define TIPC_SUB_PORTS         0x01    /* Filter for port availability  */
#define TIPC_SUB_CANCEL         0x04    /* Cancel a subscription         */

...because it doesn't know if there is the old auto endian
 swap thing being done or not being done.

Assuming it is possible to do so in some non-kludgy way,
it sounds like we want to be looking into an in-kernel change
that ensures the older user space binaries get their
functionality restored then?

Thanks,
Paul.
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