linux-kernel - Re: Linux 2.6.35/TIPC 2.0 ABI breaking changes [was: TIPC 2.0 and TIPC_SUB

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Date:	Mon, 18 Oct 2010 16:42:33 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Leandro Lucarella <luca@...cax.com.ar>
Cc:	Jon Maloy <jon.maloy@...csson.com>, davem@...hat.com,
	"tipc-discussion@...ts.sourceforge.net" 
	<tipc-discussion@...ts.sourceforge.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: Linux 2.6.35/TIPC 2.0 ABI breaking changes [was: TIPC 2.0 and TIPC_SUB_SERVICE]

On Mon, Oct 18, 2010 at 2:46 PM, Leandro Lucarella <luca@...cax.com.ar> wrote:
> Leandro Lucarella, el 18 de octubre a las 12:04 me escribiste:
>> This means D can't be used to write code that uses TIPC if you target
>> kernels >= 2.6.35 (unless they write their own bindings, or at least use
>> a custom TIPC_SUB_SERVICE constant). I could change the TIPC_SUB_SERVICE
>> value, but then people using older kernels will be screwed. Another
>> option is to use the version statement (kind of like C's #ifdef):
>>
>>       version (TIPC_2_0)
>>               const TIPC_SUB_SERVICE = 0x00;
>>       else
>>               const TIPC_SUB_SERVICE = 0x02;
>
> I keep finding problems with this change. Another, no D-related, problem
> with this change is old code could be silently (and very subtly) broken
> if they check for TIPC_SUB_SERVICE as a flag with something like:
>
>        if (s.filter & TIPC_SUB_SERVICE)
>                /* do something */
>
> Because TIPC_SUB_SERVICE has changed its semantics, not just its value,
> and the new value (0x00) will give you always 0 in that test. This one
> is really tricky, because the application code will fail silently, there
> will be no dmesg indication of a failure, nor the connection to the
> topology service be closed. You might get a compiler warning if you're
> lucky.

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.

If you have kernel code/modules, you can do the same/similar things with
the KERNEL_VERSION and its support macros.

In either case, you don't have to restrict yourself to information that is
specific to and/or exported just from TIPC itself.

Hope that helps,
Paul.

>
> --
> Leandro Lucarella (AKA luca)                     http://llucax.com.ar/
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