netdev - Re: [RFC][PATCH] ipmr: Fix struct mfcctl to be independent of MAXVIFS v2

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Message-ID: <4BBB67FE.6020209@candelatech.com>
Date:	Tue, 06 Apr 2010 09:57:34 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Patrick McHardy <kaber@...sh.net>,
	Ilia K <mail4ilia@...il.com>, Tom Goff <thomas.goff@...ing.com>
Subject: Re: [RFC][PATCH] ipmr:  Fix struct mfcctl to be independent of MAXVIFS
 v2

On 04/06/2010 08:38 AM, Eric W. Biederman wrote:
>
> Right now if you recompile the kernel increasing MAXVIFS
> to support more VIFS users of the MRT_ADD_VIF and MRT_DEL_VIF
> will break because the ABI changed.
>
> My goal is an API that works with just a recompile of existing
> applications, and an ABI that continues to work for old
> applications.
>
> The unused/dead fields at the end of struct mfcctl make this
> exercise more difficult than it should be.
>
> - Rename the existing struct mfcctl mfcctl_old.
> - Define a new and larger struct mfcctl that we can detect
>    by size.
>
>    The new and larger struct mfcctl won't have trailing garbage
>    fields so we can accept anything of that size or larger,
>    and simply ignore the entries that are above MAXVIFS.
>
> My new struct mfcctl is now 128 bytes which is noticeable on
> the stack but should still be small enough not to cause problems.
>
> v2:  Rework the support larger arrays so that most/all? existing
>     applications can simply be recompiled and work with a larger
>     maximum number of VIFS.

If we're going to change the ABI, can we not support an arbitrary
number of VIFS instead of just a larger fixed maximum?

Thanks,
Ben


-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com

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