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

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Date:	Tue, 06 Apr 2010 10:23:08 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ben Greear <greearb@...delatech.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

Ben Greear <greearb@...delatech.com> writes:

> 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?

The ABI as I have specified should work for any larger structure than
I have specified.  But like select many applications will limit themselves
to use the definition of struct mfcctl that is passed to them.

Eric
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