netdev - Re: sparse vs. skbuff.h

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Date:	Mon, 16 Nov 2009 21:23:59 +0100
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Johannes Berg <johannes@...solutions.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: sparse vs. skbuff.h

2009/11/16 Eric Dumazet <eric.dumazet@...il.com>:
> Johannes Berg a écrit :
>> On Mon, 2009-11-16 at 20:21 +0100, Johannes Berg wrote:
>>> commit 14d18a81b5171d4433e41129619c75748b4f4d26
>>> Author: Eric Dumazet <eric.dumazet@...il.com>
>>> Date:   Thu Oct 29 00:10:37 2009 +0000
>>>
>>>     net: fix kmemcheck annotations
>>>
>>>
>>> broke sparse endian checks on everything that includes skbuff.h because
>>> the first and only (because it's an error) thing sparse now reports is
>>> this:
>>>
>>> include/linux/skbuff.h:357:41: error: invalid bitfield specifier for type restricted __be16.
>>
>> Simply changing from
>>       __be16 protocol:16;
>> to
>>       __be16 protocol;
>>
>> but leaving it inside the kmemcheck annotation seems to do the right
>> thing. Except of course that kmemcheck will not properly check it now.
>> Maybe those annotations should simply be made to have no impact on
>> struct padding instead?
>>
>
> Hmm, I have really no idea of what is the right way to fix this stuff.
>
> Last time I did adding a non bitfield element inside the begin/end annotations,
> I was flamed, because a bitfield is a bitfield, not a char/short
>
> http://www.spinics.net/lists/netdev/msg108803.html
>
> Now sparse is complaining... What will be the next story ?

If by "I was flamed" you are referring to my reply:

http://www.spinics.net/lists/netdev/msg108825.html

then I am really sorry, because I had no intentions to insult you. In
fact, I am grateful that you are finding bugs and telling me about
them But I should also be allowed to disagree with a patch if I truly
believe it is the wrong thing to do.

For the issue in question: If the variable is turned into a
non-bitfield (as Johannes suggested), it would be fine, because now
GCC won't emit masking operations (AND, OR) when initializing it, but
a regular MOV. Also, the struct annotations do not by themselves do
anything, but they are used by kmemcheck_annotate_bitfield().

In other words, I think the right thing to do is to turn it into a
non-bitfield and move it _outside_ the bitfield annotation. Johannes,
can you make the patch and let us have a look? In the meantime I will
submit the patch that fixes the extraneous field padding in
KMEMCHECK=n kernels.

Vegard
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