netdev - Re: [PATCH 4/11] use ether_addr_equal

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Date:	Tue, 31 Dec 2013 08:27:02 -0800
From:	Ben Greear <greearb@...delatech.com>
To:	Julia Lawall <julia.lawall@...6.fr>
CC:	Joe Perches <joe@...ches.com>,
	Johannes Berg <johannes@...solutions.net>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	kernel-janitors@...r.kernel.org,
	Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
	Intel Linux Wireless <ilw@...ux.intel.com>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/11]  use ether_addr_equal_64bits

On 12/31/2013 08:09 AM, Julia Lawall wrote:
>
>
> On Tue, 31 Dec 2013, Ben Greear wrote:
>
>> On 12/30/2013 10:32 PM, Julia Lawall wrote:
>>>>>>> I'm just thinking of a programmer, e.g. changing a struct like this:
>>>>>>>
>>>>>>>    struct foo {
>>>>>>>      u8 addr[ETH_ALEN];
>>>>>>> -  u16 dummy;
>>>>>>>    };
>>>>
>>>> I don't know of a way to catch that.
>>>> Anyone else?
>>>
>>> Well, one could have a semantic patch that checks for that.  But the
>>> problem is that it is very slow, and it only covers the cases that I can
>>> transform automatically, which currently means no pointers, only explicit
>>> arrays.
>>>
>>> On the other hand, I am finding the structure definition, so I can easily
>>> update the structure definition with an appropriate comment.
>>>
>>> struct foo {
>>>       u8 addr[ETH_ALEN]; /* must be followed by two bytes in the structure */
>>>       u16 dummy;
>>> };
>>>
>>> Unfortunately it is kind of verbose.  Could there be an attribute?  That
>>> could even easily be checked.
>>
>> Can you not just add a build-time macro to check that sizeof(foo) >= 8
>> for each of these struct foos?  Or, is it required that the dummy field
>> be there and be not used by anything else?
>
> It doesn't matter what the field is used for.  The problem is that is it
> necessary to ensure a property of the position of addr within the
> structure.  It has to have at least 16 bytes after it.

You mean 16 bits?

>
> But maybe something with sizeof(foo) and offset_of would do?
>
> Could the macro be put near the declaration of the structure somehow?

I think that would work, but do not know all of the details of such
macros, so it's possible there is some catch.

If nothing else, then some run-time code that calculates the offset off
and asserts if it is broken in module initialization or similar might
be good enough.

Thanks,
Ben

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

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