Date: Tue, 31 Dec 2013 17:09:20 +0100 (CET)
From: Julia Lawall <julia.lawall@lip6.fr>
To: Ben Greear <greearb@candelatech.com>
cc: Julia Lawall <julia.lawall@lip6.fr>, Joe Perches <joe@perches.com>,
	Johannes Berg <johannes@sipsolutions.net>,
	Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
	kernel-janitors@vger.kernel.org,
	Emmanuel Grumbach <emmanuel.grumbach@intel.com>,
	Intel Linux Wireless <ilw@linux.intel.com>,
	"John W. Linville" <linville@tuxdriver.com>,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/11]  use ether_addr_equal_64bits
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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.

But maybe something with sizeof(foo) and offset_of would do?

Could the macro be put near the declaration of the structure somehow?

julia