netdev - Re: [PATCH v2 net-next] net: filter: export pkt_type

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Date:	Thu, 04 Sep 2014 16:33:53 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	Denis Kirjanov <kirjanov@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Denis Kirjanov <kda@...ux-powerpc.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper

On Do, 2014-09-04 at 07:32 -0700, Eric Dumazet wrote:
> On Thu, 2014-09-04 at 14:23 +0000, David Laight wrote:
> > From: Eric Dumazet
> > > On Thu, 2014-09-04 at 15:40 +0200, Hannes Frederic Sowa wrote:
> > > 
> > > > The type does not matter at all. Actually I wanted to use an empty
> > > > struct but was afraid it might not work on older compilers and didn't
> > > > want to check that with each version.
> > > 
> > > 
> > > It matters. Really.
> > > 
> > > $ cat try.c
> > > #include <stdio.h>
> > > 
> > > struct S_int {
> > > 	char a;
> > > 	int offset[0];
> > > 	char b:1;
> > > };
> > > 
> > > struct S_char {
> > > 	char a;
> > > 	char offset[0];
> > > 	char b:1;
> > > };
> > 
> > Those are also both illegal C, zero sized arrays aren't allowed.
> > 
> > 	David
> 
> 
> This does not matter, we already use such constructs in the kernel.
> 
> Take a look at kmemcheck_bitfield_begin() for example.

Which btw. also uses int, which might change alignment of structures.


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