netdev - Re: [PATCH] tcp: assign the sock correctly to an outgoing SYNACK packet

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Date:	Tue, 9 Apr 2013 15:05:42 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Paul Moore <pmoore@...hat.com>
CC:	Casey Schaufler <casey@...aufler-ca.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<mvadkert@...hat.com>, <selinux@...ho.nsa.gov>,
	<linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] tcp: assign the sock correctly to an outgoing SYNACK
 packet

On Tue, 2013-04-09 at 09:19 -0400, Paul Moore wrote:
> On Monday, April 08, 2013 06:24:59 PM Casey Schaufler wrote:
> > On 4/8/2013 6:09 PM, Eric Dumazet wrote:
> > > On Mon, 2013-04-08 at 17:59 -0700, Casey Schaufler wrote:
> > >> I don't see that with adding 4 bytes. Again, I'm willing to be
> > >> educated if I'm wrong.
> > > 
> > > Feel free to add 4 bytes without having the 'align to 8 bytes' problem
> > > on 64 bit arches. Show us your patch.
> > 
> > Recall that it's replacing an existing 4 byte value with an 8 byte value.
> > My compiler days were quite short and long ago, but it would seem that
> > an 8 byte value ought not have an 'align to 8 bytes' problem.
> > 
> > Again, I'm willing to be educated.
> 
> Armed with a cup of coffee I took a look at the sk_buff structure this morning 
> with the pahole tool and using the current sk_buff if we turn on all the 
> #ifdefs here is what I see on x86_64:
> 
> struct sk_buff {
[...]
>         sk_buff_data_t             inner_transport_header; /*   200     8 */
>         sk_buff_data_t             inner_network_header; /*   208     8 */
>         sk_buff_data_t             transport_header;     /*   216     8 */
>         sk_buff_data_t             network_header;       /*   224     8 */
>         sk_buff_data_t             mac_header;           /*   232     8 */
>         sk_buff_data_t             tail;                 /*   240     8 */
>         sk_buff_data_t             end;                  /*   248     8 */
[...]

This is wrong; sk_buff_data_t is always 32-bit:

#if BITS_PER_LONG > 32
#define NET_SKBUFF_DATA_USES_OFFSET 1
#endif

#ifdef NET_SKBUFF_DATA_USES_OFFSET
typedef unsigned int sk_buff_data_t;
#else
typedef unsigned char *sk_buff_data_t;
#endif

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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