linux-kernel - Re: [PATCH linux-next] net: batman-adv: use "__packed __aligned(2)" for each structure instead of "_

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Date:	Sun, 19 Jan 2014 01:10:39 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	Antonio Quartulli <antonio@...hcoding.com>
Cc:	Chen Gang <gang.chen.5i5j@...il.com>,
	David Miller <davem@...emloft.net>, mareklindner@...mailbox.ch,
	sw@...onwunderlich.de, b.a.t.m.a.n@...ts.open-mesh.org,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-metag@...r.kernel.org
Subject: Re: [PATCH linux-next] net: batman-adv: use "__packed __aligned(2)" for each structure instead of "__packed(2)" region

On Saturday 18 January 2014 14:03:26 Antonio Quartulli wrote:
> On 18/01/14 12:31, Chen Gang wrote:
> > Unfortunately, not all compilers assumes the structures within a pack
> > region also need be packed (e.g. metag), so need add a pack explicitly
> > to satisfy all compilers.
> > 
> > The related error (under metag with allmodconfig):
> >     MODPOST 2952 modules
> >   
> >   ERROR: "__compiletime_assert_431" [net/batman-adv/batman-adv.ko]
> >   undefined!
> >   ERROR: "__compiletime_assert_432" [net/batman-adv/batman-adv.ko]
> >   undefined!
> >   ERROR: "__compiletime_assert_429" [net/batman-adv/batman-adv.ko]
> >   undefined!
> >   ERROR: "__compiletime_assert_428" [net/batman-adv/batman-adv.ko]
> >   undefined!
> >   ERROR: "__compiletime_assert_423" [net/batman-adv/batman-adv.ko]
> >   undefined!
> > 
> > Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
> 
> David, what do you think about this change?
> 
> 
> Can "__packed __aligned(2)" generate a different structure padding than
> "#pragma pack(2)" ?
> 
> I am not really sure about the difference between the two. But if we
> have the possibility that the padding may change then this patch should
> go into net, otherwise we will have a protocol compatibility problem
> between 3.13 and 3.14.

(Chen: sorry I didn't twig what you were referring to before about #pragma 
pack not working)

It appears that the following gcc patch adds support for #pragma pack:
http://gcc.gnu.org/ml/gcc-patches/2006-10/msg01115.html

I gave it a quick spin on metag gcc (which is unfortunately stuck on an old 
version) and it seems to fix my simple test case so that #pragma pack(2) 
becomes equivalent to __packed __aligned(2) (for sizeof and __alignof__).


However, the __packed and __aligned are linux specific macros to abstract 
compiler details, whereas #pragma pack appears to be a compiler-specific WIN32 
style equivalent to GCC's __attribute__((packed)) and 
__attribute__((aligned(2))) (these are what __packed and __aligned use in 
compiler-gcc.h).

Therefore I believe using the Linux abstractions is still more correct here.

Cheers
James

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