linux-kernel - Re: [PATCH] Introduce compat_u64 and compat

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Date:	Fri, 15 Jun 2007 14:42:23 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Airlie <airlied@...ux.ie>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Introduce compat_u64 and compat_s64 types

On Friday 15 June 2007, Matthew Wilcox wrote:
> You're relying on compat_[us]64 being only used in structures which are
> already packed.  If someone uses them in a non-packed struct, they won't
> decrease the alignment.  I think it would be more effective to specify
> it as:
> 
> __attribute__((aligned(4), packed))

That's what I thought as well at first, since this is how the gcc 
documentation seems to describe it. However, recent version of gcc
complain about this:

gcc-4.1 -Wall -O2 test.c  -c
test.c:1: warning: 'packed' attribute ignored

I have tested versions 2.95, 3.3 and 4.1, an they all ignore do the
right thing when you do not specify the packed attribute.

> The other problem is that if someone defines a struct like this:
> 
> struct foo {
>         short bar;
>         compat_s64 baz;
> } __attribute__((packed))
> 
> it'll have different definitions on x86 and ia64.
> 
> So I think we should be aiming for the ((aligned, packed)) definition and
> remove the __attribute__((packed)) from the struct definitions.  What do
> you think?

There should never be an __attribute__((packed)) to solve this alignment
problem, neither in the definition of compat_s64 nor in the definition of
a data structure using it.

We might ask the gcc developers to clarify the documentation, which as of 4.1
states:

     The `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

My understanding is that this only applies to statically allocated variables,
but not to automatic stack variables and to usage of the type inside of
a data structure.

	Arnd <><
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