From: Michael Buesch <mb@bu3sch.de>
To: bcm43xx-dev@lists.berlios.de
Subject: Re: [PATCH] b43: Fix possible unaligned u32 access
Date: Fri, 5 Jun 2009 21:03:41 +0200
Cc: "John W. Linville" <linville@tuxdriver.com>,
	linux-wireless@vger.kernel.org,
	Matthieu CASTET <castet.matthieu@free.fr>
References: <200906042318.33895.mb@bu3sch.de> <200906051929.07714.mb@bu3sch.de> <20090605185047.GA7361@tuxdriver.com>
In-Reply-To: <20090605185047.GA7361@tuxdriver.com>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Message-Id: <200906052103.41381.mb@bu3sch.de>
Sender: linux-wireless-owner@vger.kernel.org

On Friday 05 June 2009 20:50:48 John W. Linville wrote:
> On Fri, Jun 05, 2009 at 07:29:07PM +0200, Michael Buesch wrote:
> > On Friday 05 June 2009 14:25:03 John W. Linville wrote:
> > > On Thu, Jun 04, 2009 at 11:18:33PM +0200, Michael Buesch wrote:
> > > > From: Matthieu CASTET <castet.matthieu@free.fr>
> > > > 
> > > > Fix possible unaligned u32 access in b43_generate_plcp_hdr().
> > > > Unaligned data is read/write with a u32 pointer instead of using the
> > > > packed structure. Some versions of gcc ignore the "packed" attribute, if the
> > > > structure element is accessed through a local pointer.
> > > > 
> > > > Signed-off-by: Matthieu CASTET <castet.matthieu@free.fr>
> > > > Signed-off-by: Michael Buesch <mb@bu3sch.de>
> > > 
> > > That seems pretty brain-dead...can you cite a source for this
> > > information?  The patch seems like a no-op...
> > > 
> > > John
> > 
> > struct foo {
> > 	int data;
> > } __attribute__((packed));
> > 
> > struct foo foo;
> > int *d = &foo->data;
> > foo->data = x;	/* Works for unaligned */
> > *d = y;		/* Does not work for unaligned */
> 
> Why not?
> 

Because some compilers don't carry the "packed" attribute of "data" though the "d" pointer.
So foo->data is a "packed" access, while *d might not.

-- 
Greetings, Michael.