Subject: Re: zd1211rw (2.6.26 sparc64): unaligned access (zd_mac_rx)
From: Johannes Berg <johannes@sipsolutions.net>
To: linux-wireless@vger.kernel.org,
	Shaddy Baddah <shaddy_baddah@hotmail.com>
In-Reply-To: <491653A6.20705@hotmail.com>
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Date: Sun, 09 Nov 2008 09:56:31 +0100
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Hi,


> So, looking into this problem, I was able to come up with a patch=20
> (attached. Note, it also includes remnants of the patch recommended by=20
> Sebastian Andrzej Siewior in=20
> http://marc.info/?l=3Dlinux-wireless&m=3D122493409906326&w=3D2). The prob=
lem=20
> occurs within compare_ether_addr() and this immediately reminded me that=20
> the patches recommended to me way back last year=20
> (http://marc.info/?l=3Dlinux-wireless&m=3D119543627712471&w=3D2) also dea=
lt=20
> with this problem.

I think you forgot to attach the patch.

> Looking into how it was solved then, all the compare_ether_addr() calls=20
> were replaced with memcmp() calls. This is what I have done with my=20
> patch as well... but this approach did not fill me with confidence. It=20
> seemed to me like a quick fix that didn't directly address the alignment=20
> problem. And IMO my subsequent findings confirm this.
>=20
> I now hit the following kernel errors:
>=20
>  > [  240.139093] Kernel unaligned access at TPC[100f7f44]
>  > sta_info_get+0x24/0x68 [mac80211]
>  > [  240.233255] Kernel unaligned access at TPC[100f7f48]
>  > sta_info_get+0x28/0x68 [mac80211]
>  > [  240.328015] Kernel unaligned access at TPC[100f7f50]
>  > sta_info_get+0x30/0x68 [mac80211]
>  > [  240.422771] Kernel unaligned access at TPC[100f7f44]
>  > sta_info_get+0x24/0x68 [mac80211]
>  > [  240.517554] Kernel unaligned access at TPC[100f7f48]
>  > sta_info_get+0x28/0x68 [mac80211]
>=20
> Looking into this, and the underlying problem is again=20
> compare_ether_addr(). Now, I'm sure that replacing this with a memcmp()=20
> is now treading on more toes than just zd1211rw users. And for this=20
> reason, I think a better solution is going to be required.
>=20
> Does that sound right? I will persist in trying to understand the code=20
> to try and come up with a fix. But obviously, I hope that I can get help=20
> with this to make the former an education exercise, not a kernel=20
> maintenance exercise.

:)
Yes, I think you're right, it appears that the 802.11 header isn't
aligned on a 2-byte boundary, which is extremely strange. Can you, with
your patch applied, do something like

	printk(KERN_DEBUG "%p\n", skb->data);

before this code in zd_mac.c:

        memcpy(skb_put(skb, length), buffer, length);

        ieee80211_rx_irqsafe(hw, skb, &stats);


johannes

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