netdev - Re: alignment faults in 3.6

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Date:	Thu, 11 Oct 2012 12:49:34 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Måns Rullgård <mans@...sr.com>,
	Jon Masters <jonathan@...masters.org>, netdev@...r.kernel.org,
	David Laight <David.Laight@...LAB.COM>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: alignment faults in 3.6

On Thu, 2012-10-11 at 11:32 +0100, Russell King - ARM Linux wrote:
> On Thu, Oct 11, 2012 at 12:22:06PM +0200, Eric Dumazet wrote:
> > I took a look, and I dont see why/how gcc could use a ldm instruction
> > 
> > Doing so assumed the alignment of the structure was 8 bytes, but its
> > not.
> > 
> > Networking stack mandates that IP headers are aligned on 4 bytes
> > boundaries, not 8 bytes.
> 
> Err, no.  ldm is "load multiple" not "load double".  It loads multiple
> 32-bit registers, and its requirement for non-faulting behaviour is for
> the pointer to be 4 byte aligned.  However, "load double" requires 8
> byte alignment.

So if you have an alignment fault, thats because IP header is not
aligned on 4 bytes ?

If so a driver is buggy and must be fixed.

Please send us full stack trace



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