netdev - RE: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get

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Date:	Fri, 10 Oct 2014 14:57:35 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	"'alexander.h.duyck@...hat.com'" <alexander.h.duyck@...hat.com>,
	"David Miller" <davem@...emloft.net>,
	"alexander.duyck@...il.com" <alexander.duyck@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH] flow-dissector: Fix alignment issue in
 __skb_flow_get_ports

From: Alexander Duyck
> On 10/09/2014 09:47 PM, David Miller wrote:
> > From: alexander.duyck@...il.com
> > Date: Thu, 09 Oct 2014 21:03:28 -0700
> >
> >> From: Alexander Duyck <alexander.h.duyck@...hat.com>
> >>
> >> This patch addresses a kernel unaligned access bug seen on a sparc64 system
> >> with an igb adapter.  Specifically the __skb_flow_get_ports was returning a
> >> be32 pointer which was then having the value directly returned.
> >>
> >> In order to keep the handling of the ports consistent with how we were
> >> handling the IPv4 and IPv6 addresses I have instead replaced the assignment
> >> with a memcpy to the flow key ports value.  This way it should stay a
> >> memcpy on systems that cannot handle unaligned access, and will likely be
> >> converted to a 32b assignment on the systems that can support it.
> >>
> >> Reported-by: David S. Miller <davem@...emloft.net>
> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@...hat.com>
> > Guess what the compiler will output for the memcpy()....
> >
> > 	*(u32 *)dest = *(u32 *)src;
> >
> > Using memcpy() is never a valid way to avoid misaligned loads and
> > stores.
> 
> If needed we could convert ports to a __be16 I suppose.

You would have to cast it somewhere where the compiler cannot
tell what the original type was.
This usually means you have to call a non-static function, which
might have to be in a different compilation unit.

...
> That is what I get for trying to come up with a fix at the end of the
> day.  Although it does leave me scratching my head why the IPv4 and IPv6
> addresses were not causing unaligned accesses or were they triggering
> them as well?

I think there is code to copy the IP and TCP headers to aligned memory
before they are parsed.

...
> 
> The problem is the igb / ixgbe / fm10k hardware doesn't have a means of
> inserting padding from its side...

Shoot the hardware engineers.

You aren't going to get the performance you expect from a 10Ge card
unless the rx buffers are 'correctly' aligned.

	David



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