netdev - Re: [PATCH] ioc3: fix incorrect use of htons/ntohs

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Date:	Mon, 15 Dec 2014 21:09:25 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Ralf Baechle <ralf@...ux-mips.org>
Cc:	Lino Sanfilippo <LinoSanfilippo@....de>, linux-mips@...ux-mips.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ioc3: fix incorrect use of htons/ntohs

On Mon, 2014-12-15 at 19:14 +0100, Ralf Baechle wrote:
> On Mon, Dec 01, 2014 at 04:09:36AM +0000, Ben Hutchings wrote:
> 
> > >  	/* Same as tx - compute csum of pseudo header  */
> > >  	csum = hwsum +
> > > -	       (ih->tot_len - (ih->ihl << 2)) +
> > > -	       htons((uint16_t)ih->protocol) +
> > > +	       (ih->tot_len - (ih->ihl << 2)) + ih->protocol +
> > >  	       (ih->saddr >> 16) + (ih->saddr & 0xffff) +
> > >  	       (ih->daddr >> 16) + (ih->daddr & 0xffff);
> > >
> > 
> > The pseudo-header is specified as:
> > 
> >                      +--------+--------+--------+--------+
> >                      |           Source Address          |
> >                      +--------+--------+--------+--------+
> >                      |         Destination Address       |
> >                      +--------+--------+--------+--------+
> >                      |  zero  |  PTCL  |    TCP Length   |
> >                      +--------+--------+--------+--------+
> > 
> > The current code zero-extends the protocol number to produce the 5th
> > 16-bit word of the pseudo-header, then uses htons() to put it in
> > big-endian order, consistent with the other fields.  (Yes, it's doing
> > addition on big-endian words; this works even on little-endian machines
> > due to the way the checksum is specified.)
> > 
> > The driver should not be doing this at all, though.  It should set
> > skb->csum = hwsum; skb->ip_summed = CHECKSUM_COMPLETE; and let the
> > network stack adjust the hardware checksum.
> 
> Really?  The IOC3 isn't the exactly the smartest NIC around; it does add up
> everything and the kitchen sink, that is ethernet headers, IP headers and
> on RX the frame's trailing CRC.

That is almost exactly what CHECKSUM_COMPLETE means on receive; only the
CRC would need to be subtracted.  Then the driver can validate
{TCP,UDP}/IPv{4,6} checksums without any header parsing.

> All that needs to be subtracted in software
> which is what this does.  I think others NICs are all smarted and don't
> need this particular piece of magic.

It may not be smart, but that allows it to cover more cases than most
smart network controllers!

On transmit, the driver should:
- Calculate the partial checksum of data up to offset csum_start
- Subtract this from the checksum at offset (csum_start + csum_offset)
It should set the NETIF_F_GEN_CSUM feature flag rather than
NETIF_F_IP_CSUM.  Then it will be able to generate {TCP,UDP}/IPv{4,6}
checksums.

Ben.

> I agree with your other comment wrt. to htons().


-- 
Ben Hutchings
The two most common things in the universe are hydrogen and stupidity.

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