Re: [PATCH] net: fix checksum verification

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Aug 09, 2013 at 10:50:19AM +0200, Uwe Kleine-König wrote:
> > D'oh. There's a bug indeed. For a good packet net_checksum returns
> > 0xffff (all ones in an u16). So the check should be:
> > 
> > 	return net_checksum(ptr, len) == 0xffff;
> with
> 
> 	return net_checksum(ptr, len) + 1
> 
> net_checksum_ok returns always something >0 (i.e. success) because both
> summands are converted to unsigned, and so never catches an error[1],
> does it?
> 
> > U-Boot has this instead:
> > 
> > 	return !((net_checksum(ptr, len) + 1) & 0xfffe);
> > 
> > From what I see both above should be equivalent so I wonder why U-Boot
> > has such a complicated code here. Some compiler optimization or is this
> > something I don't see?
> This isn't equivalent. The U-Boot code returns 1 iff net_checksum
> returns 0 or 0xffff; 0 otherwise.

Hm, indeed. Which brings me to my next question: In which cases does
net_checksum() return 0 and the result can be considered ok?

Sascha

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