From: Sascha Hauer <s.hauer@pengutronix.de>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: barebox@lists.infradead.org
Subject: Re: [PATCH] net: fix checksum verification
Date: Fri, 9 Aug 2013 10:57:35 +0200 [thread overview]
Message-ID: <20130809085735.GD26614@pengutronix.de> (raw)
In-Reply-To: <20130809085019.GT6104@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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next prev parent reply other threads:[~2013-08-09 8:57 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-09 4:01 Baruch Siach
2013-08-09 7:32 ` Sascha Hauer
2013-08-09 8:50 ` Uwe Kleine-König
2013-08-09 8:57 ` Sascha Hauer [this message]
2013-08-09 10:16 ` Baruch Siach
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