[PATCH] net: fix checksum verification

[Baruch Siach]

Hi Sascha,

On Fri, Aug 09, 2013 at 09:32:37AM +0200, Sascha Hauer wrote:
> On Fri, Aug 09, 2013 at 07:01:24AM +0300, Baruch Siach wrote:
> > Checksum verification on data including its own checksum (as is the case with
> > IP headers) should give zero. Current code works well for the correct checksum
> > case, but fails to identify (most) errors.
> > 
> > Signed-off-by: Baruch Siach <baruch at tkos.co.il>
> > ---
> > 
> > Untested. From code inspection only.
> > 
> >  net/net.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/net.c b/net/net.c
> > index 0bd9084..bd7a578 100644
> > --- a/net/net.c
> > +++ b/net/net.c
> > @@ -41,7 +41,7 @@ static unsigned int net_ip_id;
> >  
> >  int net_checksum_ok(unsigned char *ptr, int len)
> >  {
> > -	return net_checksum(ptr, len) + 1;
> > +	return net_checksum(ptr, len) == 0;
> 
> 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;

You're right, of course. I was just blindly copying the kernel that has this 
in ip_rcv():

    if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
        goto csum_error;

But I still don't understand the logic here.

baruch

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