On Sun, 20 Feb 2022, David Laight wrote:
> From: Paul Blakey
> > Sent: 20 February 2022 13:21
> >
> > Ipv6 ttl, label and tos fields are modified without first
> > pulling/pushing the ipv6 header, which would have updated
> > the hw csum (if available). This might cause csum validation
> > when sending the packet to the stack, as can be seen in
> > the trace below.
> >
> > Fix this by updating skb->csum if available.
> ...
> > +static inline __wsum
> > +csum_block_replace(__wsum csum, __wsum old, __wsum new, int offset)
> > +{
> > + return csum_block_add(csum_block_sub(csum, old, offset), new, offset);
> > +}
>
> That look computationally OTT for sub 32bit adjustments.
>
> It ought to be enough to do:
> return csum_add(old_csum, 0xf0000fff + new - old);
>
> Although it will need 'tweaking' for odd aligned 24bit values.
>
> David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1
> 1PT, UK
> Registration No: 1397386 (Wales)
>
>
I'm not comfortable with manually implementing some low-level
bit fideling checksum calculation, while covering all cases.
And as you said it also doesnt have the offset param (odd aligned).
The same could be said about csum_block_add()/sub(), replace()
just uses them. I think the csum operations are more readable and
should be inlined/optimize, so can we do it the readable way,
and then, if needed, optimize it?
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