Re: [PATCH nf-next v4 00/10] netfilter: nft_bitwise: shift support

[Jeremy Sowden <jeremy@azazel.net>]

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On 2020-01-28, at 11:00:35 +0100, Pablo Neira Ayuso wrote:
> On Mon, Jan 27, 2020 at 11:13:14AM +0000, Jeremy Sowden wrote:
> > On 2020-01-26, at 12:12:51 +0100, Pablo Neira Ayuso wrote:
> > > I've been looking into (ab)using bitwise to implement add/sub. I
> > > would like to not add nft_arith for only this, and it seems to me
> > > much of your code can be reused.
> > >
> > > Do you think something like this would work?
> >
> > Absolutely.
> >
> > A couple of questions.  What's the use-case?
>
> inc/dec ip ttl field.

If it's just a simple addition or subtraction on one value, would
this make more sense?

        for (i = 0; i < words; i++) {
	        dst[i] = src[i] + delta;
	        delta = dst[i] < src[i] ? 1 : 0;
        }

> > I find the combination of applying the delta to every u32 and having
> > a carry curious.  Do you want to support bigendian arithmetic (i.e.,
> > carrying to the left) as well?
>
> Userspace should convert to host endianess before doing arithmetics.

Yes, but if the host is bigendian, the least significant bytes will be
on the right, and we need to carry to the left, don't we?

        for (i = words; i > 0; i--) {
	        dst[i - 1] = src[i - 1] + delta;
	        delta = dst[i - 1] < src[i - 1] ? 1 : 0;
        }

J.

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