Re: [PATCH nftables 8/8] test: py: add tests for shifted nat port-ranges

[Jeremy Sowden <jeremy@azazel.net>]

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On 2023-05-03, at 22:54:11 +0200, Pablo Neira Ayuso wrote:
> On Tue, Apr 25, 2023 at 08:51:43PM +0100, Jeremy Sowden wrote:
> > On 2023-04-12, at 13:06:02 +0200, Pablo Neira Ayuso wrote:
> > > I mean, would it be possible to add a NFT_BITWISE_BOOL variant that
> > > takes _SREG2 via select_ops?
> > 
> > In an earlier version, instead of adding new boolean ops, I added
> > support for passing the mask and xor arguments in registers:
> > 
> >   https://lore.kernel.org/netfilter-devel/20200224124931.512416-1-jeremy@azazel.net/
> > 
> > Doing the same thing with one extra register is straightforward for AND
> > and XOR:
> > 
> >   AND(x, y) = (x & y) ^ 0
> >   XOR(x, y) = (x & 1) ^ y
> > 
> > since we can pass y in _SREG2 and 0 in _XOR for AND, and 1 in _MASK and
> > y in _SREG2 for XOR.  For OR:
> > 
> >   OR(x, y) = (x & ~y) ^ y
> > 
> > it's a bit more complicated.  Instead of getting both the mask and xor
> > arguments from user space, we need to do something like passing y in
> > _SREG2 alone, and then constructing the bitwise negation in the kernel.
> >
> > Obviously, this means that the kernel is no longer completely agnostic
> > about the sorts of mask-and-xor expressions user space may send.
> >
> > Since that is the case, we could go further and just perform the
> > original ope- rations.  Thus if we get an boolean op with an _SREG2
> > argument:
> > 
> >   * if there is an _XOR of 0, compute:
> > 
> >     _SREG & _SREG2
> > 
> >   * if there is a _MASK of 1, compute:
> > 
> >     _SREG ^ _SREG2
> > 
> >   * if there are no _MASK or _XOR arguments, compute:
> > 
> >     _SREG | _SREG2
> 
> OK, if my understanding is correct, these are the two options:
> 
> 1) Infer from arguments the type of operation.
> 2) Have explicit NFT_BITWISE_{AND,OR,XOR} operations.
> 
> If so, I think it is better to stick to your original patch, where
> explicit bitwise operations NFT_BITWISE_{_AND,_OR,_XOR} are added
> (which is what you proposed last time IIRC).
> 
> Thanks for explaining.

No problem.  I'll get rebasing.

J.

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