On 2023-04-12, at 13:06:02 +0200, Pablo Neira Ayuso wrote:
> On Tue, Apr 11, 2023 at 01:21:40PM +0100, Jeremy Sowden wrote:
> > On 2023-03-26, at 22:39:42 +0200, Pablo Neira Ayuso wrote:
> > > Jeremy, may I suggest you pick up on the bitwise _SREG2 support?
> > > I will post a v4 with small updates for ("mark statement support
> > > for non-constant expression") tomorrow. Probably you don't need
> > > the new AND and OR operations for this? Only the a new _SREG2 to
> > > specify that input comes from non-constant?
> >
> > Just to clarify, do you want just the `_SREG2` infrastructure from
> > the last patch series but without the new bitwise ops? That is to
> > say it would be possible to send two operands to the kernel in
> > registers, but no use would be made of it (yet). Or are you
> > proposing to update the existing mask-and-xor ops to send right hand
> > operands via registers?
>
> 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@xxxxxxxxxx/
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
J.
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