Re: [RFC PATCH bpf-next] bpf, tnums: add bitwise-not helper

[Shung-Hsi Yu <shung-hsi.yu@suse.com>]

On Mon, Nov 06, 2023 at 11:56:22AM -0800, Andrii Nakryiko wrote:
> On Sun, Nov 5, 2023 at 6:11 PM Shung-Hsi Yu <shung-hsi.yu@suse.com> wrote:
> > Note: Andrii' patch mentioned in the Link tag isn't merge yet, I'll
> >       resend this along with the proposed refactoring once it is merged.
> >       For now, sending the patch as RFC for feedback and review.
> >
> > While the BPF instruction set does not contain a bitwise-NOT
> > instruction, the verifier may still need to compute the bitwise-NOT
> > result for the value tracked in the register. One such case reference in
> > the link below is
> >
> >         u64 val;
> >         val = reg_const_value(reg2, is_jmp32);
> >         tnum_ops(..., tnum_const(~val);
> >
> > Where the value is extract of out tnum, operated with bitwise-NOT, then
> > simply turned back into tnum again; plus it has the limitation of only
> > working on constant. This commit adds the tnum_not() helper that compute
> > the bitwise-NOT result for all the values tracked within the tnum, that
> > allow us to simplify the above code to
> >
> >         tnum_ops(..., tnum_not(reg2->var_off));
> >
> > without being limited to constant, and is general enough to be reused
> > and composed with other tnum operations.
> >
> > Link: https://lore.kernel.org/bpf/ZUSwQtfjCsKpbWcL@u94a/
> > Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
> > ---

[...]

> > diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> > index 3d7127f439a1..b4f4a4beb0c9 100644
> > --- a/kernel/bpf/tnum.c
> > +++ b/kernel/bpf/tnum.c
> > @@ -111,6 +111,11 @@ struct tnum tnum_xor(struct tnum a, struct tnum b)
> >         return TNUM(v & ~mu, mu);
> >  }
> >
> > +struct tnum tnum_not(struct tnum a)
> > +{
> > +       return TNUM(~a.value & ~a.mask, a.mask);
> > +}
> > +
> 
> In isolation this does look like it's implementing the tnum version of
> ~x, so I have no objections to this. But I'm not sure it actually
> simplifies anything in my patches. But let's see, once it lands,
> please send a follow up applying this tnum_not().

Okay, will send once it lands.

[...]