Re: [PATCH bpf-next v2 03/13] bpf: track low-32 scalar equality across zero-extending movs

[Alexei Starovoitov]

On Mon Sep 21, 2026 at 7:44 PM UTC, Eduard Zingerman wrote:
> > > > > 
> > > > > Where:
> > > > > - id == 0 => no id link
> > > > > - full => all 64-bits of the register are identical to
> > > > >           all 64-bits of a scalar value `id' (let's call it X).
> > > > >           ∀ rA{.id == X, .link == full}, rB{X,full} => rA == rB
> > > > > - zext => lower 32-bits of the register are identical to
> > > > >           lower 32-bits of a scalar value X,
> > > > >           upper 32-bits of the register are null.
> > > > >           ∀ rA{.id == X, .link == ?}, rB{X,zext} => rA % 32 == rB % 32
> > > > > - sext => lower 32-bits of the register are identical to
> > > > >           lower 32-bits of a scalar value X,
> > > > >           upper 32-bits of the register are either 0 or 1,
> > > > >           depending on the bit 31 value.
> > > > >           ∀ rA{.id == X, .link == ?}, rB{X,sext} => sext(rA % 32) == sext(rB % 32)
> > > > 
> > > > hmm.
> > > > there is also 32-bit link with delta, right?
> > > 
> > > My point is that delta is independent of 32-bit/64-bit property.
> > > `delta' can be used to propagate in both directions:
> > > - full 64 bit -> 32 bit sign/zero-extened
> > > - 32 bit sign/zero-extened -> full 64-bit
> > 
> > both? how ?
> > I was under impression that in 32-bit domain delta is one way.
> > rX = ...
> > wY = wX
> > wY += 5
> > 
> > if wY == 10
> > We cannot do -5 to rX
>
> Why?
> It is still valid to transfer r32 and tnum_subreg knowledge from wY to rX.
>
>   wY + 5 == rX % 32 + 5 => hence rX % 32 knowledge can be recovered.
>
> If rX itself had some delta, e.g.:
>
>   rX = ...
>   rX += 7
>   wY = wX
>   wY += 5
>
> Then it would still be possible:
>
>   wY + 5 == (rX - 7) % 32 + 5 = rX % 32 - 2.
>
> Again, in case of this direction, only lower 32-bits of the 'full'
> register can be inferred.

Ok, so we're argeeing that it's not 'full' in your above definition.
Your enum id_link_kind needs a 4th category : apply-delta-to-lower-32bit-...
and it's not bi-directional.

rX = ...
wY = wX
wY += 5
if wY == 10
here can apply -5 to lower 32-bit of rX only.

rX = ...
wY = wX
wY += 5
if rX == 10
here can apply +5 to lower 32-bit of wY _and_ do zero extend into full rY.

I don't see a value of 'zext' kind alone that doesn't do 'add delta'.