Re: [PATCH bpf-next 5/6] bpf, arm32: Always zero extend for LDX with B/H/W

Alexei Starovoitov
2023-09-13T00:10:56Z

On Tue, Sep 12, 2023 at 4:17 PM Puranjay Mohan <puranjay12@gmail.com> wrote:
>
> On Wed, Sep 13, 2023 at 1:04 AM Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > On Tue, Sep 12, 2023 at 10:46:53PM +0000, Puranjay Mohan wrote:
> > > The JITs should not depend on the verifier for zero extending the upper
> > > 32 bits of the destination register when loading a byte, half-word, or
> > > word.
> > >
> > > A following patch will make the verifier stop patching zext instructions
> > > after LDX.
> >
> > This was introduced by:
> >
> > 163541e6ba34 ("arm: bpf: eliminate zero extension code-gen")
> >
> > along with an additional function. So three points:
> >
> > 1) the commit should probably explain why it has now become undesirable
> > to access this verifier state, whereas it appears it was explicitly
> > added to permit this optimisation.
>
> I added some details in the cover letter.
>
> For the complete discussion see: [1]
>
> > 2) you state that jits should not depend on this state, but the above
> > commit adds more references than you're removing, so aren't there still
> > references to the verifier remaining after this patch? I count a total
> > of 10, and the patch below removes three.
>
> The JITs should not depend on this state for LDX (loading
> a B/H/W.
> This patch removes the usage only for LDX.
>
> > 3) what about the bpf_jit_needs_zext() function that was added to
> > support the export of this zext state?
>
> That is still applicable, The verifier will still emit zext
> instructions for other
> instructions like BPF_ALU / BPF_ALU64
>
> >
> > Essentially, the logic stated in the commit message doesn't seem to be
> > reflected by the proposed code change.
>
> I will try to provide more information.
> Currently I have asked Alexei if we really need this in [2].
> I still think this optimization is useful and we should keep it.

Right. subreg tracking is indeed functional for narrow loads.
Let's drop this patch set.