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

Russell King (Oracle)
2023-09-12T23:04:26Z

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.
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.
3) what about the bpf_jit_needs_zext() function that was added to
support the export of this zext state?

Essentially, the logic stated in the commit message doesn't seem to be
reflected by the proposed code change.

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