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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On Wed, Sep 13, 2023 at 1:04 AM Russell King (Oracle)
<[email protected]> 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.
Thanks,
Puranjay
[1] https://lore.kernel.org/all/CANk7y0j2f-gPgZwd+YfTL71-6wfvky+f=kBC_ccqsS0EHAysyA@mail.gmail.com/
[2] https://lore.kernel.org/bpf/CANk7y0hK9sQJ-kRx3nQpVJSxpP=NzzFaLitOYq8=Pb6Dvk9fpg@mail.gmail.com/