Re: [PATCH bpf-next v4 08/20] bpf: Walk the exception unwind in the verifier

[Yonghong Song <yonghong.song@linux.dev>]

On 9/22/26 2:53 PM, Eduard Zingerman wrote:
> On Mon, 2026-09-21 at 22:25 -0700, Yonghong Song wrote:
>> On 9/21/26 9:08 PM, Alexei Starovoitov wrote:
>>> On Mon Sep 21, 2026 at 9:01 PM UTC, Yonghong Song wrote:
>>>> --- /dev/null
>>>> +++ b/include/linux/bpf_cleanup_abi.h
>>>> @@ -0,0 +1,16 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>>> +/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
>>>> +#ifndef _LINUX_BPF_CLEANUP_ABI_H
>>>> +#define _LINUX_BPF_CLEANUP_ABI_H
>>>> +
>>>> +/*
>>>> + * Value arch_bpf_run_cleanup_pad() leaves in r0 on the way into a landing pad.
>>>> + * It has to be a constant the verifier knows: LLVM names r0 as both the
>>>> + * exception pointer and the exception selector register, so every pad reads it
>>>> + * before anything else and is free to store what it read. It gets a header of
>>>> + * its own because the two sides that have to agree on it are the verifier and
>>>> + * the arch dispatchers, which are assembly.
>>>> + */
>>>> +#define BPF_PAD_ENTRY_R0	1
>>>> +
>>>> +#endif /* _LINUX_BPF_CLEANUP_ABI_H */
>>> I'm not going to read the AI reasons in commit log that it came up with
>>> to justify new .h.
>>> I bet it doesn't need new .h. If it does, please spell it out with human voice.
>>> And, in general, pls tell AI to be terse. and remember it forever.
>>>
>>> I told my clanker to be like me. Terse and to the point.
>> Okay, I added this file to be shared in x86/net/bpf_cleanup_pad.S,
>> arm64/net/bpf_cleanup_pad.S and verifier.c.  Yes, we can remove it
>> with single line comment in their respective files.
>>
> As far as I understand BPF_PAD_ENTRY_R0 is not needed at all.
> As the value is unused the jits can zero out R0 upon landing
> pad entry from throw or resume, verifier can initialize R0 as
> an unknown scalar: mark_reg_unknown(env, frame->regs, BPF_REG_0).

Agree. R0 is not really needed so make it mark_reg_unknown is
the right approach.