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

[Eduard Zingerman <eddyz87@gmail.com>]

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).