linux-kernel - Re: [PATCH bpf-next v2] bpf: Remove redundant free_verifier_state()/pop

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Message-ID: <CAADnVQKV3=S7Cs52QjiKY3ByOC08J9HxnN4wYUTmMX_ctrGVig@mail.gmail.com>
Date: Fri, 13 Jun 2025 15:06:12 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Luis Gerhorst <luis.gerhorst@....de>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpf: Remove redundant free_verifier_state()/pop_stack()

On Fri, Jun 13, 2025 at 2:17 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Fri, 2025-06-13 at 11:01 +0200, Luis Gerhorst wrote:
> > This patch removes duplicated code.
> >
> > Eduard points out [1]:
> >
> >     Same cleanup cycles are done in push_stack() and push_async_cb(),
> >     both functions are only reachable from do_check_common() via
> >     do_check() -> do_check_insn().
> >
> >     Hence, I think that cur state should not be freed in push_*()
> >     functions and pop_stack() loop there is not needed.
> >
> > This would also fix the 'symptom' for [2], but the issue also has a
> > simpler fix which was sent separately. This fix also makes sure the
> > push_*() callers always return an error for which
> > error_recoverable_with_nospec(err) is false. This is required because
> > otherwise we try to recover and access the stale `state`.
> >
> > Moving free_verifier_state() and pop_stack(..., pop_log=false) to happen
> > after the bpf_vlog_reset() call in do_check_common() is fine because the
> > pop_stack() call that is moved does not call bpf_vlog_reset() with the
> > pop_log=false parameter.
> >
> > [1] https://lore.kernel.org/all/b6931bd0dd72327c55287862f821ca6c4c3eb69a.camel@gmail.com/
> > [2] https://lore.kernel.org/all/68497853.050a0220.33aa0e.036a.GAE@google.com/
> >
> > Reported-by: Eduard Zingerman <eddyz87@...il.com>
> > Link: https://lore.kernel.org/all/b6931bd0dd72327c55287862f821ca6c4c3eb69a.camel@gmail.com/
> > Acked-by: Eduard Zingerman <eddyz87@...il.com>
> > Signed-off-by: Luis Gerhorst <luis.gerhorst@....de>
> > ---
>
> Tried v2, all looks good.
>
> [...]
>
> > @@ -22934,6 +22922,11 @@ static void free_states(struct bpf_verifier_env *env)
> >       struct bpf_scc_info *info;
> >       int i, j;
> >
> > +     WARN_ON_ONCE(!env->cur_state);
>
> Tbh I woudn't do this a warning, just an 'if (env->cur_state) ...',
> but that's immaterial. Given current way do_check_common() is written
> env->cur_state != NULL at this point, so the patch is safe to land.

I removed it while applying, since it's useless.
If do_check_common() changes in the future and cur_state is NULL
here the warn will warn, but won't prevent the crash in the next line.

Also for tricky things we switched to verifier_bug() instead of WARN.
So no new WARNs allowed.