linux-kernel - Re: [PATCH bpf-next 01/11] bpf: Move insn if/else into do_check

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Message-ID: <293dbe3950a782b8eb3b87b71d7a967e120191fd.camel@gmail.com>
Date: Fri, 14 Mar 2025 15:47:00 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: Luis Gerhorst <luis.gerhorst@....de>, Alexei Starovoitov
 <ast@...nel.org>,  Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,  Song Liu
 <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, John Fastabend	
 <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, Stanislav
 Fomichev	 <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>,  Puranjay Mohan <puranjay@...nel.org>, Xu Kuohai
 <xukuohai@...weicloud.com>, Catalin Marinas	 <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Hari Bathini	 <hbathini@...ux.ibm.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,  Naveen N Rao
 <naveen@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael
 Ellerman	 <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, Mykola
 Lysenko	 <mykolal@...com>, Shuah Khan <shuah@...nel.org>, Henriette Herzog	
 <henriette.herzog@....de>, Cupertino Miranda
 <cupertino.miranda@...cle.com>,  Matan Shachnai <m.shachnai@...il.com>,
 Dimitar Kanaliev <dimitar.kanaliev@...eground.com>, Shung-Hsi Yu	
 <shung-hsi.yu@...e.com>, Daniel Xu <dxu@...uu.xyz>, bpf@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, linux-kselftest@...r.kernel.org, George Guo	
 <guodongtai@...inos.cn>, WANG Xuerui <git@...0n.name>, Tiezhu Yang	
 <yangtiezhu@...ngson.cn>
Cc: Maximilian Ott <ott@...fau.de>, Milan Stephan <milan.stephan@....de>
Subject: Re: [PATCH bpf-next 01/11] bpf: Move insn if/else into
 do_check_insn()

On Thu, 2025-03-13 at 18:21 +0100, Luis Gerhorst wrote:
> This is required to catch the errors later and fall back to a nospec if
> on a speculative path.
> 
> Move code into do_check_insn(), replace
> * "continue" with "return INSN_IDX_MODIFIED"
> * "goto process_bpf_exit" with "return PROCESS_BPF_EXIT"
> * "do_print_state = " with "*do_print_state = "
> 
> Signed-off-by: Luis Gerhorst <luis.gerhorst@....de>
> Acked-by: Henriette Herzog <henriette.herzog@....de>
> Cc: Maximilian Ott <ott@...fau.de>
> Cc: Milan Stephan <milan.stephan@....de>
> ---

This refactoring is a long overdue, thank you!
A few nits below.

[...]

> +		err = do_check_insn(env, insn, pop_log, &do_print_state, regs, state,
> +				    &prev_insn_idx);

- `regs` remains declared in do_check(), while nothing prevents
  pushing its declaration to do_check_insn().
- `state` is `env->cur_state`, so I'd avoid passing it as a parameter
  (just to reduce count);
- `prev_insn_idx` is unused by `do_check_insn`;
- `pop_log` is not used by `do_check_insn`;
- given that `insn` is presumed to correspond to `env->insn_idx` in
  many places down the stack not sure about this parameter.

> +		if (err < 0) {
> +			return err;
> +		} else if (err == INSN_IDX_MODIFIED) {

Also, I'd get rid of `INSN_IDX_MODIFIED` and move `env->insn_idx++`
into `do_check_insn()`. This would save a few mental cycles when
looking at the code with full patch-set applied:

		} else if (err == INSN_IDX_MODIFIED) {
			continue;
		} else if (err == PROCESS_BPF_EXIT) {
			goto process_bpf_exit;
		}
		WARN_ON_ONCE(err);

		if (state->speculative && cur_aux(env)->nospec_result) {
			... bunch of actions ...
		}

		env->insn_idx++;

One needs to stop for a moment and think why "bunch of actions" is
performed for regular index increment, but not for INSN_IDX_MODIFIED.

> +			continue;
> +		} else if (err == PROCESS_BPF_EXIT) {

[...]