linux-kernel - Re: [PATCH RFC bpf-next v6 4/6] bpf/helpers: mark the callback of bpf_timer_set_sleepable

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Date: Tue, 09 Apr 2024 01:35:55 +0300
From: Eduard Zingerman <eddyz87@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>, 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@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>,  Mykola Lysenko <mykolal@...com>, Shuah Khan
 <shuah@...nel.org>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next v6 4/6] bpf/helpers: mark the callback of
 bpf_timer_set_sleepable_cb() as sleepable

On Mon, 2024-04-08 at 10:09 +0200, Benjamin Tissoires wrote:
> Now that we have bpf_timer_set_sleepable_cb() available and working, we
> can tag the attached callback as sleepable, and let the verifier check
> in the correct context the calls and kfuncs.
> 
> Signed-off-by: Benjamin Tissoires <bentiss@...nel.org>
> 
> ---

I think this patch is fine with one nit regarding in_sleepable().
Acked-by: Eduard Zingerman <eddyz87@...il.com>

> @@ -5279,7 +5281,8 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
>  
>  static bool in_sleepable(struct bpf_verifier_env *env)
>  {
> -	return env->prog->sleepable;
> +	return env->prog->sleepable ||
> +	       (env->cur_state && env->cur_state->in_sleepable);
>  }

Sorry, I already raised this before.
As far as I understand the 'env->cur_state' check is needed because
this function is used from do_misc_fixups():

		if (is_storage_get_function(insn->imm)) {
			if (!in_sleepable(env) ||
			    env->insn_aux_data[i + delta].storage_get_func_atomic)
				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_ATOMIC);
			else
				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_KERNEL);
			insn_buf[1] = *insn;
			cnt = 2;
			...
		}

For a timer callback function 'env->prog->sleepable' would be false.
Which means that inside sleepable callback function GFP_ATOMIC would
be used in cases where GFP_KERNEL would be sufficient.
An alternative would be to check (and set) sleepable flag not for a
full program but for a subprogram.

Whether or not this is something worth addressing I don't know.

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