linux-kernel - Re: [PATCH bpf-next v4 4/6] bpf/helpers: mark the callback of bpf_timer_set_sleepable

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Date: Tue, 19 Mar 2024 01:54:36 +0200
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 bpf-next v4 4/6] bpf/helpers: mark the callback of
 bpf_timer_set_sleepable_cb() as sleepable

On Fri, 2024-03-15 at 15:29 +0100, Benjamin Tissoires wrote:
[...]

> @@ -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);
>  }

I was curious why 'env->cur_state &&' check was needed and found that
removing it caused an error in the following fragment:

static int do_misc_fixups(struct bpf_verifier_env *env)
{
		...
		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);
			...
		}
		...
}

When do_misc_fixups() is done env->cur_state is NULL.
Current implementation would use GFP_ATOMIC allocation even for
sleepable callbacks, where GFP_KERNEL is sufficient.
Is this is something we want to address?

>  
>  /* The non-sleepable programs and sleepable programs with explicit bpf_rcu_read_lock()