linux-kernel - Re: [PATCH RFC bpf-next v3 07/16] bpf/helpers: mark the callback of bpf_timer_set_sleepable

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Date: Fri, 23 Feb 2024 17:35:19 +0200
From: Eduard Zingerman <eddyz87@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>, 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@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>, Jiri Kosina <jikos@...nel.org>,  Benjamin Tissoires
 <benjamin.tissoires@...hat.com>, Jonathan Corbet <corbet@....net>, Shuah
 Khan <shuah@...nel.org>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-input@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next v3 07/16] bpf/helpers: mark the callback of
 bpf_timer_set_sleepable_cb() as sleepable

On Wed, 2024-02-21 at 17:25 +0100, Benjamin Tissoires wrote:

[...]

> @@ -626,6 +627,7 @@ struct bpf_subprog_info {
>  	bool is_async_cb: 1;
>  	bool is_exception_cb: 1;
>  	bool args_cached: 1;
> +	bool is_sleepable: 1;
>  
>  	u8 arg_cnt;
>  	struct bpf_subprog_arg_info args[MAX_BPF_FUNC_REG_ARGS];

[...]

> @@ -2421,6 +2424,7 @@ static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
>  	 * Initialize it similar to do_check_common().
>  	 */
>  	elem->st.branches = 1;
> +	elem->st.in_sleepable = env->subprog_info[subprog].is_sleepable;
>  	frame = kzalloc(sizeof(*frame), GFP_KERNEL);
>  	if (!frame)
>  		goto err;

[...]

> @@ -9478,6 +9483,7 @@ static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins
>  
>  		/* there is no real recursion here. timer callbacks are async */
>  		env->subprog_info[subprog].is_async_cb = true;
> +		env->subprog_info[subprog].is_sleepable = is_bpf_timer_set_sleepable_cb_kfunc(insn->imm);
>  		async_cb = push_async_cb(env, env->subprog_info[subprog].start,
>  					 insn_idx, subprog);

I'd make is_sleepable a parameter for push_async_cb() instead of a field
in struct bpf_subprog_info.
I had to spend some time convincing myself that bpf_subprog_info->is_sleepable
does not have to be computed before do_check() in check_cfg(),
or what would happen if same callback is passed as both sleepable and
non-sleepable callback. These questions won't arise if this is a parameter.

[...]