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: Mon, 8 Apr 2024 20:14:00 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: 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>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <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, Apr 8, 2024 at 3:36 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> 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.

At this point all subprograms are still part of the main program.
jit_subprogs() hasn't been called yet.
So there is only one 'prog' object.
So cannot really set prog->sleepable for callback subprog.

But you've raised a good point.
We can remove "!in_sleepable(env)" part in do_misc_fixups() with:
-                if (in_sleepable(env) && is_storage_get_function(func_id))
-
env->insn_aux_data[insn_idx].storage_get_func_atomic = true;

+ if (is_storage_get_function(func_id))
+   env->insn_aux_data[insn_idx].storage_get_func_atomic = !in_sleepable(env);