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Message-ID: <55177311ccdc24a74811d4a291ee1880044a5227.camel@gmail.com>
Subject: Re: [PATCH RFC bpf-next v3 08/16] bpf/verifier: do_misc_fixups for
 is_bpf_timer_set_sleepable_cb_kfunc
From: Eduard Zingerman <eddyz87@gmail.com>
To: Benjamin Tissoires <bentiss@kernel.org>, Alexei Starovoitov
 <ast@kernel.org>,  Daniel Borkmann <daniel@iogearbox.net>, John Fastabend
 <john.fastabend@gmail.com>, Andrii Nakryiko <andrii@kernel.org>, Martin
 KaFai Lau <martin.lau@linux.dev>, Song Liu <song@kernel.org>, Yonghong Song
 <yonghong.song@linux.dev>, KP Singh <kpsingh@kernel.org>, Stanislav
 Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, Jiri Olsa
 <jolsa@kernel.org>, Jiri Kosina <jikos@kernel.org>,  Benjamin Tissoires
 <benjamin.tissoires@redhat.com>, Jonathan Corbet <corbet@lwn.net>, Shuah
 Khan <shuah@kernel.org>
Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-input@vger.kernel.org, linux-doc@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
Date: Fri, 23 Feb 2024 18:00:51 +0200
In-Reply-To: <20240221-hid-bpf-sleepable-v3-8-1fb378ca6301@kernel.org>
References: <20240221-hid-bpf-sleepable-v3-0-1fb378ca6301@kernel.org>
	 <20240221-hid-bpf-sleepable-v3-8-1fb378ca6301@kernel.org>
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On Wed, 2024-02-21 at 17:25 +0100, Benjamin Tissoires wrote:
> This is still a WIP, but I think this can be dropped as we never
> get to this instruction. So what should we do here?

As Alexei replied in a separate sub-thread you probably want this
for sleepable timers. Here is full source code block:

        if (insn->imm =3D=3D BPF_FUNC_timer_set_callback ||
            is_bpf_timer_set_sleepable_cb_kfunc(insn->imm)) {
            ...
            struct bpf_insn ld_addrs[2] =3D {
                BPF_LD_IMM64(BPF_REG_3, (long)prog->aux),
            };

            insn_buf[0] =3D ld_addrs[0];
            insn_buf[1] =3D ld_addrs[1];
            insn_buf[2] =3D *insn;
            cnt =3D 3;

            new_prog =3D bpf_patch_insn_data(env, i + delta, insn_buf, cnt)=
;
            ...
        }

Effectively, it sets up third function call parameter (R3)
for timer_set_callback() to be prog->aux.
E.g. before bpf_patch_insn_data():

   r1 =3D ... timer ...
   r2 =3D ... callback address ...
   call timer_set_callback

After bpf_patch_insn_data():

   r1 =3D ... timer ...
   r2 =3D ... callback address ...
   r3 =3D prog->aux ll
   call timer_set_callback

This way it won't be necessary to walk stack in search for ctx.aux
in bpf_timer_set_sleepable_cb().