linux-kernel - Re: [PATCH 2/8] bpf: Add bpf_get_func

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Date:   Mon, 7 Feb 2022 13:01:29 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jiri Olsa <olsajiri@...il.com>
Subject: Re: [PATCH 2/8] bpf: Add bpf_get_func_ip kprobe helper for fprobe link

On Mon, Feb 7, 2022 at 10:59 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Feb 2, 2022 at 5:53 AM Jiri Olsa <jolsa@...hat.com> wrote:
> >
> > Adding support to call get_func_ip_fprobe helper from kprobe
> > programs attached by fprobe link.
> >
> > Also adding support to inline it, because it's single load
> > instruction.
> >
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  kernel/bpf/verifier.c    | 19 ++++++++++++++++++-
> >  kernel/trace/bpf_trace.c | 16 +++++++++++++++-
> >  2 files changed, 33 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 1ae41d0cf96c..a745ded00635 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -13625,7 +13625,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >                         continue;
> >                 }
> >
> > -               /* Implement bpf_get_func_ip inline. */
> > +               /* Implement tracing bpf_get_func_ip inline. */
> >                 if (prog_type == BPF_PROG_TYPE_TRACING &&
> >                     insn->imm == BPF_FUNC_get_func_ip) {
> >                         /* Load IP address from ctx - 16 */
> > @@ -13640,6 +13640,23 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >                         continue;
> >                 }
> >
> > +               /* Implement kprobe/fprobe bpf_get_func_ip inline. */
> > +               if (prog_type == BPF_PROG_TYPE_KPROBE &&
> > +                   eatype == BPF_TRACE_FPROBE &&
> > +                   insn->imm == BPF_FUNC_get_func_ip) {
> > +                       /* Load IP address from ctx (struct pt_regs) ip */
> > +                       insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1,
> > +                                                 offsetof(struct pt_regs, ip));
>
> Isn't this architecture-specific? I'm starting to dislike this
> inlining whole more and more. It's just a complication in verifier
> without clear real-world benefits. We are clearly prematurely
> optimizing here. In practice you'll just call bpf_get_func_ip() once
> and that's it. Function call overhead will be negligible compare to
> other *userful* work you'll be doing in your BPF program.

We should be doing inlining when we can.
Every bit of performance matters.