Re: [PATCH 2/8] bpf: Add bpf_get_func_ip kprobe helper for fprobe link

Alexei Starovoitov
2022-02-07T21:01:44Z

On Mon, Feb 7, 2022 at 10:59 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Feb 2, 2022 at 5:53 AM Jiri Olsa <jolsa@redhat.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@kernel.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.