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

[Andrii Nakryiko]

On Wed, Feb 2, 2022 at 5:53 AM Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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.


> +
> +                       new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
> +                       if (!new_prog)
> +                               return -ENOMEM;
> +
> +                       env->prog = prog = new_prog;
> +                       insn      = new_prog->insnsi + i + delta;
> +                       continue;
> +               }
> +
>  patch_call_imm:
>                 fn = env->ops->get_func_proto(insn->imm, env->prog);
>                 /* all functions that have prototype and verifier allowed
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index a2024ba32a20..28e59e31e3db 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1036,6 +1036,19 @@ static const struct bpf_func_proto bpf_get_func_ip_proto_kprobe = {
>         .arg1_type      = ARG_PTR_TO_CTX,
>  };
>
> +BPF_CALL_1(bpf_get_func_ip_fprobe, struct pt_regs *, regs)
> +{
> +       /* This helper call is inlined by verifier. */
> +       return regs->ip;
> +}
> +
> +static const struct bpf_func_proto bpf_get_func_ip_proto_fprobe = {
> +       .func           = bpf_get_func_ip_fprobe,
> +       .gpl_only       = false,
> +       .ret_type       = RET_INTEGER,
> +       .arg1_type      = ARG_PTR_TO_CTX,
> +};
> +
>  BPF_CALL_1(bpf_get_attach_cookie_trace, void *, ctx)
>  {
>         struct bpf_trace_run_ctx *run_ctx;
> @@ -1279,7 +1292,8 @@ kprobe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>                 return &bpf_override_return_proto;
>  #endif
>         case BPF_FUNC_get_func_ip:
> -               return &bpf_get_func_ip_proto_kprobe;
> +               return prog->expected_attach_type == BPF_TRACE_FPROBE ?
> +                       &bpf_get_func_ip_proto_fprobe : &bpf_get_func_ip_proto_kprobe;
>         case BPF_FUNC_get_attach_cookie:
>                 return &bpf_get_attach_cookie_proto_trace;
>         default:
> --
> 2.34.1
>