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

Andrii Nakryiko
2022-02-07T19:04:36Z

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.

> +
> + 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
>