linux-kernel - Re: [RFC PATCH] libbpf: Add bpf_program__set_section

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Message-ID: <CAEf4Bzag72gYXWJ2+mPnWaysyTK0OC3NjnasxNsb+N0NxXY2fw@mail.gmail.com>
Date: Mon, 25 Mar 2024 10:25:34 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Tao Chen <chen.dylane@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] libbpf: Add bpf_program__set_section_name api

On Mon, Mar 25, 2024 at 8:48 AM Tao Chen <chen.dylane@...il.com> wrote:
>
> The new api can be used to reset the function we want to trace in
> runtime. So we need not to change the code again when we just do
> minor changes to the trace function in kprobe or other ebpf event
> type. We can replace the old section with new section passed in via
> parameter. Maybe the following scenario we can use the api:
>
> 1. solve "*.isra.o" issue caused by compiler in new kernel
> obj = offcputime_bpf__open();
> bpf_program__set_section_name(*(obj->skeleton->obj),
> "kprobe/finish_task_switch", "kprobe/finish_task_switch.isra.0");
>
> 2. dynamic adjustment for trace function offset
> obj = offcputime_bpf__open();
> bpf_program__set_section_name(*(obj->skeleton->obj),
> "kprobe/finish_task_switch+23", "kprobe/finish_task_switch+45");
>
> Signed-off-by: Tao Chen <chen.dylane@...il.com>
> ---
>  tools/lib/bpf/libbpf.c   | 24 ++++++++++++++++++++++++
>  tools/lib/bpf/libbpf.h   |  2 ++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 27 insertions(+)
>

This is a wrong approach. SEC() is immutable and serves as a hint to
libbpf on program type and possibly some attach parameters. But after
that libbpf allows to override all of them though APIs like
bpf_program__set_type() and others. Attach APIs always allow to
specify all the target parameters, including kprobe function name and
so on.

Please check all of the libbpf APIs in libbpf.h.

pw-bot: cr

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 96ff1aa4bf6a..94f32e845c61 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8576,6 +8576,30 @@ const char *bpf_program__section_name(const struct bpf_program *prog)
>         return prog->sec_name;
>  }
>

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