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

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Date: Tue, 26 Mar 2024 22:54:53 +0800
From: Tao Chen <chen.dylane@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...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

Hi, Andrii, thank you for your reply. I read the apis again in libbpf.h, 
as you say, it seems that this patch no need. Thanks again.

在 2024/3/26 上午1:25, Andrii Nakryiko 写道:
> 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;
>>   }
>>
> 
> [...]