netdev - Re: [PATCH bpf-next v2 06/18] bpf: tracing: add support to record and check the accessed args

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Message-ID: <0027bec0-e10f-4c7d-9a56-1c9be7737f6a@linux.dev>
Date: Tue, 15 Jul 2025 07:45:16 +0800
From: Menglong Dong <menglong.dong@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>,
 Menglong Dong <menglong8.dong@...il.com>
Cc: alexei.starovoitov@...il.com, rostedt@...dmis.org, jolsa@...nel.org,
 bpf@...r.kernel.org, Menglong Dong <dongml2@...natelecom.cn>,
 John Fastabend <john.fastabend@...il.com>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Simon Horman <horms@...nel.org>, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 06/18] bpf: tracing: add support to record and
 check the accessed args


On 2025/7/15 06:07, Andrii Nakryiko wrote:
> On Thu, Jul 3, 2025 at 5:20 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>> In this commit, we add the 'accessed_args' field to struct bpf_prog_aux,
>> which is used to record the accessed index of the function args in
>> btf_ctx_access().
> Do we need to bother giving access to arguments through direct ctx[i]
> access for these multi-fentry/fexit programs? We have
> bpf_get_func_arg_cnt() and bpf_get_func_arg() which can be used to get
> any given argument at runtime.


Hi Andrii. This commit is not for that purpose. We remember all the accessed
args to bpf_prog_aux->accessed_args. And when we attach the tracing-multi
prog to the kernel functions, we will check if the accessed arguments are
consistent between all the target functions.

The bpf_prog_aux->accessed_args will be used in
https://lore.kernel.org/bpf/20250703121521.1874196-12-dongml2@chinatelecom.cn/

in bpf_tracing_check_multi() to do such checking.

With such checking, the target functions don't need to have
the same prototype, which makes tracing-multi more flexible.

Thanks!
Menglong Dong


>
>> Meanwhile, we add the function btf_check_func_part_match() to compare the
>> accessed function args of two function prototype. This function will be
>> used in the following commit.
>>
>> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
>> ---
>>   include/linux/bpf.h   |   4 ++
>>   include/linux/btf.h   |   3 +-
>>   kernel/bpf/btf.c      | 108 +++++++++++++++++++++++++++++++++++++++++-
>>   net/sched/bpf_qdisc.c |   2 +-
>>   4 files changed, 113 insertions(+), 4 deletions(-)
>>
> [...]
>