linux-kernel - Re: [PATCH bpf-next 1/4] bpf: Allow get_func_[arg|arg

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Message-ID: <e2bd2bd0-cb0f-4eef-896e-f1569b40243f@linux.dev>
Date: Tue, 13 May 2025 00:01:50 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
 Kafai Wan <mannkafai@...il.com>, Song Liu <song@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Matt Bobrowski <mattbobrowski@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
 bpf <bpf@...r.kernel.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>,
 Network Development <netdev@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/4] bpf: Allow get_func_[arg|arg_cnt] helpers in
 raw tracepoint programs

On 2025/5/12 23:25, Alexei Starovoitov wrote:
> On Mon, May 12, 2025 at 4:12 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
>>

[...]

>>
>> However, since bpf_rdonly_cast() is a kfunc, it causes registers r1–r5
>> to be considered volatile.
> 
> It is not.
> See:
> BTF_ID_FLAGS(func, bpf_rdonly_cast, KF_FASTCALL)
> and relevant commits.

Thanks for the reminder — you're right, bpf_rdonly_cast() is marked with
KF_FASTCALL, so it doesn't make r1–r5 volatile.

Thanks,
Leon