netdev - Re: [PATCH bpf-next v2 3/5] tcp: Use skb__nullable in trace_tcp_send

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Message-ID: <CAADnVQL1Z3LGc+7W1+NrffaGp7idefpbnKPQTeHS8xbQme5Paw@mail.gmail.com>
Date: Thu, 5 Sep 2024 17:26:42 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Philo Lu <lulie@...ux.alibaba.com>
Cc: bpf <bpf@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Eddy Z <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	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>, 
	Jiri Olsa <jolsa@...nel.org>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Alexandre Torgue <alexandre.torgue@...s.st.com>, 
	Kui-Feng Lee <thinker.li@...il.com>, Juntong Deng <juntong.deng@...look.com>, jrife@...gle.com, 
	Alan Maguire <alan.maguire@...cle.com>, Dave Marchevsky <davemarchevsky@...com>, 
	Daniel Xu <dxu@...uu.xyz>, Viktor Malik <vmalik@...hat.com>, 
	Cupertino Miranda <cupertino.miranda@...cle.com>, Matt Bobrowski <mattbobrowski@...gle.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Network Development <netdev@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 3/5] tcp: Use skb__nullable in trace_tcp_send_reset

On Thu, Sep 5, 2024 at 12:56 AM Philo Lu <lulie@...ux.alibaba.com> wrote:
>
> Replace skb with skb__nullable as the argument name. The suffix tells
> bpf verifier through btf that the arg could be NULL and should be
> checked in tp_btf prog.
>
> For now, this is the only nullable argument in tcp tracepoints.
>
> Signed-off-by: Philo Lu <lulie@...ux.alibaba.com>
> ---
>  include/trace/events/tcp.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
> index 1c8bd8e186b89..a27c4b619dffd 100644
> --- a/include/trace/events/tcp.h
> +++ b/include/trace/events/tcp.h
> @@ -91,10 +91,10 @@ DEFINE_RST_REASON(FN, FN)
>  TRACE_EVENT(tcp_send_reset,
>
>         TP_PROTO(const struct sock *sk,
> -                const struct sk_buff *skb,
> +                const struct sk_buff *skb__nullable,
>                  const enum sk_rst_reason reason),
>
> -       TP_ARGS(sk, skb, reason),
> +       TP_ARGS(sk, skb__nullable, reason),

netdev folks pls ack this patch.

Yes, it's a bit of a whack a mole and eventually we can get rid of it
with a smarter verifier (likely) or smarter objtool (unlikely).
Long term we should be able to analyze body of TP_fast_assign
automatically and conclude whether it's handling NULL for pointer
arguments or not. bpf verifier can easily do it for bpf code.
We just need to compile TP_fast_assign() as a tiny bpf snippet.
This is work in progress.

For now we have to use a suffix like __nullable.