linux-kernel - Re: [PATCH bpf-next] bpf: Add rcu ptr in btf_id_sock_common

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Message-ID: <80cb3d4b-cebb-4f08-865d-354110a54467@linux.dev>
Date: Tue, 8 Oct 2024 12:05:59 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Philo Lu <lulie@...ux.alibaba.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
 andrii@...nel.org, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, xuanzhuo@...ux.alibaba.com,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Add rcu ptr in btf_id_sock_common_types

On 10/8/24 1:09 AM, Philo Lu wrote:
> Sometimes sk is dereferenced as an rcu ptr, such as skb->sk in tp_btf,
> which is a valid type of sock common. Then helpers like bpf_skc_to_*()
> can be used with skb->sk.
> 
> For example, the following prog will be rejected without this patch:
> ```
> SEC("tp_btf/tcp_bad_csum")
> int BPF_PROG(tcp_bad_csum, struct sk_buff* skb)
> {
> 	struct sock *sk = skb->sk;
> 	struct tcp_sock *tp;
> 
> 	if (!sk)
> 		return 0;
> 	tp = bpf_skc_to_tcp_sock(sk);

If the use case is for reading the fields in tp, please use the bpf_core_cast 
from the libbpf's bpf_core_read.h. bpf_core_cast is using the bpf_rdonly_cast 
kfunc underneath.

pw-bot: cr