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

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Message-ID: <2e3f676a-ef03-4618-852d-ceb3b620a640@linux.alibaba.com>
Date: Wed, 9 Oct 2024 10:23:48 +0800
From: Philo Lu <lulie@...ux.alibaba.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
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 2024/10/9 03:05, Martin KaFai Lau wrote:
> 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.
> 

Thank you! This works for me so this patch is unnecessary then.

Just curious is there any technical issue to include rcu_ptr into 
btf_id_sock_common_types? AFAICT rcu_ptr should also be a valid ptr 
type, and then btf_id_sock_common_types will behave like (PTR_TO_BTF_ID 
+ &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON]) in bpf_func_proto.

Thanks.
-- 
Philo