Re: [PATCH bpf-next] bpf: Add rcu ptr in btf_id_sock_common_types

[Philo Lu]

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