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

[Philo Lu]

On 2024/10/11 06:07, Martin KaFai Lau wrote:
> On 10/8/24 7:23 PM, Philo Lu wrote:
> > 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.
>
> bpf_skc_to_*() returns a PTR_TO_BTF_ID which can be passed into other 
> helpers that takes ARG_PTR_TO_BTF_ID_SOCK_COMMON. There are helpers that 
> change the sk. e.g. bpf_setsockopt() changes the sk and needs sk to be 
> locked. Other non tracing hooks do have a hold on the skb also. I did 
> take a quick look at the bpf_setsockopt situation and looks ok. I am 
> positive there are other helpers that need to audit first.
>
> Tracing use case should only read the sk. bpf_core_cast() is the correct 
> one to use. The bpf_sk_storage_{get,delete}() should be the only allowed 
> helper that can change the sk.

Thank you for explanation, Martin. This helps me a lot.
--
Philo