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

[Martin KaFai Lau]

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.