Re: [PATCH mptcp-next 2/2] selftests/bpf: Add getsockopt to inspect mptcp subflow

[Martin KaFai Lau <martin.lau@linux.dev>]

On 8/26/24 2:24 AM, Geliang Tang wrote:
>> I'm not an expert in this, but I guess it means you cannot use
>> 'mptcp_subflow_active()', because it can modify the 'subflow'
>> structure
>> that you got with bpf_core_cast() for a read-only usage.
> 
> A read-only function will get the same error.
> 
> I added a read-only function mptcp_subflow_get_scheduled() for testing:
> 
> bool mptcp_subflow_get_scheduled(struct mptcp_subflow_context *subflow)
> {
>          return subflow->scheduled;
> }
> 
> And invoke it from BPF in mptcp_for_each_subflow() loop:
> 
> int BPF_PROG(bpf_first_get_subflow, struct mptcp_sock *msk,
>               struct mptcp_sched_data *data)
> {
>          struct mptcp_subflow_context *subflow, *tmp;
> 
>          mptcp_for_each_subflow(msk, tmp) {
>                  subflow = bpf_core_cast(tmp, struct
> mptcp_subflow_context);
>                  mptcp_subflow_get_scheduled(subflow);
>          }
>          return 0;
> }
> 
> The same "arg#0 is untrusted_ptr_ expected ptr_ or socket" occurs.
> 
> Hope Martin can give us a solution for this issue.

I don't know the context for this list walking + modify-by-kfunc usage, so the 
following could be a grain of salt.

It seems like fitting the bpf_iter use case. Take a look at some recent bpf_iter 
additions, e.g. bpf_iter_{task,css}_next(). Also the bpf_for_each macro usage in 
selftests. There may be some secondary things that need to consider, e.g. how 
the walked subflow is protected, rcu, refcnt...etc.