Re: [PATCH net-next v5 03/15] bpf: introduce timestamp_used to allow UDP socket fetched in bpf prog

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

On 1/15/25 5:12 PM, Jason Xing wrote:
>>> Also, I need to set allow_direct_access to one as long as there is
>>> "sock_ops.is_fullsock = 1;" in the existing callbacks.
>> Only set allow_direct_access when the sk is fullsock in the "existing" sockops
>> callback.
> Only "existing"? Then how can the bpf program access those members of
> the tcp socket structure in the current/new timestamping callbacks?

There is at least one sk write:

	case offsetof(struct bpf_sock_ops, sk_txhash):
		SOCK_OPS_GET_OR_SET_FIELD(sk_txhash, sk_txhash,
					 struct sock, type);

afaict, the kernel always writes sk->sk_txhash with the sk lock held. The new 
timestamping callbacks cannot write because it does not hold the lock.
Otherwise, it needs another flag in bpf_sock_ops_kern to say read only or not. 
imo, it is too complicated to be worth it.

It is fine for the new timestamping callbacks not able to access the tcp_sock 
fields through the bpf_sock_ops. We are not losing anything. The accessible 
tcp_sock fields through the bpf_sock_ops is limited and the  bpf_sock_ops api is 
pretty much frozen. The bpf prog should use the bpf_core_cast(skops->sk, struct 
tcp_sock). The future UDP timestamping support will likely need to use the 
bpf_core_cast anyway because we are not extending "struct bpf_sock_ops" for the 
udp_sock specific fields.