Squash to "bpf: Add mptcp_subflow bpf_iter"

Drop the NULL check for 'msk' as Martin suggested, add more checks
for 'sk'.

Use the "struct sock *sk" instead of "struct mptcp-sock *msk" as the
argument in the bpf_iter_mptcp_subflow_new as Martin suggested.

v5:
 - check bpf_iter_task in mptcp_subflow_new()

v4:
 - drop sock_owned_by_user_nocheck and spin_is_locked. According to
   comments from Mat [2] and Martin [1], in this set mptcp_subflow
   bpf_iter only used from a cg sockopt bpf prog, no need to add these
   check at this moment.

[1]
https://lore.kernel.org/all/[email protected]/
[2]
https://patchwork.kernel.org/project/mptcp/patch/f6469225598beecbf0bda12a4c33fafa86c0ff15.1739787744.git.tanggeliang@kylinos.cn/

v3:
 - continue to use sock_owned_by_user_nocheck and spin_is_locked
checks instead of using msk_owned_by_me().

v2:
 - check the owner before assigning the msk as Mat suggested.

Signed-off-by: Geliang Tang <[email protected]>

Author: Geliang Tang

net/mptcp/bpf.c

@@ -249,24 +249,33 @@ bpf_mptcp_subflow_ctx(const struct sock *sk)
 
 __bpf_kfunc static int
 bpf_iter_mptcp_subflow_new(struct bpf_iter_mptcp_subflow *it,
-			   struct mptcp_sock *msk)
+			   struct sock *sk)
 {
 	struct bpf_iter_mptcp_subflow_kern *kit = (void *)it;
-	struct sock *sk = (struct sock *)msk;
+	struct task_struct *task;
+	struct mptcp_sock *msk;
 
 	BUILD_BUG_ON(sizeof(struct bpf_iter_mptcp_subflow_kern) >
 		     sizeof(struct bpf_iter_mptcp_subflow));
 	BUILD_BUG_ON(__alignof__(struct bpf_iter_mptcp_subflow_kern) !=
 		     __alignof__(struct bpf_iter_mptcp_subflow));
 
-	kit->msk = msk;
-	if (!msk)
+	if (unlikely(!sk || !sk_fullsock(sk)))
 		return -EINVAL;
 
-	if (!sock_owned_by_user_nocheck(sk) &&
-	    !spin_is_locked(&sk->sk_lock.slock))
+	if (sk->sk_protocol != IPPROTO_MPTCP)
 		return -EINVAL;
 
+	msk = mptcp_sk(sk);
+	task = mptcp_get_bpf_iter_task(msk);
+	if (!task || task != current)
+		return -EINVAL;
+
+	mptcp_clear_bpf_iter_task(msk);
+
+	msk_owned_by_me(msk);
+
+	kit->msk = msk;
 	kit->pos = &msk->conn_list;
 	return 0;
 }