mptcp: add bpf_iter_task for mptcp_sock

To make sure the mptcp_subflow bpf_iter is running in the
MPTCP context. This patch adds a simplified version of tracking
for it:

1. Add a 'struct task_struct *bpf_iter_task' field to struct
mptcp_sock.

2. Do a WRITE_ONCE(msk->bpf_iter_task, current) before calling
a MPTCP BPF hook, and WRITE_ONCE(msk->bpf_iter_task, NULL) after
the hook returns.

3. In bpf_iter_mptcp_subflow_new(), check

	"READ_ONCE(msk->bpf_scheduler_task) == current"

to confirm the correct task, return -EINVAL if it doesn't match.

Also creates helpers for setting, clearing and checking that value.

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

Author: Geliang Tang

net/mptcp/protocol.c

@@ -2726,6 +2726,7 @@ static void __mptcp_init_sock(struct sock *sk)
 	msk->scaling_ratio = TCP_DEFAULT_SCALING_RATIO;
 
 	WRITE_ONCE(msk->first, NULL);
+	WRITE_ONCE(msk->bpf_iter_task, NULL);
 	inet_csk(sk)->icsk_sync_mss = mptcp_sync_mss;
 	WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
 	WRITE_ONCE(msk->allow_infinite_fallback, true);

net/mptcp/protocol.h

@@ -327,6 +327,7 @@ struct mptcp_sock {
 	struct list_head conn_list;
 	struct list_head rtx_queue;
 	struct mptcp_data_frag *first_pending;
+	struct task_struct *bpf_iter_task;
 	struct list_head join_list;
 	struct sock	*first; /* The mptcp ops can safely dereference, using suitable
 				 * ONCE annotation, the subflow outside the socket
@@ -1291,4 +1292,19 @@ mptcp_token_join_cookie_init_state(struct mptcp_subflow_request_sock *subflow_re
 static inline void mptcp_join_cookie_init(void) {}
 #endif
 
+static inline void mptcp_set_bpf_iter_task(struct mptcp_sock *msk)
+{
+	WRITE_ONCE(msk->bpf_iter_task, current);
+}
+
+static inline void mptcp_clear_bpf_iter_task(struct mptcp_sock *msk)
+{
+	WRITE_ONCE(msk->bpf_iter_task, NULL);
+}
+
+static inline struct task_struct *mptcp_get_bpf_iter_task(struct mptcp_sock *msk)
+{
+	return READ_ONCE(msk->bpf_iter_task);
+}
+
 #endif /* __MPTCP_PROTOCOL_H */