mptcp: reset when MPTCP opts are dropped after join

Before this patch, if the checksum was not used, the subflow was only
reset if map_data_len was != 0. If there were no MPTCP options or an
invalid mapping, map_data_len was not set to the data len, and then the
subflow was not reset as it should have been, leaving the MPTCP
connection in a wrong fallback mode.

This map_data_len condition has been introduced to handle the reception
of the infinite mapping. Instead, a new dedicated mapping error could
have been returned and treated as a special case. However, the commit
31bf11d ("mptcp: introduce MAPPING_BAD_CSUM") has been introduced
by Paolo Abeni soon after, and backported later on to stable. It better
handle the csum case, and it means the exception for valid_csum_seen in
subflow_can_fallback(), plus this one for the infinite mapping in
subflow_check_data_avail(), are no longer needed.

In other words, the code can be simplified there: a fallback should only
be done if msk->allow_infinite_fallback is set. This boolean is set to
false once MPTCP-specific operations acting on the whole MPTCP
connection vs the initial path have been done, e.g. a second path has
been created, or an MPTCP re-injection -- yes, possible even with a
single subflow. The subflow_can_fallback() helper can then be dropped,
and replaced by this single condition.

This also makes the code clearer: a fallback should only be done if it
is possible to do so.

While at it, no need to set map_data_len to 0 in get_mapping_status()
for the infinite mapping case: it will be set to skb->len just after, at
the end of subflow_check_data_avail(), and not read in between.

Fixes: f8d4bca ("mptcp: infinite mapping receiving")
Cc: stable@vger.kernel.org
Reported-by: Chester A. Unal <chester.a.unal@xpedite-tech.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/544
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Tested-by: Chester A. Unal <chester.a.unal@xpedite-tech.com>
Link: https://patch.msgid.link/20250224-net-mptcp-misc-fixes-v1-2-f550f636b435@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/subflow.c

@@ -1142,7 +1142,6 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 	if (data_len == 0) {
 		pr_debug("infinite mapping received\n");
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_INFINITEMAPRX);
-		subflow->map_data_len = 0;
 		return MAPPING_INVALID;
 	}
 
@@ -1286,18 +1285,6 @@ static void subflow_sched_work_if_closed(struct mptcp_sock *msk, struct sock *ss
 		mptcp_schedule_work(sk);
 }
 
-static bool subflow_can_fallback(struct mptcp_subflow_context *subflow)
-{
-	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
-
-	if (subflow->mp_join)
-		return false;
-	else if (READ_ONCE(msk->csum_enabled))
-		return !subflow->valid_csum_seen;
-	else
-		return READ_ONCE(msk->allow_infinite_fallback);
-}
-
 static void mptcp_subflow_fail(struct mptcp_sock *msk, struct sock *ssk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
@@ -1393,7 +1380,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			return true;
 		}
 
-		if (!subflow_can_fallback(subflow) && subflow->map_data_len) {
+		if (!READ_ONCE(msk->allow_infinite_fallback)) {
 			/* fatal protocol error, close the socket.
 			 * subflow_error_report() will introduce the appropriate barriers
 			 */