mptcp: Use struct_group() to avoid cross-field memset()

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use struct_group() to capture the fields to be reset, so that memset()
can be appropriately bounds-checked by the compiler.

Cc: Matthieu Baerts <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Mat Martineau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/mptcp/protocol.h

···
 struct mptcp_subflow_context {
 	struct	list_head node;/* conn_list of subflows */
 
-	char	reset_start[0];
+	struct_group(reset,
 
 	unsigned long avg_pacing_rate; /* protected by msk socket lock */
 	u64	local_key;
···
 
 	long	delegated_status;
 
-	char	reset_end[0];
+	);
 
 	struct	list_head delegated_node;   /* link into delegated_action, protected by local BH */
 
···
 static inline void
 mptcp_subflow_ctx_reset(struct mptcp_subflow_context *subflow)
 {
-	memset(subflow->reset_start, 0, subflow->reset_end - subflow->reset_start);
+	memset(&subflow->reset, 0, sizeof(subflow->reset));
 	subflow->request_mptcp = 1;
 }