sched: ipipe: announce signal receipt

Emit IPIPE_KEVT_SIGWAKE when the target task is about to receive a
(regular) signal. The co-kernel may decide to schedule a transition of
the recipient to the root domain in order to have it handle that
signal asap, which is commonly required for keeping the kernel sane.

This notification is always sent from the context of the issuer.

kernel/signal.c

@@ -32,6 +32,7 @@
 #include <linux/tracehook.h>
 #include <linux/capability.h>
 #include <linux/freezer.h>
+#include <linux/ipipe.h>
 #include <linux/pid_namespace.h>
 #include <linux/nsproxy.h>
 #include <linux/user_namespace.h>
@@ -737,6 +738,10 @@ still_pending:
 void signal_wake_up_state(struct task_struct *t, unsigned int state)
 {
 	set_tsk_thread_flag(t, TIF_SIGPENDING);
+
+	/* TIF_SIGPENDING must be prior to reporting. */
+	__ipipe_report_sigwake(t);
+
 	/*
 	 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
 	 * case. We don't check t->state here because there is a race with it
@@ -957,8 +962,11 @@ static inline bool wants_signal(int sig, struct task_struct *p)
 	if (sig == SIGKILL)
 		return true;
 
-	if (task_is_stopped_or_traced(p))
+	if (task_is_stopped_or_traced(p)) {
+		if (!signal_pending(p))
+			__ipipe_report_sigwake(p);
 		return false;
+	}
 
 	return task_curr(p) || !signal_pending(p);
 }