Starting with r223966 in 9-CURRENT, a new interface is available
(RFTSIGZMB).  It is supported in eglibc 2.13-11 or later.  Use this
patch if you need support for older eglibc.

--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -480,7 +480,13 @@
 	if (flags & RFTSIGZMB)
 	        p2->p_sigparent = RFTSIGNUM(flags);
 	else if (flags & RFLINUXTHPN)
-	        p2->p_sigparent = SIGUSR1;
+	{
+		int sig;
+		sig = RFTHPNSIGNUM(flags);
+	        if (sig == 0) sig = SIGUSR1;
+	        if (sig == SIGCHLD) sig = 0;
+	        p2->p_sigparent = sig;
+	}
 	else
 	        p2->p_sigparent = SIGCHLD;
 
@@ -723,12 +729,16 @@
 	static struct timeval lastfail;
 
 	/* Check for the undefined or unimplemented flags. */
-	if ((flags & ~(RFFLAGS | RFTSIGFLAGS(RFTSIGMASK))) != 0)
+	if ((flags & ~(RFFLAGS | RFTSIGFLAGS(RFTSIGMASK) | (RFTHPNMASK << RFTHPNSHIFT))) != 0)
 		return (EINVAL);
 
-	/* Signal value requires RFTSIGZMB. */
-	if ((flags & RFTSIGFLAGS(RFTSIGMASK)) != 0 && (flags & RFTSIGZMB) == 0)
-		return (EINVAL);
+	/* Signal value requires either RFTSIGZMB or RFLINUXTHPN. */
+	if ((flags & RFTSIGZMB) == 0 && (flags & RFLINUXTHPN) == 0) {
+		if ((flags & RFTSIGFLAGS(RFTSIGMASK)) != 0)
+			return (EINVAL);
+		if ((flags & (RFTHPNMASK << RFTHPNSHIFT)) != 0)
+			return (EINVAL);
+	}
 
 	/* Can't copy and clear. */
 	if ((flags & (RFFDG|RFCFDG)) == (RFFDG|RFCFDG))
--- a/sys/sys/unistd.h
+++ b/sys/sys/unistd.h
@@ -191,6 +191,11 @@
     RFTHREAD | RFSIGSHARE | RFLINUXTHPN | RFSTOPPED | RFHIGHPID | RFTSIGZMB | \
     RFPPWAIT)
 
+#define RFTHPNSHIFT	24	/* reserve bits 24-30 */
+#define RFTHPNMASK	0x7F    /* for compatibility with linuxthreads/clone()   */
+				/* allow to specify  "clone exit parent notification" signal */
+#define RFTHPNSIGNUM(flags)	(((flags) >> RFTHPNSHIFT) & RFTHPNMASK)
+
 #endif /* __BSD_VISIBLE */
 
 #endif /* !_SYS_UNISTD_H_ */