Index: sys/kern/kern_exec.c
===================================================================
--- sys/kern/kern_exec.c
+++ sys/kern/kern_exec.c
@@ -119,6 +119,7 @@
 
 static int sysctl_kern_ps_strings(SYSCTL_HANDLER_ARGS);
 static int sysctl_kern_usrstack(SYSCTL_HANDLER_ARGS);
+static int sysctl_kern_stacktop(SYSCTL_HANDLER_ARGS);
 static int sysctl_kern_stackprot(SYSCTL_HANDLER_ARGS);
 static int do_execve(struct thread *td, struct image_args *args,
     struct mac *mac_p, struct vmspace *oldvmspace);
@@ -133,6 +134,10 @@
     CTLFLAG_CAPRD|CTLFLAG_MPSAFE, NULL, 0, sysctl_kern_usrstack, "LU",
     "Top of process stack");
 
+SYSCTL_PROC(_kern, KERN_STACKTOP, stacktop, CTLTYPE_ULONG|CTLFLAG_RD|
+    CTLFLAG_CAPRD|CTLFLAG_MPSAFE, NULL, 0, sysctl_kern_stacktop, "LU",
+    "Top of process stack with stack gap.");
+
 SYSCTL_PROC(_kern, OID_AUTO, stackprot, CTLTYPE_INT|CTLFLAG_RD|CTLFLAG_MPSAFE,
     NULL, 0, sysctl_kern_stackprot, "I",
     "Stack memory permissions");
@@ -189,6 +194,29 @@
 	return error;
 }
 
+static int
+sysctl_kern_stacktop(SYSCTL_HANDLER_ARGS)
+{
+	vm_offset_t stacktop;
+	struct proc *p;
+	int error;
+
+	p = curproc;
+#ifdef SCTL_MASK32
+	if (req->flags & SCTL_MASK32) {
+		unsigned int val;
+		val = (unsigned int)p->p_sysent->sv_usrstack
+		    - (unsigned int)p->p_vmspace->vm_stkgap;
+		error = SYSCTL_OUT(req, &val, sizeof(val));
+	} else
+#endif
+	{
+		stacktop = p->p_sysent->sv_usrstack - p->p_vmspace->vm_stkgap;
+		error = SYSCTL_OUT(req, &stacktop, sizeof(stacktop));
+	}
+	return error;
+}
+
 static int
 sysctl_kern_stackprot(SYSCTL_HANDLER_ARGS)
 {
Index: sys/sys/sysctl.h
===================================================================
--- sys/sys/sysctl.h
+++ sys/sys/sysctl.h
@@ -982,6 +982,7 @@
 #define	KERN_HOSTUUID		36	/* string: host UUID identifier */
 #define	KERN_ARND		37	/* int: from arc4rand() */
 #define	KERN_MAXPHYS		38	/* int: MAXPHYS value */
+#define	KERN_STACKTOP		39	/* int: USRSTACK - stack gap */
 /*
  * KERN_PROC subtypes
  */