diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index 11c0334181e1..eea624019fff 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -279,7 +279,7 @@ static void
 ast_sig(struct thread *td, int tda)
 {
 	struct proc *p;
-	int sig;
+	int old_boundary, sig;
 	bool resched_sigs;
 
 	p = td->td_proc;
@@ -321,12 +321,15 @@ ast_sig(struct thread *td, int tda)
 	    !SIGISEMPTY(p->p_siglist)) {
 		sigfastblock_fetch(td);
 		PROC_LOCK(p);
+		old_boundary = ~TDB_BOUNDARY | (td->td_dbgflags & TDB_BOUNDARY);
+		td->td_dbgflags |= TDB_BOUNDARY;
 		mtx_lock(&p->p_sigacts->ps_mtx);
 		while ((sig = cursig(td)) != 0) {
 			KASSERT(sig >= 0, ("sig %d", sig));
 			postsig(sig);
 		}
 		mtx_unlock(&p->p_sigacts->ps_mtx);
+		td->td_dbgflags &= old_boundary;
 		PROC_UNLOCK(p);
 		resched_sigs = true;
 	} else {
diff --git a/sys/kern/subr_syscall.c b/sys/kern/subr_syscall.c
index 33dd50d3d50a..36830a13e596 100644
--- a/sys/kern/subr_syscall.c
+++ b/sys/kern/subr_syscall.c
@@ -73,6 +73,7 @@ syscallenter(struct thread *td)
 	traced = (p->p_flag & P_TRACED) != 0;
 	if (__predict_false(traced || td->td_dbgflags & TDB_USERWR)) {
 		PROC_LOCK(p);
+		MPASS((td->td_dbgflags & TDB_BOUNDARY) == 0);
 		td->td_dbgflags &= ~TDB_USERWR;
 		if (traced)
 			td->td_dbgflags |= TDB_SCE;
@@ -201,7 +202,7 @@ syscallenter(struct thread *td)
 	    td->td_retval[1]);
 	if (__predict_false(traced)) {
 		PROC_LOCK(p);
-		td->td_dbgflags &= ~TDB_SCE;
+		td->td_dbgflags &= ~(TDB_SCE | TDB_BOUNDARY);
 		PROC_UNLOCK(p);
 	}
 	(p->p_sysent->sv_set_syscall_retval)(td, error);
@@ -280,9 +281,13 @@ syscallret(struct thread *td)
 		 */
 		if (traced &&
 		    ((td->td_dbgflags & (TDB_FORK | TDB_EXEC)) != 0 ||
-		    (p->p_ptevents & PTRACE_SCX) != 0))
+		    (p->p_ptevents & PTRACE_SCX) != 0)) {
+			MPASS((td->td_dbgflags & TDB_BOUNDARY) == 0);
+			td->td_dbgflags |= TDB_BOUNDARY;
 			ptracestop(td, SIGTRAP, NULL);
-		td->td_dbgflags &= ~(TDB_SCX | TDB_EXEC | TDB_FORK);
+		}
+		td->td_dbgflags &= ~(TDB_SCX | TDB_EXEC | TDB_FORK |
+		    TDB_BOUNDARY);
 		PROC_UNLOCK(p);
 	}
 }
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
index 1552da628b11..f17207d741c1 100644
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -522,7 +522,8 @@ enum {
 #define	TDB_FSTP	0x00001000 /* The thread is PT_ATTACH leader */
 #define	TDB_STEP	0x00002000 /* (x86) PSL_T set for PT_STEP */
 #define	TDB_SSWITCH	0x00004000 /* Suspended in ptracestop */
-#define	TDB_COREDUMPREQ	0x00008000 /* Coredump request */
+#define	TDB_BOUNDARY	0x00008000 /* ptracestop() at boundary */
+#define	TDB_COREDUMPREQ	0x00010000 /* Coredump request */
 
 /*
  * "Private" flags kept in td_pflags: