Index: lib/libc/sys/ptrace.2
===================================================================
--- lib/libc/sys/ptrace.2
+++ lib/libc/sys/ptrace.2
@@ -2,7 +2,7 @@
 .\"	$NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
 .\"
 .\" This file is in the public domain.
-.Dd December 1, 2017
+.Dd May 22, 2018
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -606,6 +606,8 @@
 is the count of array entries filled in.
 .It Dv PT_SETSTEP
 This request will turn on single stepping of the specified process.
+The steeping mode is automatically turned off when the trap due to
+the step is catched.
 .It Dv PT_CLEARSTEP
 This request will turn off single stepping of the specified process.
 .It Dv PT_SUSPEND
Index: sys/amd64/amd64/machdep.c
===================================================================
--- sys/amd64/amd64/machdep.c
+++ sys/amd64/amd64/machdep.c
@@ -1978,13 +1978,20 @@
 int
 ptrace_single_step(struct thread *td)
 {
-	td->td_frame->tf_rflags |= PSL_T;
+
+	if ((td->td_frame->tf_rflags & PSL_T) == 0) {
+		td->td_frame->tf_rflags |= PSL_T;
+		PROC_LOCK(td->td_proc);
+		td->td_dbgflags |= TDB_STEP;
+		PROC_UNLOCK(td->td_proc);
+	}
 	return (0);
 }
 
 int
 ptrace_clear_single_step(struct thread *td)
 {
+
 	td->td_frame->tf_rflags &= ~PSL_T;
 	return (0);
 }
Index: sys/amd64/amd64/trap.c
===================================================================
--- sys/amd64/amd64/trap.c
+++ sys/amd64/amd64/trap.c
@@ -285,8 +285,14 @@
 			signo = SIGTRAP;
 			ucode = TRAP_TRACE;
 			dr6 = rdr6();
-			if (dr6 & DBREG_DR6_BS)
-				frame->tf_rflags &= ~PSL_T;
+			if ((dr6 & DBREG_DR6_BS) != 0) {
+				PROC_LOCK(td->td_proc);
+				if ((td->td_dbgflags & TDB_STEP) != 0) {
+					td->td_frame->tf_rflags &= ~PSL_T;
+					td->td_dbgflags &= ~TDB_STEP;
+				}
+				PROC_UNLOCK(td->td_proc);
+			}
 			break;
 
 		case T_ARITHTRAP:	/* arithmetic trap */
Index: sys/i386/i386/machdep.c
===================================================================
--- sys/i386/i386/machdep.c
+++ sys/i386/i386/machdep.c
@@ -2764,13 +2764,20 @@
 int
 ptrace_single_step(struct thread *td)
 {
-	td->td_frame->tf_eflags |= PSL_T;
+
+	if ((td->td_frame->tf_eflags & PSL_T) == 0) {
+		td->td_frame->tf_eflags |= PSL_T;
+		PROC_LOCK(td->td_proc);
+		td->td_dbgflags |= TDB_STEP;
+		PROC_UNLOCK(td->td_proc);
+	}
 	return (0);
 }
 
 int
 ptrace_clear_single_step(struct thread *td)
 {
+
 	td->td_frame->tf_eflags &= ~PSL_T;
 	return (0);
 }
Index: sys/i386/i386/trap.c
===================================================================
--- sys/i386/i386/trap.c
+++ sys/i386/i386/trap.c
@@ -337,8 +337,14 @@
 			signo = SIGTRAP;
 			ucode = TRAP_TRACE;
 			dr6 = rdr6();
-			if (dr6 & DBREG_DR6_BS)
-				frame->tf_eflags &= ~PSL_T;
+			if ((dr6 & DBREG_DR6_BS) != 0) {
+				PROC_LOCK(td->td_proc);
+				if ((td->td_dbgflags & TDB_STEP) != 0) {
+					td->td_frame->tf_eflags &= ~PSL_T;
+					td->td_dbgflags &= ~TDB_STEP;
+				}
+				PROC_UNLOCK(td->td_proc);
+			}
 			break;
 
 		case T_ARITHTRAP:	/* arithmetic trap */
@@ -357,7 +363,6 @@
 			if (frame->tf_eflags & PSL_VM) {
 				signo = vm86_emulate((struct vm86frame *)frame);
 				if (signo == SIGTRAP) {
-					type = T_TRCTRAP;
 					load_dr6(rdr6() | 0x4000);
 					goto user_trctrap_out;
 				}
Index: sys/sys/proc.h
===================================================================
--- sys/sys/proc.h
+++ sys/sys/proc.h
@@ -465,6 +465,7 @@
 #define	TDB_EXIT	0x00000400 /* Exiting LWP indicator for ptrace() */
 #define	TDB_VFORK	0x00000800 /* vfork indicator for ptrace() */
 #define	TDB_FSTP	0x00001000 /* The thread is PT_ATTACH leader */
+#define	TDB_STEP	0x00002000 /* (x86) PSL_T set for PT_STEP */
 
 /*
  * "Private" flags kept in td_pflags: