Index: sys/arch/amd64/include/ptrace.h
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/include/ptrace.h,v
retrieving revision 1.10
diff -u -r1.10 ptrace.h
--- sys/arch/amd64/include/ptrace.h	23 Feb 2017 03:34:22 -0000	1.10
+++ sys/arch/amd64/include/ptrace.h	5 Apr 2017 21:50:52 -0000
@@ -43,6 +43,8 @@
 #define	PT_SETFPREGS		(PT_FIRSTMACH + 4)
 #define	PT_GETDBREGS		(PT_FIRSTMACH + 5)
 #define	PT_SETDBREGS		(PT_FIRSTMACH + 6)
+#define	PT_SETSTEP		(PT_FIRSTMACH + 7)
+#define	PT_CLEARSTEP		(PT_FIRSTMACH + 8)
 
 #define PT_MACHDEP_STRINGS \
 	"PT_STEP", \
@@ -51,7 +53,8 @@
 	"PT_GETFPREGS", \
 	"PT_SETFPREGS", \
 	"PT_GETDBREGS", \
-	"PT_SETDBREGS",
+	"PT_SETSTEP", \
+	"PT_CLEARSTEP",
 
 #include <machine/reg.h>
 #define PTRACE_REG_PC(r)	(r)->regs[_REG_RIP]
Index: sys/kern/sys_ptrace_common.c
===================================================================
RCS file: /cvsroot/src/sys/kern/sys_ptrace_common.c,v
retrieving revision 1.20
diff -u -r1.20 sys_ptrace_common.c
--- sys/kern/sys_ptrace_common.c	29 Mar 2017 22:48:03 -0000	1.20
+++ sys/kern/sys_ptrace_common.c	5 Apr 2017 21:51:22 -0000
@@ -229,6 +229,8 @@
 
 #ifdef PT_STEP
 	case PT_STEP:
+	case PT_SETSTEP:
+	case PT_CLEARSTEP:
 #endif
 	case PT_CONTINUE:
 	case PT_KILL:
@@ -452,6 +454,8 @@
 	case  PT_DUMPCORE:
 #ifdef PT_STEP
 	case  PT_STEP:
+	case  PT_SETSTEP:
+	case  PT_CLEARSTEP:
 #endif
 	case  PT_SET_EVENT_MASK:
 	case  PT_GET_EVENT_MASK:
@@ -532,6 +536,8 @@
 	switch (req) {
 #ifdef PT_STEP
 	case PT_STEP:
+	case PT_SETSTEP:
+	case PT_CLEARSTEP:
 #endif
 	case PT_CONTINUE:
 	case PT_DETACH:
@@ -797,13 +803,18 @@
 		 * the requested thread, and clear it for other threads.
 		 */
 		LIST_FOREACH(lt2, &t->p_lwps, l_sibling) {
-			if (lt != lt2) {
+			if (ISSET(lt2->l_pflag, LP_SINGLESTEP)) {
+				lwp_lock(lt2);
+				process_sstep(lt2, 1);
+				lwp_unlock(lt2);
+			} else if (lt != lt2) {
 				lwp_lock(lt2);
 				process_sstep(lt2, 0);
 				lwp_unlock(lt2);
 			}
 		}
-		error = process_sstep(lt, req == PT_STEP);
+		error = process_sstep(lt,
+		    ISSET(lt->l_pflag, LP_SINGLESTEP) || req == PT_STEP);
 		if (error)
 			break;
 #endif
@@ -818,6 +829,12 @@
 
 			/* not being traced any more */
 			t->p_opptr = NULL;
+
+			/* clear single step */
+			LIST_FOREACH(lt2, &t->p_lwps, l_sibling) {
+				CLR(lt2->l_pflag, LP_SINGLESTEP);
+			}
+			CLR(lt->l_pflag, LP_SINGLESTEP);
 		}
 	sendsig:
 		t->p_fpid = 0;
@@ -862,6 +879,36 @@
 		SET(t->p_slflag, PSL_SYSCALLEMU);
 		break;
 
+#ifdef PT_STEP
+	case  PT_SETSTEP:
+		write = 1;
+
+	case  PT_CLEARSTEP:
+		/* write = 0 done above. */
+
+		tmp = data;
+		if (tmp != 0 && t->p_nlwps > 1) {
+			lwp_delref(lt);
+			mutex_enter(t->p_lock);
+			lt = lwp_find(t, tmp);
+			if (lt == NULL) {
+				mutex_exit(t->p_lock);
+				error = ESRCH;
+				break;
+			}
+			lwp_addref(lt);
+			mutex_exit(t->p_lock);
+		}
+
+		if (ISSET(lt->l_flag, LW_SYSTEM))
+			error = EINVAL;
+		else if (write)
+			SET(lt->l_pflag, LP_SINGLESTEP);
+		else
+			CLR(lt->l_pflag, LP_SINGLESTEP);
+		break;
+#endif
+
 	case  PT_KILL:
 		/* just send the process a KILL signal. */
 		signo = SIGKILL;
Index: sys/sys/lwp.h
===================================================================
RCS file: /cvsroot/src/sys/sys/lwp.h,v
retrieving revision 1.172
diff -u -r1.172 lwp.h
--- sys/sys/lwp.h	3 Jul 2016 14:24:59 -0000	1.172
+++ sys/sys/lwp.h	5 Apr 2017 21:51:22 -0000
@@ -255,6 +255,7 @@
 #define	LP_SYSCTLWRITE	0x00000080 /* sysctl write lock held */
 #define	LP_MUSTJOIN	0x00000100 /* Must join kthread on exit */
 #define	LP_VFORKWAIT	0x00000200 /* Waiting at vfork() for a child */
+#define	LP_SINGLESTEP	0x00000400 /* Single step thread in ptrace(2) */
 #define	LP_TIMEINTR	0x00010000 /* Time this soft interrupt */
 #define	LP_RUNNING	0x20000000 /* Active on a CPU */
 #define	LP_BOUND	0x80000000 /* Bound to a CPU */
Index: tests/lib/libc/sys/t_ptrace_wait.c
===================================================================
RCS file: /cvsroot/src/tests/lib/libc/sys/t_ptrace_wait.c,v
retrieving revision 1.1
diff -u -r1.1 t_ptrace_wait.c
--- tests/lib/libc/sys/t_ptrace_wait.c	2 Apr 2017 21:44:00 -0000	1.1
+++ tests/lib/libc/sys/t_ptrace_wait.c	5 Apr 2017 21:51:23 -0000
@@ -4396,7 +4396,7 @@
 
 #if defined(PT_STEP)
 static void
-ptrace_step(int N)
+ptrace_step(int N, int setstep)
 {
 	const int exitval = 5;
 	const int sigval = SIGSTOP;
@@ -4435,15 +4435,32 @@
 	validate_status_stopped(status, sigval);
 
 	while (N --> 0) {
-		printf("Before resuming the child process where it left off "
-		    "and without signal to be sent (use PT_STEP)\n");
-		ATF_REQUIRE(ptrace(PT_STEP, child, (void *)1, 0) != -1);
+		if (setstep) {
+			printf("Before resuming the child process where it "
+			    "left off and without signal to be sent (use "
+			    "PT_STEP)\n");
+			ATF_REQUIRE(ptrace(PT_SETSTEP, child, (void *)1, 0)
+			    != -1);
+			ATF_REQUIRE(ptrace(PT_CONTINUE, child, (void *)1, 0)
+			    != -1);
+		} else {
+			printf("Before resuming the child process where it "
+			    "left off and without signal to be sent (use "
+			    "PT_STEP)\n");
+			ATF_REQUIRE(ptrace(PT_STEP, child, (void *)1, 0)
+			    != -1);
+		}
 
 		printf("Before calling %s() for the child\n", TWAIT_FNAME);
 		TWAIT_REQUIRE_SUCCESS(wpid = TWAIT_GENERIC(child, &status, 0),
 		    child);
 
 		validate_status_stopped(status, SIGTRAP);
+
+		if (setstep) {
+			ATF_REQUIRE(ptrace(PT_CLEARSTEP, child, (void *)1, 0)
+			    != -1);
+		}
 	}
 
 	printf("Before resuming the child process where it left off and "
@@ -4470,7 +4487,7 @@
 
 ATF_TC_BODY(step1, tc)
 {
-	ptrace_step(1);
+	ptrace_step(1, 0);
 }
 #endif
 
@@ -4484,7 +4501,7 @@
 
 ATF_TC_BODY(step2, tc)
 {
-	ptrace_step(2);
+	ptrace_step(2, 0);
 }
 #endif
 
@@ -4498,7 +4515,7 @@
 
 ATF_TC_BODY(step3, tc)
 {
-	ptrace_step(3);
+	ptrace_step(3, 0);
 }
 #endif
 
@@ -4512,7 +4529,63 @@
 
 ATF_TC_BODY(step4, tc)
 {
-	ptrace_step(3);
+	ptrace_step(4, 0);
+}
+#endif
+
+#if defined(PT_STEP)
+ATF_TC(setstep1);
+ATF_TC_HEAD(setstep1, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Verify single PT_SETSTEP call");
+}
+
+ATF_TC_BODY(setstep1, tc)
+{
+	ptrace_step(1, 1);
+}
+#endif
+
+#if defined(PT_STEP)
+ATF_TC(setstep2);
+ATF_TC_HEAD(setstep2, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Verify PT_SETSTEP called twice");
+}
+
+ATF_TC_BODY(setstep2, tc)
+{
+	ptrace_step(2, 1);
+}
+#endif
+
+#if defined(PT_STEP)
+ATF_TC(setstep3);
+ATF_TC_HEAD(setstep3, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Verify PT_SETSTEP called three times");
+}
+
+ATF_TC_BODY(setstep3, tc)
+{
+	ptrace_step(3, 1);
+}
+#endif
+
+#if defined(PT_STEP)
+ATF_TC(setstep4);
+ATF_TC_HEAD(setstep4, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Verify PT_SETSTEP called four times");
+}
+
+ATF_TC_BODY(setstep4, tc)
+{
+	ptrace_step(4, 1);
 }
 #endif
 
@@ -7535,6 +7608,11 @@
 	ATF_TP_ADD_TC_PT_STEP(tp, step3);
 	ATF_TP_ADD_TC_PT_STEP(tp, step4);
 
+	ATF_TP_ADD_TC_PT_STEP(tp, setstep1);
+	ATF_TP_ADD_TC_PT_STEP(tp, setstep2);
+	ATF_TP_ADD_TC_PT_STEP(tp, setstep3);
+	ATF_TP_ADD_TC_PT_STEP(tp, setstep4);
+
 	ATF_TP_ADD_TC(tp, kill1);
 	ATF_TP_ADD_TC(tp, kill2);