Index: head/lib/libc/sys/ptrace.2
===================================================================
--- head/lib/libc/sys/ptrace.2
+++ head/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 August 29, 2016
+.Dd June 11, 2017
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -643,6 +643,26 @@
 .Fa data
 arguments are used the same as for
 .Dv PT_CONTINUE.
+.It Dv PT_GET_SC_ARGS
+For the thread which is stopped in either
+.Dv PL_FLAG_SCE
+or
+.Dv PL_FLAG_SCX
+state, that is, on entry or exit to a syscall,
+this request fetches the syscall arguments.
+.Pp
+The arguments are copied out into the buffer pointed to by the
+.Fa addr
+pointer, sequentially.
+Each syscall argument is stored as the machine word.
+Kernel copies out as many arguments as the syscall accepts,
+see the
+.Va pl_syscall_narg
+member of the
+.Vt struct ptrace_lwpinfo ,
+but not more than the
+.Fa data
+bytes in total are copied.
 .It Dv PT_FOLLOW_FORK
 This request controls tracing for new child processes of a traced process.
 If
Index: head/sys/kern/sys_process.c
===================================================================
--- head/sys/kern/sys_process.c
+++ head/sys/kern/sys_process.c
@@ -586,6 +586,7 @@
 		struct ptrace_lwpinfo32 pl32;
 		struct ptrace_vm_entry32 pve32;
 #endif
+		char args[nitems(td->td_sa.args) * sizeof(register_t)];
 		int ptevents;
 	} r;
 	void *addr;
@@ -606,6 +607,7 @@
 	case PT_GETFPREGS:
 	case PT_GETDBREGS:
 	case PT_LWPINFO:
+	case PT_GET_SC_ARGS:
 		break;
 	case PT_SETREGS:
 		error = COPYIN(uap->addr, &r.reg, sizeof r.reg);
@@ -663,6 +665,10 @@
 		/* NB: The size in uap->data is validated in kern_ptrace(). */
 		error = copyout(&r.pl, uap->addr, uap->data);
 		break;
+	case PT_GET_SC_ARGS:
+		error = copyout(r.args, uap->addr, MIN(uap->data,
+		    sizeof(r.args)));
+		break;
 	}
 
 	return (error);
@@ -739,6 +745,7 @@
 	case PT_GET_EVENT_MASK:
 	case PT_SET_EVENT_MASK:
 	case PT_DETACH:
+	case PT_GET_SC_ARGS:
 		sx_xlock(&proctree_lock);
 		proctree_locked = 1;
 		break;
@@ -1008,6 +1015,28 @@
 		CTR3(KTR_PTRACE, "PT_SET_EVENT_MASK: pid %d mask %#x -> %#x",
 		    p->p_pid, p->p_ptevents, tmp);
 		p->p_ptevents = tmp;
+		break;
+
+	case PT_GET_SC_ARGS:
+		CTR1(KTR_PTRACE, "PT_GET_SC_ARGS: pid %d", p->p_pid);
+		if ((td2->td_dbgflags & (TDB_SCE | TDB_SCX)) == 0
+#ifdef COMPAT_FREEBSD32
+		    || (wrap32 && !safe)
+#endif
+		    ) {
+			error = EINVAL;
+			break;
+		}
+		bzero(addr, sizeof(td2->td_sa.args));
+#ifdef COMPAT_FREEBSD32
+		if (wrap32)
+			for (num = 0; num < nitems(td2->td_sa.args); num++)
+				((uint32_t *)addr)[num] = (uint32_t)
+				    td2->td_sa.args[num];
+		else
+#endif
+			bcopy(td2->td_sa.args, addr, td2->td_sa.narg *
+			    sizeof(register_t));
 		break;
 		
 	case PT_STEP:
Index: head/sys/sys/ptrace.h
===================================================================
--- head/sys/sys/ptrace.h
+++ head/sys/sys/ptrace.h
@@ -69,6 +69,8 @@
 #define	PT_GET_EVENT_MASK 25	/* get mask of optional events */
 #define	PT_SET_EVENT_MASK 26	/* set mask of optional events */
 
+#define	PT_GET_SC_ARGS	27	/* fetch syscall args */
+
 #define PT_GETREGS      33	/* get general-purpose registers */
 #define PT_SETREGS      34	/* set general-purpose registers */
 #define PT_GETFPREGS    35	/* get floating-point registers */