Index: sys/gdb/gdb_int.h
===================================================================
--- sys/gdb/gdb_int.h
+++ sys/gdb/gdb_int.h
@@ -130,6 +130,13 @@
 		*gdb_txp++ = *s++;
 }
 
+static __inline void
+gdb_tx_hexstr(const char *s)
+{
+	while (*s)
+		gdb_tx_hex(*s++, 2);
+}
+
 static __inline void
 gdb_tx_varhex(uintmax_t n)
 {
Index: sys/gdb/gdb_main.c
===================================================================
--- sys/gdb/gdb_main.c
+++ sys/gdb/gdb_main.c
@@ -47,6 +47,10 @@
 SYSCTL_NODE(_debug, OID_AUTO, gdb, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "GDB settings");
 
+static bool gdb_can_resume = false;
+SYSCTL_BOOL(_debug_gdb, OID_AUTO, can_resume, CTLFLAG_RWTUN, &gdb_can_resume,
+    0, "GDB can resume execution");
+
 static dbbe_init_f gdb_init;
 static dbbe_trap_f gdb_trap;
 
@@ -608,7 +612,7 @@
 		gdb_cur->gdb_term();
 
 #ifdef DDB
-	if (!gdb_return_to_ddb)
+	if (!gdb_return_to_ddb && gdb_can_resume)
 		return;
 
 	gdb_return_to_ddb = false;
@@ -712,6 +716,48 @@
 	return;
 }
 
+/*
+ * Send a T packet.  We currently do not support watchpoints (the awatch,
+ * rwatch, or watch elements).
+ */
+static void
+gdb_tx_stop(int type, int code, bool full)
+{
+	gdb_tx_begin('T');
+	gdb_tx_hex(gdb_cpu_signal(type, code), 2);
+	if (full) {
+		gdb_tx_varhex(GDB_REG_PC);
+		gdb_tx_char(':');
+		gdb_tx_reg(GDB_REG_PC);
+		gdb_tx_char(';');
+		gdb_cpu_stop_reason(type, code);
+	}
+	gdb_tx_str("thread:");
+	gdb_tx_varhex((long)kdb_thread->td_tid);
+	gdb_tx_char(';');
+	gdb_tx_end();			/* XXX check error condition. */
+}
+
+static bool
+gdb_allow_resume(int type, int code)
+{
+	if (gdb_can_resume)
+		return (true);
+
+	gdb_tx_begin('O');
+	gdb_tx_hexstr("GDB is not allowed to resume execution.\n");
+	gdb_tx_hexstr("Detach and use DDB, or: set gdb_can_resume = 1\n");
+	gdb_tx_end();
+
+	/*
+	 * A strict reading of the protocol spec does not allow an error reply.
+	 * Tell GDB the target stopped in the same way as last time.
+	 */
+	gdb_tx_stop(type, code, true);
+
+	return (false);
+}
+
 static int
 gdb_trap(int type, int code)
 {
@@ -731,35 +777,18 @@
 	gdb_listening = 0;
 	gdb_ackmode = true;
 
-	/*
-	 * Send a T packet. We currently do not support watchpoints (the
-	 * awatch, rwatch or watch elements).
-	 */
-	gdb_tx_begin('T');
-	gdb_tx_hex(gdb_cpu_signal(type, code), 2);
-	gdb_tx_varhex(GDB_REG_PC);
-	gdb_tx_char(':');
-	gdb_tx_reg(GDB_REG_PC);
-	gdb_tx_char(';');
-	gdb_cpu_stop_reason(type, code);
-	gdb_tx_str("thread:");
-	gdb_tx_varhex((uintmax_t)kdb_thread->td_tid);
-	gdb_tx_char(';');
-	gdb_tx_end();			/* XXX check error condition. */
+	gdb_tx_stop(type, code, true);
 
 	thr_iter = NULL;
 	while (gdb_rx_begin() == 0) {
 		/* printf("GDB: got '%s'\n", gdb_rxp); */
 		switch (gdb_rx_char()) {
 		case '?':	/* Last signal. */
-			gdb_tx_begin('T');
-			gdb_tx_hex(gdb_cpu_signal(type, code), 2);
-			gdb_tx_str("thread:");
-			gdb_tx_varhex((long)kdb_thread->td_tid);
-			gdb_tx_char(';');
-			gdb_tx_end();
+			gdb_tx_stop(type, code, false);
 			break;
 		case 'c': {	/* Continue. */
+			if (!gdb_allow_resume(type, code))
+				break;
 			uintmax_t addr;
 			register_t pc;
 			if (!gdb_rx_varhex(&addr)) {
@@ -771,6 +800,8 @@
 			return (1);
 		}
 		case 'C': {	/* Continue with signal. */
+			if (!gdb_allow_resume(type, code))
+				break;
 			uintmax_t addr, sig;
 			register_t pc;
 			if (!gdb_rx_varhex(&sig) && gdb_rx_char() == ';' &&
@@ -929,6 +960,8 @@
 				gdb_tx_empty();
 			break;
 		case 's': {	/* Step. */
+			if (!gdb_allow_resume(type, code))
+				break;
 			uintmax_t addr;
 			register_t pc;
 			if (!gdb_rx_varhex(&addr)) {
@@ -940,6 +973,8 @@
 			return (1);
 		}
 		case 'S': {	/* Step with signal. */
+			if (!gdb_allow_resume(type, code))
+				break;
 			uintmax_t addr, sig;
 			register_t pc;
 			if (!gdb_rx_varhex(&sig) && gdb_rx_char() == ';' &&