Add BTI exceptions

We could hit these when executing code marked as using BTI but jumps
to a non-branch target instruction.

Sponsored by:	Arm Ltd
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39450

sys/arm64/arm64/locore.S

@@ -864,7 +864,7 @@ sctlr_set:
 	.quad (SCTLR_LSMAOE | SCTLR_nTLSMD | SCTLR_UCI | SCTLR_SPAN | \
 	    SCTLR_nTWE | SCTLR_nTWI | SCTLR_UCT | SCTLR_DZE | \
 	    SCTLR_I | SCTLR_SED | SCTLR_SA0 | SCTLR_SA | SCTLR_C | \
-	    SCTLR_M | SCTLR_CP15BEN)
+	    SCTLR_M | SCTLR_CP15BEN | SCTLR_BT1 | SCTLR_BT0)
 sctlr_clear:
 	/* Bits to clear */
 	.quad (SCTLR_EE | SCTLR_E0E | SCTLR_IESB | SCTLR_WXN | SCTLR_UMA | \

sys/arm64/arm64/trap.c

@@ -559,6 +559,11 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 		panic("Undefined instruction: %08x",
 		    *(uint32_t *)frame->tf_elr);
 		break;
+	case EXCP_BTI:
+		print_registers(frame);
+		print_gp_register("far", far);
+		panic("Branch Target exception");
+		break;
 	default:
 		print_registers(frame);
 		print_gp_register("far", far);
@@ -699,6 +704,11 @@ do_el0_sync(struct thread *td, struct trapframe *frame)
 		    (void *)frame->tf_elr, exception);
 		userret(td, frame);
 		break;
+	case EXCP_BTI:
+		call_trapsignal(td, SIGILL, ILL_ILLOPC, (void *)frame->tf_elr,
+		    exception);
+		userret(td, frame);
+		break;
 	default:
 		call_trapsignal(td, SIGBUS, BUS_OBJERR, (void *)frame->tf_elr,
 		    exception);

sys/arm64/include/armreg.h

@@ -408,6 +408,7 @@
 #define	 EXCP_UNKNOWN		0x00	/* Unkwn exception */
 #define	 EXCP_TRAP_WFI_WFE	0x01	/* Trapped WFI or WFE */
 #define	 EXCP_FP_SIMD		0x07	/* VFP/SIMD trap */
+#define	 EXCP_BTI		0x0d	/* Branch Target Exception */
 #define	 EXCP_ILL_STATE		0x0e	/* Illegal execution state */
 #define	 EXCP_SVC32		0x11	/* SVC trap for AArch32 */
 #define	 EXCP_SVC64		0x15	/* SVC trap for AArch64 */