arm64: Use the fault handler when one is provided

In align_abort() and tag_check_abort(), if we got a fault while in kernel,
do not panic if a fault handler has been provided. We may get such a fault
when trying to read or write userland data, it can at least happen with
_umtx_op() if an unaligned pointer is provided. Instead, just let the
fault handler deal with it.

MFC After: 1 week
Approved by: andrew
Differential Revision: https://reviews.freebsd.org/D58426

Author: cognet

sys/arm64/arm64/trap.c

@@ -217,6 +217,17 @@ align_abort(struct thread *td, struct trapframe *frame, uint64_t esr,
     uint64_t far, int lower)
 {
 	if (!lower) {
+		/*
+		 * Accessing unaligned memory may fault when using atomics.
+		 * Make sure we don't panic if we're doing an unaligned
+		 * access to userland memory, as can happen with _umtx_op()
+		 */
+		if (td->td_intr_nesting_level == 0 &&
+		    td->td_pcb->pcb_onfault != 0) {
+			frame->tf_elr = td->td_pcb->pcb_onfault;
+			return;
+		}
+
 		print_registers(frame);
 		print_gp_register("far", far);
 		printf(" esr: 0x%.16lx\n", esr);
@@ -262,6 +273,14 @@ tag_check_abort(struct thread *td, struct trapframe *frame, uint64_t esr,
 	 * at EL0 and a kernel panic if at EL1.
 	 */
 	if (!lower) {
+		/*
+		 * If we have a fault handler, let it decide what to do.
+		 */
+		if (td->td_intr_nesting_level == 0 &&
+		    td->td_pcb->pcb_onfault != 0) {
+			frame->tf_elr = td->td_pcb->pcb_onfault;
+			return;
+		}
 		print_registers(frame);
 		print_gp_register("far", far);
 		printf(" esr: 0x%.16lx\n", esr);