runtime: fix abort handling on arm64

The implementation of runtime.abort on arm64 currently branches to
address 0, which results in a signal from PC 0, rather than from
runtime.abort, so the runtime fails to recognize it as an abort.

Fix runtime.abort on arm64 to read from address 0 like what other
architectures do and recognize this in the signal handler.

Should fix the linux/arm64 build.

Change-Id: I960ab630daaeadc9190287604d4d8337b1ea3853
Reviewed-on: https://go-review.googlesource.com/99895
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/asm_arm64.s

@@ -710,7 +710,8 @@ TEXT runtime·getcallerpc(SB),NOSPLIT|NOFRAME,$0-8
 	RET
 
 TEXT runtime·abort(SB),NOSPLIT|NOFRAME,$0-0
-	B	(ZR)
+	MOVD	ZR, R0
+	MOVD	(R0), R0
 	UNDEF
 
 TEXT runtime·return0(SB), NOSPLIT, $0

src/runtime/os3_plan9.go

@@ -35,7 +35,7 @@ func sighandler(_ureg *ureg, note *byte, gp *g) int {
 		print("sighandler: note is longer than ERRMAX\n")
 		goto Throw
 	}
-	if c.pc() == funcPC(abort) || (GOARCH == "arm" && c.pc() == funcPC(abort)+4) {
+	if isAbortPC(c.pc()) {
 		// Never turn abort into a panic.
 		goto Throw
 	}

src/runtime/panic.go

@@ -837,3 +837,9 @@ func shouldPushSigpanic(gp *g, pc, lr uintptr) bool {
 	// will work.
 	return true
 }
+
+// isAbortPC returns true if pc is the program counter at which
+// runtime.abort raises a signal.
+func isAbortPC(pc uintptr) bool {
+	return pc == funcPC(abort) || ((GOARCH == "arm" || GOARCH == "arm64") && pc == funcPC(abort)+sys.PCQuantum)
+}

src/runtime/signal_sighandler.go

@@ -43,7 +43,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		// stack. Abort in the signal handler instead.
 		flags = (flags &^ _SigPanic) | _SigThrow
 	}
-	if c.sigpc() == funcPC(abort) || (GOARCH == "arm" && c.sigpc() == funcPC(abort)+4) {
+	if isAbortPC(c.sigpc()) {
 		// On many architectures, the abort function just
 		// causes a memory fault. Don't turn that into a panic.
 		flags = _SigThrow

src/runtime/signal_windows.go

@@ -46,7 +46,7 @@ func isgoexception(info *exceptionrecord, r *context) bool {
 		return false
 	}
 
-	if r.ip() == funcPC(abort) || (GOARCH == "arm" && r.ip() == funcPC(abort)+4) {
+	if isAbortPC(r.ip()) {
 		// Never turn abort into a panic.
 		return false
 	}