runtime: windows/arm fix tracebacks printed from sigpanic

The exception handler modifies the stack and continuation context so
it looks like the faulting code calls sigpanic() directly. The call was
not set up correctly on ARM, because it did not handle the link register
correctly. This change handles the link register correctly for ARM.

Updates #28854

Change-Id: I7ccf838adfc05cd968a5edd7d19ebba6a2478360
Reviewed-on: https://go-review.googlesource.com/c/150957
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/defs_windows_386.go

@@ -105,10 +105,11 @@ func (c *context) ip() uintptr { return uintptr(c.eip) }
 func (c *context) sp() uintptr { return uintptr(c.esp) }
 
 // 386 does not have link register, so this returns 0.
-func (c *context) lr() uintptr { return 0 }
+func (c *context) lr() uintptr      { return 0 }
+func (c *context) set_lr(x uintptr) {}
 
-func (c *context) setip(x uintptr) { c.eip = uint32(x) }
-func (c *context) setsp(x uintptr) { c.esp = uint32(x) }
+func (c *context) set_ip(x uintptr) { c.eip = uint32(x) }
+func (c *context) set_sp(x uintptr) { c.esp = uint32(x) }
 
 func dumpregs(r *context) {
 	print("eax     ", hex(r.eax), "\n")

src/runtime/defs_windows_amd64.go

@@ -120,10 +120,11 @@ func (c *context) ip() uintptr { return uintptr(c.rip) }
 func (c *context) sp() uintptr { return uintptr(c.rsp) }
 
 // Amd64 does not have link register, so this returns 0.
-func (c *context) lr() uintptr { return 0 }
+func (c *context) lr() uintptr      { return 0 }
+func (c *context) set_lr(x uintptr) {}
 
-func (c *context) setip(x uintptr) { c.rip = uint64(x) }
-func (c *context) setsp(x uintptr) { c.rsp = uint64(x) }
+func (c *context) set_ip(x uintptr) { c.rip = uint64(x) }
+func (c *context) set_sp(x uintptr) { c.rsp = uint64(x) }
 
 func dumpregs(r *context) {
 	print("rax     ", hex(r.rax), "\n")

src/runtime/defs_windows_arm.go

@@ -104,8 +104,9 @@ func (c *context) ip() uintptr { return uintptr(c.pc) }
 func (c *context) sp() uintptr { return uintptr(c.spr) }
 func (c *context) lr() uintptr { return uintptr(c.lrr) }
 
-func (c *context) setip(x uintptr) { c.pc = uint32(x) }
-func (c *context) setsp(x uintptr) { c.spr = uint32(x) }
+func (c *context) set_ip(x uintptr) { c.pc = uint32(x) }
+func (c *context) set_sp(x uintptr) { c.spr = uint32(x) }
+func (c *context) set_lr(x uintptr) { c.lrr = uint32(x) }
 
 func dumpregs(r *context) {
 	print("r0   ", hex(r.r0), "\n")

src/runtime/signal_windows.go

@@ -117,10 +117,18 @@ func exceptionhandler(info *exceptionrecord, r *context, gp *g) int32 {
 	if r.ip() != 0 {
 		sp := unsafe.Pointer(r.sp())
 		sp = add(sp, ^(unsafe.Sizeof(uintptr(0)) - 1)) // sp--
-		*((*uintptr)(sp)) = r.ip()
-		r.setsp(uintptr(sp))
+		r.set_sp(uintptr(sp))
+		switch GOARCH {
+		default:
+			panic("unsupported architecture")
+		case "386", "amd64":
+			*((*uintptr)(sp)) = r.ip()
+		case "arm":
+			*((*uintptr)(sp)) = r.lr()
+			r.set_lr(r.ip())
+		}
 	}
-	r.setip(funcPC(sigpanic))
+	r.set_ip(funcPC(sigpanic))
 	return _EXCEPTION_CONTINUE_EXECUTION
 }