runtime: fix unsafe.Pointer alignment on Linux

Caught by go test -a -short -gcflags=all=-d=checkptr runtime

TestMincoreErrorSign intentionally uses uintptr(1) to get -EINVAL,
but it violates unsafe pointer rules 2. So use another misaligned
pointer add(new(int32), 1), but do not violate unsafe pointer rules.

TestEpollctlErrorSign passes an unsafe.Pointer of &struct{}{} to
Epollctl, which is then casted to epollevent, causes mis-alignment.
Fixing it by exporting epollevent on runtime_test package, so it can be
passed to Epollctl.

Updates #34972

Change-Id: I78ebfbeaf706fd1d372272af0bbc4e2cabca4631
Reviewed-on: https://go-review.googlesource.com/c/go/+/202157
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/export_linux_test.go

@@ -10,6 +10,9 @@ import "unsafe"
 
 var NewOSProc0 = newosproc0
 var Mincore = mincore
+var Add = add
+
+type EpollEvent epollevent
 
 func Epollctl(epfd, op, fd int32, ev unsafe.Pointer) int32 {
 	return epollctl(epfd, op, fd, (*epollevent)(ev))

src/runtime/runtime_linux_test.go

@@ -41,11 +41,11 @@ func TestLockOSThread(t *testing.T) {
 	}
 }
 
-// Test that error values are negative. Use address 1 (a misaligned
-// pointer) to get -EINVAL.
+// Test that error values are negative.
+// Use a misaligned pointer to get -EINVAL.
 func TestMincoreErrorSign(t *testing.T) {
 	var dst byte
-	v := Mincore(unsafe.Pointer(uintptr(1)), 1, &dst)
+	v := Mincore(Add(unsafe.Pointer(new(int32)), 1), 1, &dst)
 
 	const EINVAL = 0x16
 	if v != -EINVAL {
@@ -54,7 +54,7 @@ func TestMincoreErrorSign(t *testing.T) {
 }
 
 func TestEpollctlErrorSign(t *testing.T) {
-	v := Epollctl(-1, 1, -1, unsafe.Pointer(&struct{}{}))
+	v := Epollctl(-1, 1, -1, unsafe.Pointer(&EpollEvent{}))
 
 	const EBADF = 0x09
 	if v != -EBADF {