runtime: add test for mincore's return value sign on Linux

Updates #14297

Change-Id: I6b5f5020af5efaaa71280bdeb2ff99785ee9b959
Reviewed-on: https://go-review.googlesource.com/19457
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/export_test.go

@@ -28,6 +28,8 @@ var Exitsyscall = exitsyscall
 var LockedOSThread = lockedOSThread
 var Xadduintptr = atomic.Xadduintptr
 
+var Mincore = mincore
+
 var FuncPC = funcPC
 
 var Fastlog2 = fastlog2

src/runtime/runtime_linux_test.go

@@ -8,6 +8,7 @@ import (
 	. "runtime"
 	"syscall"
 	"testing"
+	"unsafe"
 )
 
 var pid, tid int
@@ -27,3 +28,15 @@ func TestLockOSThread(t *testing.T) {
 		t.Fatalf("pid=%d but tid=%d", pid, tid)
 	}
 }
+
+// Test that error values are negative. Use address 1 (a misaligned
+// pointer) to get -EINVAL.
+func TestMincoreErrorSign(t *testing.T) {
+	var dst byte
+	v := Mincore(unsafe.Pointer(uintptr(1)), 1, &dst)
+
+	const EINVAL = 0x16
+	if v != -EINVAL {
+		t.Errorf("mincore = %v, want %v", v, -EINVAL)
+	}
+}