unix: add support for OpenBSD unveil(2)

Now as `SYS_UNVEIL` has landed in `x/sys`, it's time to have a convenient handle for it too.

Change-Id: I61db9cc23fa9f2e10bac601959002319ea9c854b
GitHub-Last-Rev: eb9487242e1fafa06c057ef49444ea5a711382df
GitHub-Pull-Request: golang/sys#19
Reviewed-on: https://go-review.googlesource.com/c/142317Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/openbsd_unveil.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build openbsd
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// Unveil implements the unveil syscall.
+// For more information see unveil(2).
+// Note that the special case of blocking further
+// unveil calls is handled by UnveilBlock.
+func Unveil(path string, flags string) error {
+	pathPtr, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	flagsPtr, err := syscall.BytePtrFromString(flags)
+	if err != nil {
+		return err
+	}
+	_, _, e := syscall.Syscall(SYS_UNVEIL, uintptr(unsafe.Pointer(pathPtr)), uintptr(unsafe.Pointer(flagsPtr)), 0)
+	if e != 0 {
+		return e
+	}
+	return nil
+}
+
+// UnveilBlock blocks future unveil calls.
+// For more information see unveil(2).
+func UnveilBlock() error {
+	// Both pointers must be nil.
+	var pathUnsafe, flagsUnsafe unsafe.Pointer
+	_, _, e := syscall.Syscall(SYS_UNVEIL, uintptr(pathUnsafe), uintptr(flagsUnsafe), 0)
+	if e != 0 {
+		return e
+	}
+	return nil
+}