unix: export Ifreq and add IoctlIfreq function

This CL expands upon CL 340369 by exporting the Ifreq type along with methods
for setting and getting data to/from the ifreq union in a type-safe way.
ifreqData remains unexported as we can keep adding helpers similar to the
IoctlGetEthtoolDrvinfo to expose those operations in a less error-prone way.

A test is also added to verify interface index data using IoctlIfreq against
the modern rtnetlink API used by the standard library.

Change-Id: Ic6980cbcd3792cc341cd614061cce32fa1f851e7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/340370
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/ifreq_linux_test.go

@@ -0,0 +1,142 @@
+// Copyright 2021 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.
+
+//go:build linux
+// +build linux
+
+package unix
+
+import (
+	"testing"
+	"unsafe"
+)
+
+// An ifreqUnion is shorthand for a byte array matching the
+// architecture-dependent size of an ifreq's union field.
+type ifreqUnion = [len(ifreq{}.Ifru)]byte
+
+func TestNewIfreq(t *testing.T) {
+	// Interface name too long.
+	if _, err := NewIfreq("abcdefghijklmnop"); err != EINVAL {
+		t.Fatalf("expected error EINVAL, but got: %v", err)
+	}
+}
+
+func TestIfreqSize(t *testing.T) {
+	// Ensure ifreq (generated) and Ifreq/ifreqData (hand-written to create a
+	// safe wrapper and store a pointer field) are identical in size.
+	want := unsafe.Sizeof(ifreq{})
+	if got := unsafe.Sizeof(Ifreq{}); want != got {
+		t.Fatalf("unexpected Ifreq size: got: %d, want: %d", got, want)
+	}
+
+	if got := unsafe.Sizeof(ifreqData{}); want != got {
+		t.Fatalf("unexpected IfreqData size: got: %d, want: %d", got, want)
+	}
+}
+
+func TestIfreqName(t *testing.T) {
+	// Invalid ifreq (no NULL terminator), so expect empty string.
+	var name [IFNAMSIZ]byte
+	for i := range name {
+		name[i] = 0xff
+	}
+
+	bad := &Ifreq{raw: ifreq{Ifrn: name}}
+	if got := bad.Name(); got != "" {
+		t.Fatalf("expected empty ifreq name, but got: %q", got)
+	}
+
+	// Valid ifreq, expect the hard-coded testIfreq name.
+	ifr := testIfreq(t)
+	if want, got := ifreqName, ifr.Name(); want != got {
+		t.Fatalf("unexpected ifreq name: got: %q, want: %q", got, want)
+	}
+}
+
+func TestIfreqWithData(t *testing.T) {
+	ifr := testIfreq(t)
+
+	// Store pointer data in the ifreq so we can retrieve it and cast back later
+	// for comparison.
+	want := [5]byte{'h', 'e', 'l', 'l', 'o'}
+	ifrd := ifr.withData(unsafe.Pointer(&want[0]))
+
+	// Ensure the memory of the original Ifreq was not modified by SetData.
+	if ifr.raw.Ifru != (ifreqUnion{}) {
+		t.Fatalf("ifreq was unexpectedly modified: % #x", ifr.raw.Ifru)
+	}
+
+	got := *(*[5]byte)(ifrd.data)
+	if want != got {
+		t.Fatalf("unexpected ifreq data bytes:\n got: % #x\nwant: % #x", got, want)
+	}
+}
+
+func TestIfreqUint16(t *testing.T) {
+	ifr := testIfreq(t)
+	const in = 0x0102
+	ifr.SetUint16(in)
+
+	// The layout of the bytes depends on the machine's endianness.
+	var want ifreqUnion
+	if isBigEndian {
+		want[0] = 0x01
+		want[1] = 0x02
+	} else {
+		want[0] = 0x02
+		want[1] = 0x01
+	}
+
+	if got := ifr.raw.Ifru; want != got {
+		t.Fatalf("unexpected ifreq uint16 bytes:\n got: % #x\nwant: % #x", got, want)
+	}
+
+	if got := ifr.Uint16(); in != got {
+		t.Fatalf("unexpected ifreq uint16: got: %d, want: %d", got, in)
+	}
+}
+
+func TestIfreqUint32(t *testing.T) {
+	ifr := testIfreq(t)
+	const in = 0x01020304
+	ifr.SetUint32(in)
+
+	// The layout of the bytes depends on the machine's endianness.
+	var want ifreqUnion
+	if isBigEndian {
+		want[0] = 0x01
+		want[1] = 0x02
+		want[2] = 0x03
+		want[3] = 0x04
+	} else {
+		want[0] = 0x04
+		want[1] = 0x03
+		want[2] = 0x02
+		want[3] = 0x01
+	}
+
+	if got := ifr.raw.Ifru; want != got {
+		t.Fatalf("unexpected ifreq uint32 bytes:\n got: % #x\nwant: % #x", got, want)
+	}
+
+	if got := ifr.Uint32(); in != got {
+		t.Fatalf("unexpected ifreq uint32: got: %d, want: %d", got, in)
+	}
+}
+
+// ifreqName is a hard-coded name for testIfreq.
+const ifreqName = "eth0"
+
+// testIfreq returns an Ifreq with a populated interface name.
+func testIfreq(t *testing.T) *Ifreq {
+	t.Helper()
+
+	ifr, err := NewIfreq(ifreqName)
+	if err != nil {
+		t.Fatalf("failed to create ifreq: %v", err)
+	}
+
+	return ifr
+}