unix: align Ifreq so its union accessors cannot fault

unix/ifreq_linux.go

@@ -22,7 +22,13 @@ import (
 // fields can be get and set using the following methods:
 //   - Uint16/SetUint16: flags
 //   - Uint32/SetUint32: ifindex, metric, mtu
-type Ifreq struct{ raw ifreq }
+type Ifreq struct {
+	// Aligns the union for the accessors below, which cast it in place;
+	// the generated ifreq is all byte arrays, so its alignment is one.
+	_ [0]int64
+
+	raw ifreq
+}
 
 // NewIfreq creates an Ifreq with the input network interface name after
 // validating the name does not exceed IFNAMSIZ-1 (trailing NULL required)

unix/ifreq_linux_test.go

@@ -172,3 +172,18 @@ func testIfreq(t *testing.T) *Ifreq {
 
 	return ifr
 }
+
+func TestIfreqAlignment(t *testing.T) {
+	// The accessors cast the union in place, so it must be aligned for the
+	// widest type they use. Merely slow on amd64; a fault on sparc64.
+	want := unsafe.Alignof(uint32(0))
+	if a := unsafe.Alignof(RawSockaddrInet4{}); a > want {
+		want = a
+	}
+	if got := unsafe.Alignof(Ifreq{}); got%want != 0 {
+		t.Errorf("unsafe.Alignof(Ifreq{}) = %d, want a multiple of %d", got, want)
+	}
+	if off := unsafe.Offsetof(Ifreq{}.raw) + unsafe.Offsetof(ifreq{}.Ifru); off%want != 0 {
+		t.Errorf("union lies at offset %d within Ifreq, want a multiple of %d", off, want)
+	}
+}