Make netstack (//pkg/tcpip) buildable for 32 bit

Doing so involved breaking dependencies between //pkg/tcpip and the rest
of gVisor, which are discouraged anyways.

Tested on the Go branch via:
  gvisor.dev/gvisor/pkg/tcpip/...

Addresses #1446.

PiperOrigin-RevId: 363081778

pkg/sentry/socket/netstack/BUILD

@@ -12,6 +12,7 @@ go_library(
         "provider_vfs2.go",
         "save_restore.go",
         "stack.go",
+        "tun.go",
     ],
     visibility = [
         "//pkg/sentry:internal",
@@ -42,6 +43,7 @@ go_library(
         "//pkg/syserror",
         "//pkg/tcpip",
         "//pkg/tcpip/header",
+        "//pkg/tcpip/link/tun",
         "//pkg/tcpip/network/ipv4",
         "//pkg/tcpip/network/ipv6",
         "//pkg/tcpip/stack",

pkg/sentry/socket/netstack/tun.go

@@ -0,0 +1,51 @@
+// Copyright 2021 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package netstack
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/tcpip/link/tun"
+)
+
+// TUNFlagsToLinux converts a tun.Flags to Linux TUN flags.
+func TUNFlagsToLinux(flags tun.Flags) uint16 {
+	ret := uint16(linux.IFF_NOFILTER)
+	if flags.TAP {
+		ret |= linux.IFF_TAP
+	}
+	if flags.TUN {
+		ret |= linux.IFF_TUN
+	}
+	if flags.NoPacketInfo {
+		ret |= linux.IFF_NO_PI
+	}
+	return ret
+}
+
+// LinuxToTUNFlags converts Linux TUN flags to a tun.Flags.
+func LinuxToTUNFlags(flags uint16) (tun.Flags, error) {
+	// Linux adds IFF_NOFILTER (the same value as IFF_NO_PI unfortunately)
+	// when there is no sk_filter. See __tun_chr_ioctl() in
+	// net/drivers/tun.c.
+	if flags&^uint16(linux.IFF_TUN|linux.IFF_TAP|linux.IFF_NO_PI) != 0 {
+		return tun.Flags{}, syserror.EINVAL
+	}
+	return tun.Flags{
+		TUN:          flags&linux.IFF_TUN != 0,
+		TAP:          flags&linux.IFF_TAP != 0,
+		NoPacketInfo: flags&linux.IFF_NO_PI != 0,
+	}, nil
+}