Add DiscoveryNetworkMonitor implementation

chrome/browser/media/router/discovery/discovery_network_list_wifi_linux.cc

+// Copyright (c) 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/media/router/discovery/discovery_network_list_wifi.h"
+
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+
+#include <linux/wireless.h>
+
+#include "base/files/scoped_file.h"
+#include "base/logging.h"
+#include "net/base/network_interfaces_linux.h"
+
+bool MaybeGetWifiSSID(const char* if_name, std::string* ssid_out) {
+  DCHECK(if_name);

[mark a. foltz, 2017/04/26 22:25:12]

DCHECK(ssid_out)

[btolsch, 2017/05/03 22:03:09]

Done.

+
+  base::ScopedFD ioctl_socket(socket(AF_INET, SOCK_DGRAM, 0));
+  if (!ioctl_socket.is_valid())
+    return false;
+  struct iwreq wreq = {};
+  strncpy(wreq.ifr_name, if_name, IFNAMSIZ - 1);

[mark a. foltz, 2017/04/26 22:25:11]

Is wreq.ifr_name guaranteed to hold IFNAMSIZE bytes?

Consider using base::strlcpy to guarantee null termination of ifr_name.

[btolsch, 2017/05/03 22:03:09]

Yes, wreq.ifr_name is defined as IFNAMSIZ bytes (unless this were defined as a
different value in a different kernel, then we'd be in trouble).  wreq = {};
strncpy(..., IFNAMSIZ - 1) also guarantees null-termination.

+
+  char ssid[IW_ESSID_MAX_SIZE + 1] = {0};
+  std::string maybe_ssid;
+  wreq.u.essid.pointer = ssid;
+  wreq.u.essid.length = IW_ESSID_MAX_SIZE;
+  if (ioctl(ioctl_socket.get(), SIOCGIWESSID, &wreq) != -1)
+    maybe_ssid.assign(ssid);

[mark a. foltz, 2017/04/26 22:25:12]

Are you certain ssid will be null terminated?

[btolsch, 2017/05/03 22:03:09]

Yes.  |ssid| above is defined with an extra byte which the ioctl won't touch,
unless you think I should double-defend and check that the kernel didn't write
beyond the length I gave it.

+
+  if (maybe_ssid.size() > 0) {
+    *ssid_out = std::move(maybe_ssid);

[mark a. foltz, 2017/04/26 22:25:12]

Can you just assign ssid to ssid_out directly?

[btolsch, 2017/05/03 22:03:09]

D'oh, bad inlining by me.  Done.

+    return true;
+  }
+  return false;
+}