Ensure native library isn't actually loaded, and no IO on net thread

net/udp/udp_socket_posix.cc

 // Copyright (c) 2012 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 "net/udp/udp_socket_posix.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <net/if.h>
 #include <netdb.h>
 #include <netinet/in.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
 
 #include "base/callback.h"
 #include "base/debug/alias.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/rand_util.h"
+#include "base/threading/thread_restrictions.h"
 #include "base/trace_event/trace_event.h"
 #include "net/base/io_buffer.h"
 #include "net/base/ip_address.h"
 #include "net/base/ip_endpoint.h"
 #include "net/base/net_errors.h"
 #include "net/base/network_activity_monitor.h"
 #include "net/base/sockaddr_storage.h"
 #include "net/log/net_log.h"
 #include "net/socket/socket_descriptor.h"
 #include "net/udp/udp_net_log_parameters.h"
@@ skipping 311 matching lines @@
   // NOTE(pauljensen): This does rely on Android implementation details, but
   // these details are unlikely to change.
   typedef int (*SetNetworkForSocket)(unsigned netId, int socketFd);
   static SetNetworkForSocket setNetworkForSocket;
   // This is racy, but all racers should come out with the same answer so it
   // shouldn't matter.
   if (setNetworkForSocket == nullptr) {
     // Android's netd client library should always be loaded in our address
     // space as it shims libc functions like connect().
     base::FilePath file(base::FilePath::FromUTF16Unsafe(
         base::GetNativeLibraryName(base::ASCIIToUTF16("netd_client"))));

[xunjieli, 2016/06/27 18:54:16]

If we already have netd_client loaded in Chromium's address space, why do we
need to load it here? In other words, when will setNetworkForSocket == nullptr?

[pauljensen, 2016/06/27 19:07:10]

setNetworkForSocket is nullptr when the executable starts executing.  It's set
by line 361 below.

libnetd_client.so will be loaded indirectly by the executable calling socket()
in UDPSocketPosix::Open().
See how socket() in libc.so calls into libnetd_client.so:
http://androidxref.com/6.0.1_r10/xref/bionic/libc/bionic/socket.cpp#22
http://androidxref.com/6.0.1_r10/xref/bionic/libc/bionic/NetdClient.cpp#37

I just need to get the base::NativeLibrary pointer here so I can pass it to
base::GetFunctionPointerFromNativeLibrary().  Normally dlopen() goes and loads
the library, but then the library is already loaded it just gives back the
handle to it.  dlopen() man page says "If  the same library is loaded again with
dlopen(), the same file handle is returned."  I've verified this by reading
Android's dlopen code and also I wrote an example and strace'd it to verify, see
my notes in the linked bug.

+    // Disable IO assertion in LoadNativeLibrary as the library will already
+    // be loaded by the socket() call (which goes through libnetd_client) that
+    // created |socket_|, so no IO will actually be performed. crbug.com/623555
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
     base::NativeLibrary lib = base::LoadNativeLibrary(file, nullptr);
     setNetworkForSocket = reinterpret_cast<SetNetworkForSocket>(
         base::GetFunctionPointerFromNativeLibrary(lib, "setNetworkForSocket"));
   }
   if (setNetworkForSocket == nullptr)
     return ERR_NOT_IMPLEMENTED;
   int rv = setNetworkForSocket(network, socket_);
   // If |network| has since disconnected, |rv| will be ENONET.  Surface this as
   // ERR_NETWORK_CHANGED, rather than MapSystemError(ENONET) which gives back
   // the less descriptive ERR_FAILED.
@@ skipping 438 matching lines @@
     return MapSystemError(errno);
 
   return OK;
 }
 
 void UDPSocketPosix::DetachFromThread() {
   base::NonThreadSafe::DetachFromThread();
 }
 
 }  // namespace net