Document why we choose SOCK_SEQPACKET.

chrome/browser/zygote_host_linux.cc

 // Copyright (c) 2009 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/zygote_host_linux.h"
 
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
@@ skipping 36 matching lines @@
 
 ZygoteHost::ZygoteHost() {
   std::wstring chrome_path;
   CHECK(PathService::Get(base::FILE_EXE, &chrome_path));
   CommandLine cmd_line(chrome_path);
 
   cmd_line.AppendSwitchWithValue(switches::kProcessType,
                                  switches::kZygoteProcess);
 
   int fds[2];
+
+  // SOCK_SEQPACKET is used rather than SOCK_DGRAM because SOCK_DGRAM
+  // allows sendto()/sendmsg() to send to arbitrary addresses, and the
+  // SUID sandbox cannot prevent that. Apart from this security
+  // problem, SOCK_DGRAM also has the correct semantics for this
+  // socket pair.
   CHECK(socketpair(PF_UNIX, SOCK_SEQPACKET, 0, fds) == 0);
+
   base::file_handle_mapping_vector fds_to_map;
   fds_to_map.push_back(std::make_pair(fds[1], 3));
 
   const CommandLine& browser_command_line = *CommandLine::ForCurrentProcess();
   if (browser_command_line.HasSwitch(switches::kZygoteCmdPrefix)) {
     const std::wstring prefix =
         browser_command_line.GetSwitchValue(switches::kZygoteCmdPrefix);
     cmd_line.PrependWrapper(prefix);
   }
   // Append any switches from the browser process that need to be forwarded on
@@ skipping 129 matching lines @@
       !read_pickle.ReadBool(&iter, &tmp_child_exited)) {
     LOG(WARNING) << "Error parsing DidProcessCrash response from zygote.";
     return false;
   }
 
   if (child_exited)
     *child_exited = tmp_child_exited;
 
   return did_crash;
 }