Zygote: create variables for magic fd numbers

The magic number "7" was used across source files to denote the file
descriptor for the dummy fd.

BUG=None
TEST=None
NOTRY=true


Review URL: https://chromiumcodereview.appspot.com/10392176

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@138269 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/zygote_host_impl_linux.cc

@@ -103,7 +103,7 @@ void ZygoteHostImpl::Init(const std::string& sandbox_cmd) {
   CHECK(socketpair(PF_UNIX, SOCK_SEQPACKET, 0, fds) == 0);
 #endif
   base::FileHandleMappingVector fds_to_map;
-  fds_to_map.push_back(std::make_pair(fds[1], 3));
+  fds_to_map.push_back(std::make_pair(fds[1], content::kZygoteSocketPairFd));
 
   const CommandLine& browser_command_line = *CommandLine::ForCurrentProcess();
   if (browser_command_line.HasSwitch(switches::kZygoteCmdPrefix)) {
@@ -161,13 +161,14 @@ void ZygoteHostImpl::Init(const std::string& sandbox_cmd) {
   // Start up the sandbox host process and get the file descriptor for the
   // renderers to talk to it.
   const int sfd = RenderSandboxHostLinux::GetInstance()->GetRendererSocket();
-  fds_to_map.push_back(std::make_pair(sfd, 5));
+  fds_to_map.push_back(std::make_pair(sfd, content::kZygoteRendererSocketFd));
 
   int dummy_fd = -1;
   if (using_suid_sandbox_) {
     dummy_fd = socket(PF_UNIX, SOCK_DGRAM, 0);
     CHECK(dummy_fd >= 0);
-    fds_to_map.push_back(std::make_pair(dummy_fd, 7));
+    fds_to_map.push_back(std::make_pair(dummy_fd,
+                                        content::kZygoteIdFd));
   }
 
   base::ProcessHandle process = -1;

content/common/zygote_commands_linux.h

@@ -11,6 +11,17 @@ namespace content {
 // is ready to go.
 static const char kZygoteHelloMessage[] = "ZYGOTE_OK";
 
+// File descriptors initialized by the Zygote Host
+const int kZygoteSocketPairFd = 3;
+const int kZygoteRendererSocketFd = 5;
+// This file descriptor is special. It is passed to the Zygote and a setuid
+// helper will be called to locate the process of the Zygote on the system.
+// This mechanism is used when multiple PID namespaces exist because of the
+// setuid sandbox.
+// It is very important that this file descriptor does not exist in multiple
+// processes.
+const int kZygoteIdFd = 7;
+
 // These are the command codes used on the wire between the browser and the
 // zygote.
 enum {

content/zygote/zygote_linux.cc

@@ -35,8 +35,6 @@ namespace content {
 
 namespace {
 
-const int kZygoteIdDescriptor = 7;
-
 // NOP function. See below where this handler is installed.
 void SIGCHLDHandler(int signal) {
 }
@@ -413,7 +411,7 @@ base::ProcessId Zygote::ReadArgsAndFork(const Pickle& pickle,
 
     close(kBrowserDescriptor);  // Our socket from the browser.
     if (UsingSUIDSandbox())
-      close(kZygoteIdDescriptor);  // Another socket from the browser.
+      close(kZygoteIdFd);  // Another socket from the browser.
     base::GlobalDescriptors::GetInstance()->Reset(mapping);
 
 #if defined(CHROMIUM_SELINUX)