Use RecvMsgWithPid to find real PID for zygote children

content/browser/zygote_host/zygote_host_impl_linux.cc

Index: content/browser/zygote_host/zygote_host_impl_linux.cc
diff --git a/content/browser/zygote_host/zygote_host_impl_linux.cc b/content/browser/zygote_host/zygote_host_impl_linux.cc
index 8e6cf8978427479e826f14f1a373155eab8b644b..679fb81d94d0acc3ab7137be98b5af3758c9def2 100644
--- a/content/browser/zygote_host/zygote_host_impl_linux.cc
+++ b/content/browser/zygote_host/zygote_host_impl_linux.cc
@@ -301,6 +301,15 @@ pid_t ZygoteHostImpl::ForkRequest(
   DCHECK(init_);
   Pickle pickle;
 
+  int raw_socks[2];
+  if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, raw_socks) != 0) {

[jln (very slow on Chromium), 2014/05/02 18:25:00]

I think it's fine to PCHECK here, this looks like a catastrophic case.

[mdempsky, 2014/05/02 23:36:01]

Done.

+    LOG(ERROR) << "Failed to create socketpair";
+    return base::kNullProcessHandle;
+  }
+  base::ScopedFD my_sock(raw_socks[0]);
+  base::ScopedFD peer_sock(raw_socks[1]);
+  CHECK(UnixDomainSocket::EnableReceiveProcessId(my_sock.get()));
+
   pickle.WriteInt(kZygoteCommandFork);
   pickle.WriteString(process_type);
   pickle.WriteInt(argv.size());
@@ -311,9 +320,12 @@ pid_t ZygoteHostImpl::ForkRequest(
   pickle.WriteInt(mapping.size());
 
   std::vector<int> fds;
-  // Scoped pointers cannot be stored in containers, so we have to use a
-  // linked_ptr.
-  std::vector<linked_ptr<base::ScopedFD> > autodelete_fds;
+  ScopedVector<base::ScopedFD> autoclose_fds;
+
+  // First FD to send is peer_sock.
+  fds.push_back(peer_sock.get());
+  autoclose_fds.push_back(new base::ScopedFD(peer_sock.Pass()));
+
   for (std::vector<FileDescriptorInfo>::const_iterator
        i = mapping.begin(); i != mapping.end(); ++i) {
     pickle.WriteUInt32(i->id);
@@ -321,8 +333,7 @@ pid_t ZygoteHostImpl::ForkRequest(
     if (i->fd.auto_close) {
       // Auto-close means we need to close the FDs after they have been passed
       // to the other process.
-      linked_ptr<base::ScopedFD> ptr(new base::ScopedFD(fds.back()));
-      autodelete_fds.push_back(ptr);
+      autoclose_fds.push_back(new base::ScopedFD(i->fd.fd));
     }
   }
 
@@ -331,10 +342,28 @@ pid_t ZygoteHostImpl::ForkRequest(
     base::AutoLock lock(control_lock_);
     if (!SendMessage(pickle, &fds))
       return base::kNullProcessHandle;
+    autoclose_fds.clear();

[jln (very slow on Chromium), 2014/05/02 18:25:00]

Hmmm, I sense a new SendMessage that takes ownership might be useful :) (For
another CL)

[mdempsky, 2014/05/02 23:36:01]

Deferred.  This is a bit tricky because we're only closing a subset of the
descriptors we're sending, so we can't simply pass ownership of all the
descriptors.

 
-    // Read the reply, which pickles the PID and an optional UMA enumeration.
     static const unsigned kMaxReplyLength = 2048;
     char buf[kMaxReplyLength];
+
+    {
+      ScopedVector<base::ScopedFD> recv_fds;
+      base::ProcessId real_pid;
+
+      if (UnixDomainSocket::RecvMsgWithPid(
+              my_sock.get(), buf, sizeof(buf), &recv_fds, &real_pid) <= 0) {
+        LOG(ERROR) << "Failed to receive PID for zygote child";
+        real_pid = -1;
+      }
+
+      // Always write back PID to zygote.
+      const ssize_t n =
+          HANDLE_EINTR(write(control_fd_, &real_pid, sizeof(real_pid)));
+      CHECK_EQ(sizeof(real_pid), static_cast<size_t>(n));
+    }
+
+    // Read the reply, which pickles the PID and an optional UMA enumeration.
     const ssize_t len = ReadReply(buf, sizeof(buf));
 
     Pickle reply_pickle(buf, len);