Redirect stdin, stdout and stderr to /dev/null when launching LLGS process.

[ovyalov]

Summary

Closing std handles (but without redirecting them) allows kernel to "reuse" 0,1, 2 file handle ids for subsequent file handle open operations.
This issue may lead to junk data sent over TCP connection from lldb-gdbserver to lldb because stderr file handle might be assigned to the socket handle:

int fd = connection_up->GetReadObject()->GetWaitableHandle();
assert(fd == STDERR_FILENO); // llgs doesn't crash on such assertion without this code change in.

By redirecting std handles to /dev/null we prevent OS from such std handles reuse.

[clayborg]

Can we remove:

launch_info.AppendCloseFileAction (STDIN_FILENO);
launch_info.AppendCloseFileAction (STDOUT_FILENO);
launch_info.AppendCloseFileAction (STDERR_FILENO);

As they are not needed if you are suppressing immediately afterward?

[ovyalov]

I left close actions because I found such comment for posix_spawn_file_actions_addopen (http://linux.die.net/man/3/posix_spawn_file_actions_addopen):

"If fildes was already an open file descriptor, it shall be closed before the new file is opened." - my understanding that open without prior close in such situation may lead to handle leakage.

Thank you.

[ovyalov]

Greg, please let me know whether it's okay to leave combination of close and suppress file actions here.

Thank you.

[clayborg]

It is ok as long as the test suite on MacOSX is still happy with it.

[ovyalov]

I ran OSX test suite on my side w/ and w/o the patch - no regressions noticed.

Thank you!

[ovyalov]

Shawn, if the CL looks good to you could you submit it?

Thanks.

[Eugene.Zelenko]

Committed in r221324.

source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp

             }
         } while (has_env_var);
 
-        // Close STDIN, STDOUT and STDERR. We might need to redirect them
-        // to "/dev/null" if we run into any problems.
+        // Close STDIN, STDOUT and STDERR.
         launch_info.AppendCloseFileAction (STDIN_FILENO);
         launch_info.AppendCloseFileAction (STDOUT_FILENO);
         launch_info.AppendCloseFileAction (STDERR_FILENO);
+
+        // Redirect STDIN, STDOUT and STDERR to "/dev/null".
+        launch_info.AppendSuppressFileAction (STDIN_FILENO, true, false);
+        launch_info.AppendSuppressFileAction (STDOUT_FILENO, false, true);
+        launch_info.AppendSuppressFileAction (STDERR_FILENO, false, true);
         
         error = Host::LaunchProcess(launch_info);