Re: [RFC PATCH 3/3] gdb/nat/linux: Fix attaching to process when it has zombie threads

[Pedro Alves <pedro@palves.net>]

On 2024-03-21 23:11, Thiago Jung Bauermann wrote:
> When GDB attaches to a multi-threaded process, it calls
> linux_proc_attach_tgid_threads () to go through all threads found in
> /proc/PID/task/ and call attach_proc_task_lwp_callback () on each of
> them.  If it does that twice without the callback reporting that a new
> thread was found, then it considers that all inferior threads have been
> found and returns.
> 
> The problem is that the callback considers any thread that it hasn't
> attached to yet as new.  This causes problems if the process has one or
> more zombie threads, because GDB can't attach to it and the loop will
> always "find" a new thread (the zombie one), and get stuck in an
> infinite loop.
> 
> This is easy to trigger (at least on aarch64-linux and powerpc64le-linux)
> with the gdb.threads/attach-many-short-lived-threads.exp testcase, because
> its test program constantly creates and finishes joinable threads so the
> chance of having zombie threads is high.

Hmm.  How about simply not restarting the loop if attach_lwp tries to attach to
a zombie lwp (and silently fails)?

I.e., in gdb, make attach_proc_task_lwp_callback return false/0 here:

      if (ptrace (PTRACE_ATTACH, lwpid, 0, 0) < 0)
	{
	  int err = errno;

	  /* Be quiet if we simply raced with the thread exiting.
	     EPERM is returned if the thread's task still exists, and
	     is marked as exited or zombie, as well as other
	     conditions, so in that case, confirm the status in
	     /proc/PID/status.  */
	  if (err == ESRCH
	      || (err == EPERM && linux_proc_pid_is_gone (lwpid)))
	    {
	      linux_nat_debug_printf
		("Cannot attach to lwp %d: thread is gone (%d: %s)",
		 lwpid, err, safe_strerror (err));

              return 0;        <<<< NEW RETURN
	    }

Similar thing for gdbserver, of course.

Pedro Alves