Re: [PATCH 1/4][REPOST] Remote Linux ptrace exit events

[Yao Qi <yao at codesourcery dot com>]

Don,
I am not the right person to review this patch series and approve them,
but I can do a primary review.  Hope it helps to relieve feeling of the
long time review :)

On 05/01/2014 03:17 AM, Don Breazeal wrote:
> This patch implements support for the extended ptrace event
> PTRACE_EVENT_EXIT on Linux.  This is a preparatory patch for exec event
> support.
> 
> The use of this event is entirely internal to gdbserver; these events are
> not reported to GDB or the user.  If an existing thread is not the last
> thread in a process, its lwp entry is simply deleted, since this is what
> happens in the absence of exit events.  If it is the last thread of a
> process, the wait status is set to the actual wait status of the thread,
> instead of the status that indicates the extended event, and the existing
> mechanisms for handling thread exit proceed as usual.
> 
> The only purpose in using the exit events instead of the existing wait
> mechanisms is to ensure that the exit of a thread group leader is detected
> reliably when a non-leader thread calls exec.

Can you elaborate a little please?  why existing wait mechanism is not
reliable?

>  gdb/common/linux-ptrace.c |   42 ++++++++++++++++++-
>  gdb/gdbserver/linux-low.c |   99 +++++++++++++++++++++++++++++++++++++++------
>  2 files changed, 125 insertions(+), 16 deletions(-)
> 
> diff --git a/gdb/common/linux-ptrace.c b/gdb/common/linux-ptrace.c
> index 7c1b78a..e3fc705 100644
> --- a/gdb/common/linux-ptrace.c
> +++ b/gdb/common/linux-ptrace.c
> @@ -414,7 +414,7 @@ linux_test_for_tracefork (int child_pid)
>    ret = ptrace (PTRACE_CONT, child_pid, (PTRACE_TYPE_ARG3) 0,
>  		(PTRACE_TYPE_ARG4) 0);
>    if (ret != 0)
> -    warning (_("linux_test_for_tracefork: failed to resume child"));
> +    warning (_("linux_test_for_tracefork: failed to resume child (a)"));
>  
>    ret = my_waitpid (child_pid, &status, 0);
>  
> @@ -455,10 +455,46 @@ linux_test_for_tracefork (int child_pid)
>  		       "failed to kill second child"));
>  	  my_waitpid (second_pid, &status, 0);
>  	}
> +      else
> +	{
> +	  /* Fork events are not supported.  */
> +	  return;
> +	}
>      }
>    else
> -    warning (_("linux_test_for_tracefork: unexpected result from waitpid "
> -	     "(%d, status 0x%x)"), ret, status);
> +    {
> +      warning (_("linux_test_for_tracefork: unexpected result from waitpid "
> +	       "(%d, status 0x%x)"), ret, status);
> +      return;
> +    }
> +
> +#ifdef GDBSERVER
> +  /* PTRACE_O_FORK is supported, so now test for PTRACE_O_TRACEEXIT.
> +     First try to set the option.  If this fails, we know for sure that
> +     it is not supported.  */
> +  ret = ptrace (PTRACE_SETOPTIONS, child_pid, (PTRACE_TYPE_ARG3) 0,
> +		(PTRACE_TYPE_ARG4) PTRACE_O_TRACEEXIT);
> +  if (ret != 0)
> +    return;
> +
> +  /* We don't know for sure that the feature is available; old
> +     versions of PTRACE_SETOPTIONS ignored unknown options.  So
> +     see if the process exit will generate a PTRACE_EVENT_EXIT.  */
> +  ret = ptrace (PTRACE_CONT, child_pid, (PTRACE_TYPE_ARG3) 0,
> +		(PTRACE_TYPE_ARG4) 0);
> +  if (ret != 0)
> +    warning (_("linux_test_for_tracefork: failed to resume child (b)"));
> +
> +  ret = my_waitpid (child_pid, &status, 0);
> +
> +  /* Check if we received an exit event notification.  */
> +  if (ret == child_pid && WIFSTOPPED (status)
> +      && status >> 16 == PTRACE_EVENT_EXIT)
> +    {
> +        /* PTRACE_O_TRACEEXIT is supported.  */
> +        current_ptrace_options |= PTRACE_O_TRACEEXIT;
> +    }
> +#endif
>  }
>  

The code looks clear to me, but it is strange to put them in function
linux_test_for_tracefork.  IWBN to move them to a new function.

>  /* Enable reporting of all currently supported ptrace events.  */
> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
> index 2d8d5f5..90e7b15 100644
> --- a/gdb/gdbserver/linux-low.c
> +++ b/gdb/gdbserver/linux-low.c
> @@ -224,6 +224,7 @@ static void proceed_all_lwps (void);
>  static int finish_step_over (struct lwp_info *lwp);
>  static CORE_ADDR get_stop_pc (struct lwp_info *lwp);
>  static int kill_lwp (unsigned long lwpid, int signo);
> +static int num_lwps (int pid);
>  
>  /* True if the low target can hardware single-step.  Such targets
>     don't need a BREAKPOINT_REINSERT_ADDR callback.  */
> @@ -367,13 +368,32 @@ linux_add_process (int pid, int attached)
>    return proc;
>  }
>  
> +/* Check wait status for extended event */

Period and one space is missing at the end of the comment.

> +
> +static int
> +is_extended_waitstatus (int wstat)
> +{
> +    return wstat >> 16 != 0;
   ^^^^
Two spaces instead of four.

> +}
> +
> +/* Check wait status for extended event */

Likewise.

> +
> +static int
> +is_extended_exit (int wstat)
> +{
> +    return wstat >> 16 == PTRACE_EVENT_EXIT;
   ^^^^
Likewise.

> +}
> +
>  /* Handle a GNU/Linux extended wait response.  If we see a clone
>     event, we need to add the new LWP to our list (and not report the

I know it is not about your change, but better to replace LWP with
EVENT_CHILD.  We also need to update it for the exit event.

> -   trap to higher layers).  */
> +   trap to higher layers).  This function returns non-zero if the
> +   event should be ignored and we should wait again.  The wait status
> +   in WSTATP may be modified if an exit event occurred.  */
>  
> -static void
> -handle_extended_wait (struct lwp_info *event_child, int wstat)
> +static int
> +handle_extended_wait (struct lwp_info *event_child, int *wstatp)
>  {
> +  int wstat = *wstatp;
>    int event = wstat >> 16;
>    struct thread_info *event_thr = get_lwp_thread (event_child);
>    struct lwp_info *new_lwp;
> @@ -448,11 +468,54 @@ handle_extended_wait (struct lwp_info *event_child, int wstat)
>  	    linux_resume_one_lwp (new_lwp, 0, WSTOPSIG (status), NULL);
>  	}
>  
> +      /* Enable extended events for the new thread.  */
> +      linux_enable_event_reporting (new_pid);
> +

This change isn't mentioned in ChangeLog entry.  I also wonder why is it
needed here.

>        /* Always resume the current thread.  If we are stopping
>  	 threads, it will have a pending SIGSTOP; we may as well
>  	 collect it now.  */
>        linux_resume_one_lwp (event_child, event_child->stepping, 0, NULL);
> +
> +      /* Don't report the event.  */
> +      return 1;
> +     }
> +  else if (event == PTRACE_EVENT_EXIT)
> +    {
> +      unsigned long exit_status;
> +      unsigned long lwpid = lwpid_of (event_thr);
> +      int ret;
> +
> +      if (debug_threads)
> +        debug_printf ("LHEW: Got exit event from LWP %ld\n",

s/LHEW/HEW/

> +                      lwpid_of (event_thr));
> +
> +      ptrace (PTRACE_GETEVENTMSG, lwpid_of (event_thr), (PTRACE_TYPE_ARG3) 0,
> +	      &exit_status);
> +
> +      if (num_lwps (pid_of (event_thr)) > 1)
> +        {
> +	  /* If there is at least one more LWP, then the program still
> +	     exists and the exit should not be reported to GDB.  */
> +          delete_lwp (event_child);
> +          ret = 1;
> +        }
> +      else
> +        {
> +          /* Set the exit status to the actual exit status, so normal
> +             WIFEXITED/WIFSIGNALLED processing and reporting for the
> +             last lwp in the process can proceed from here.  */
> +          *wstatp = exit_status;
> +          ret = 0;
> +        }
> +
> +      /* Resume the thread so that it actually exits.  Subsequent exit
> +         events for LWPs that were deleted above will be ignored.  */
> +      ptrace (PTRACE_CONT, lwpid, (PTRACE_TYPE_ARG3) 0,
> +              (PTRACE_TYPE_ARG4) 0);
> +      return ret;
>      }
> +  internal_error (__FILE__, __LINE__,
> +	          _("unknown ptrace event %d"), event);

This should be mentioned in ChangeLog too.

-- 
Yao (éå)