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

Breazeal, Don
2014-05-09T21:03:42Z

Hi Yao,
Thanks for the review.

On 5/7/2014 10:20 PM, Yao Qi wrote:
> 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?

I'm working on a write-up that I will include in the next version of the
patch. The short version is that there is a race condition in the
current implementation that can leave a dangling entry in the lwp list
(an entry that doesn't have a corresponding actual lwp). In this case
gdbserver will hang waiting for the non-existent lwp to stop. Using the
exit events eliminates this race condition.

>
>> 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.

I had gone back and forth on this issue. I will make a new function for
this.

>
>> /* 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.

I will clean up everything you mentioned from here to the bottom.
thanks
--Don

>
>> +
>> +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.
>