Re: [PATCH] kernel/signal: Signal-based pre-coredump notification

[Enke Chen]

Hi, Oleg:

On 10/16/18 7:14 AM, Oleg Nesterov wrote:
> On 10/15, Enke Chen wrote:
>>
>>> I don't understand why we need valid_predump_signal() at all.
>>
>> Most of the signals have well-defined semantics, and would not be appropriate
>> for this purpose.
> 
> you are going to change the rules anyway.
> 
>> That is why it is limited to only SIGCHLD, SIGUSR1, SIGUSR2.
> 
> Which do not queue. So the parent won't get the 2nd signal if 2 children
> crash at the same time.
> 
>>>>  		if (sig_kernel_coredump(signr)) {
>>>> +			/*
>>>> +			 * Notify the parent prior to the coredump if the
>>>> +			 * parent is interested in such a notificaiton.
>>>> +			 */
>>>> +			int p_sig = current->real_parent->predump_signal;
>>>> +
>>>> +			if (valid_predump_signal(p_sig)) {
>>>> +				read_lock(&tasklist_lock);
>>>> +				do_notify_parent_predump(current);
>>>> +				read_unlock(&tasklist_lock);
>>>> +				cond_resched();
>>>
>>> perhaps this should be called by do_coredump() after coredump_wait() kills
>>> all the sub-threads?
>>
>> proc_coredump_connector(current) is located here, they should stay together.
> 
> Why?
> 
> Once again, other threads are still alive. So if the parent restarts the service
> after it recieves -predump_signal, the new process can "race" with the old thread.

Yes, it is a good idea to do the signal notification in do_coredump() after
coredump_wait(). Will make the change as suggested.

Thanks.  -- Enke