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

[Eric W. Biederman]

Oleg Nesterov <oleg@xxxxxxxxxx> writes:

> 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.

I will just add that CLD_XXX is only valid with SIGCHLD as they are
signal specific si_codes.  In conjunction with another signal like
SIGUSR it will have another meaning.  I would really appreciate it
if new code does not further complicate siginfo_layout.

>> 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.

We do best effort queueing but we don't guarantee anything.  So yes
this makes signals a very louzy interface for sending this kind of
information.

>> >>  		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 isn't until do_coredump calls coredump_wait that all of the
threads are killed.

Eric