[CRIU] [PATCHSET 0/2] PF_IO_WORKER signal tweaks

[Eric W. Biederman]

Jens Axboe <axboe at kernel.dk> writes:

> On 3/20/21 4:08 PM, Eric W. Biederman wrote:
>> 
>> Added criu because I just realized that io_uring (which can open files
>> from an io worker thread) looks to require some special handling for
>> stopping and freezing processes.  If not in the SIGSTOP case in the
>> related cgroup freezer case.
>> 
>> Linus Torvalds <torvalds at linux-foundation.org> writes:
>> 
>>> On Sat, Mar 20, 2021 at 10:51 AM Linus Torvalds
>>> <torvalds at linux-foundation.org> wrote:
>>>>
>>>> Alternatively, make it not use
>>>> CLONE_SIGHAND|CLONE_THREAD at all, but that would make it
>>>> unnecessarily allocate its own signal state, so that's "cleaner" but
>>>> not great either.
>>>
>>> Thinking some more about that, it would be problematic for things like
>>> the resource counters too. They'd be much better shared.
>>>
>>> Not adding it to the thread list etc might be clever, but feels a bit too scary.
>>>
>>> So on the whole I think Jens' minor patches to just not have IO helper
>>> threads accept signals are probably the right thing to do.
>> 
>> The way I see it we have two options:
>> 
>> 1) Don't ask PF_IO_WORKERs to stop do_signal_stop and in
>>    task_join_group_stop.
>> 
>>    The easiest comprehensive implementation looks like just
>>    updating task_set_jobctl_pending to treat PF_IO_WORKER
>>    as it treats PF_EXITING.
>> 
>> 2) Have the main loop of the kernel thread test for JOBCTL_STOP_PENDING
>>    and call into do_signal_stop.
>> 
>> It is a wee bit trickier to modify the io_workers to stop, but it does
>> not look prohibitively difficult.
>> 
>> All of the work performed by the io worker is work scheduled via
>> io_uring by the process being stopped.
>> 
>> - Is the amount of work performed by the io worker thread sufficiently
>>   negligible that we don't care?
>> 
>> - Or is the amount of work performed by the io worker so great that it
>>   becomes a way for an errant process to escape SIGSTOP?
>> 
>> As the code is all intermingled with the cgroup_freezer.  I am also
>> wondering creating checkpoints needs additional stopping guarantees.
>
> The work done is the same a syscall, basically. So it could be long
> running and essentially not doing anything (eg read from a socket, no
> data is there), or it's pretty short lived (eg read from a file, just
> waiting on DMA).
>
> This is outside of my domain of expertise, which is exactly why I added
> you and Linus to make some calls on what the best approach here would
> be. My two patches obviously go route #1 in terms of STOP. And fwiw,
> I tested this:
>
>> To solve the issue that SIGSTOP is simply broken right now I am totally
>> fine with something like:
>> 
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index ba4d1ef39a9e..cb9acdfb32fa 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -288,7 +288,8 @@ bool task_set_jobctl_pending(struct task_struct *task, unsigned long mask)
>>  			JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING));
>>  	BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK));
>>  
>> -	if (unlikely(fatal_signal_pending(task) || (task->flags & PF_EXITING)))
>> +	if (unlikely(fatal_signal_pending(task) ||
>> +		     (task->flags & (PF_EXITING | PF_IO_WORKER))))
>>  		return false;
>>  
>>  	if (mask & JOBCTL_STOP_SIGMASK)
>
> and can confirm it works fine for me with 2/2 reverted and this applied
> instead.
>
>> Which just keeps from creating unstoppable processes today.  I am just
>> not convinced that is what we want as a long term solution.
>
> How about we go with either my 2/2 or yours above to at least ensure we
> don't leave workers looping as schedule() is a nop with sigpending? If
> there's a longer timeline concern that "evading" SIGSTOP is a concern, I
> have absolutely no qualms with making the IO threads participate. But
> since it seems conceptually simple but with potentially lurking minor
> issues, probably not the ideal approach for right now.


Here is the signoff for mine.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>

Yours misses the joining of group stop during fork.  So we better use
mine.

As far as I can see that fixes the outstanding bugs.

Jens can you make a proper patch out of it and send it to Linus for
-rc4?  I unfortunately have other commitments and this is all I can do
for today.

Eric