[PATCH] powerpc: don't try to copy ppc for task with NULL pt_regs

[Jens Axboe]

On 3/27/23 12:36?AM, Nicholas Piggin wrote:
> On Mon Mar 27, 2023 at 8:15 AM AEST, Jens Axboe wrote:
>> Powerpc sets up PF_KTHREAD and PF_IO_WORKER with a NULL pt_regs, which
>> from my (arguably very short) checking is not commonly done for other
>> archs. This is fine, except when PF_IO_WORKER's have been created and
>> the task does something that causes a coredump to be generated. Then we
>> get this crash:
> 
> Hey Jens,
> 
> Thanks for the testing and the patch.
> 
> I think your patch would work, but I'd be inclined to give the IO worker
> a pt_regs so it looks more like other archs and a regular user thread.

Yep I think that'd be a better idea. No better way to get a good patch
than to send out a bad one :-)

> Your IO worker bug reminded me to resurrect some copy_thread patches I
> had and I think they should do that
> 
> https://lists.ozlabs.org/pipermail/linuxppc-dev/2023-March/256271.html
> 
> I wouldn't ask you to test it until I've at least tried, do you have a
> test case that triggers this?

I can test them pretty easily. I did write a test case that is 100%
reliable for me, attached. Just do:

$ gcc -Wall -o ppc-crash ppc-crash.c -luring
$ ulimit -c10000000
$ ./ppc-crash

and it'll bomb while trying to write that coredump.

-- 
Jens Axboe
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