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

[Christophe Leroy]

Le 27/03/2023 à 08:36, Nicholas Piggin a écrit :
> 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.
> 
> 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 fact, most architectures don't have thread.regs, but rely on something 
like:

#define task_pt_regs(p) \
	((struct pt_regs *)(THREAD_SIZE + task_stack_page(p)) - 1)


In powerpc, thread.regs was there because of the regs not being at the 
same place in the stack depending on which interrupt it was.

However with the two commits below, we now have stable fixed location 
for the regs, so thread.regs shouldn't be needed anymore:
- db297c3b07af ("powerpc/32: Don't save thread.regs on interrupt entry")
- b5cfc9cd7b04 ("powerpc/32: Fix critical and debug interrupts on BOOKE")

But in the meantime, thread.regs started to be used for additional 
purpose, like knowing if it is a user thread or a kernel thread by using 
thread.regs nullity.


Now that thread.regs doesn't change anymore at each interrupt, it would 
probably be worth dropping it and falling back to task_pt_regs() as 
defined on most architecture.
Knowing whether a thread is a kernel or user thread can for instance be 
known with PF_KTHREAD flag, so no need of thread.regs for that.

Christophe