Re: [PATCH v2 0/5] x86/fpu: don't abuse x86_task_fpu(PF_USER_WORKER) in .regset_get() paths

[Oleg Nesterov <oleg@redhat.com>]

On 08/22, Edgecombe, Rick P wrote:
>
> On Fri, 2025-08-22 at 21:21 +0200, Oleg Nesterov wrote:
> > > PKRU affects kernel accesses to userspace. io threads and vhost access
> > > userspace. So why don't we want PKRU state to be inherited for user workers?
> >
> > Sorry I don't follow... Again, this is not my area, I am sure I've missed
> > something.
> > But could you please explain how can this series affect the PKRU logic?
> >
> > > I guess it is not today
>
> I'm sorry, this is incorrect. PKRU is not kept in the FPU structs anymore. So it
> should be inherited over clone I guess.

Yes,

> But despite not being in the actual FPU
> buffer, for compatibility it's left in the uabi xstate copy stuff that the
> regsets use.

Yes, and copy_xstate_to_uabi_buf() still reports target->thread.pkru for
io threads.

So this series doesn't make any difference in this respect...

> > > But again, what exactly is the problem here? Is there a crash or something
> > > for
> > > user workers?
> >
> > Well. I already tried to to explain this in the previous discussions.
> > Apperently I wasn't clear, my fault. So I guess this needs yet another email
> > which I'll write tomorrow, becauase I am already sleeping today.
>
> I believe you said something like "sorry my fault and I'll explain in another
> mail"[0]. Did I miss it?
>
> [0]
> https://lore.kernel.org/lkml/20250815191306.GK11549@redhat.com/

I tried to add more details in this "[PATCH v2 0/5]" cover letter, in particular
to explain why does this series include "[PATCH v2 4/5] x86/shstk: don't create the
shadow stack for PF_USER_WORKERs". I thought that your were asking to explain this
part...

So. Sorry if it wasn't clear, this series is not a bug fix or something like this.
This starts the cleanups I was thinking about year ago, see

	https://lore.kernel.org/all/20240606120038.GB22450@redhat.com/

Then later we can probably make more changes so that the kernel threads
(PF_KTHREADs and PF_USER_WORKERs) will run without "struct fpu" attached
to task_struct, so that x86_task_fpu() should return NULL regardless of
CONFIG_X86_DEBUG_FPU.

But even the WARN_ON_ONCE(task->flags & (PF_KTHREAD|PF_USER_WORKER)) in
x86_task_fpu() makes sense to me.

Say, switch_fpu() has no reason to check "PF_KTHREAD | PF_USER_WORKER",
this check should die. But if something goes wrong, it would be nice to
have a warning for io threads as well.

But as I said, I understand that cleanups are always subjective. It seems
that nobody is interested, and the only reviewer (you ;) doesn't like these
changes. I am going to give up.

That said... Could you explain why do you dislike 4/5 ?

Oleg.