linux-kernel - Re: [PATCH v2 0/5] x86/fpu: don't abuse x86_task_fpu(PF_USER_WORKER) in .regset

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Message-ID: <b483759593fb83ec977c318d02ea1865f4052eb7.camel@intel.com>
Date: Fri, 22 Aug 2025 20:01:57 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "oleg@...hat.com" <oleg@...hat.com>
CC: "debug@...osinc.com" <debug@...osinc.com>, "mingo@...nel.org"
	<mingo@...nel.org>, "bp@...en8.de" <bp@...en8.de>, "broonie@...nel.org"
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	"hpa@...or.com" <hpa@...or.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "Mehta, Sohil"
	<sohil.mehta@...el.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 0/5] x86/fpu: don't abuse x86_task_fpu(PF_USER_WORKER)
 in .regset_get() paths

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

> > , but to me, conceptually we maybe don't want a special case for them? So
> > rather than add more special handling, could we actually just remove special
> > handling to make it consistent?
> 
> Could you spell please?

PKRU is for "protection keys". These are memory permissions that can be applied
per-thread. So you can make a virtual address toggleable for read or write
without affecting the other threads. The kernel is supposed to have these
permissions enforced just like the normal ones (read-only, etc). So it's an
example of user FPU state that applied to the kernel. Except that, as above, it
had to be moved out of the actual FPU state because it was special in that way.

But I think you could argue that it should be part of ptrace regsets. A debugger
may want to inspect what PKRU enforcement was happening for the io thread.

> 
> > 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/