linux-kernel - Re: PF_USER_WORKERs and shadow stack

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Message-ID: <20250813192025.GB26754@redhat.com>
Date: Wed, 13 Aug 2025 21:20:26 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, Jens Axboe <axboe@...nel.dk>,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: PF_USER_WORKERs and shadow stack

On 08/13, Oleg Nesterov wrote:
>
> On 08/13, Dave Hansen wrote:
> >
> > On 8/13/25 09:28, Oleg Nesterov wrote:
> > > But it seems that if a features_enabled(ARCH_SHSTK_SHSTK) thread creates a
> > > PF_USER_WORKER thread, shstk_alloc_thread_stack() will allocate the shadow
> > > stack for no reason.
> >
> > Is this costing us anything other than some CPU cycles and 160 bytes of
> > memory for a VMA?
>
> Well, I guess no, but I do have another reason for "something-like-this" cleanup.
> I am working on other changes which should eliminate x86_task_fpu(PF_USER_WORKER).
> Hopefully I'll send the patches tomorrow. To remind, see
> https://lore.kernel.org/all/20250812125700.GA11290@redhat.com/
>
> So I'd like to ensure that ssp_active() can't return T in ssp_get().

Sorry for noise, in case I wasn't clear...

I meant, can't return T if target->flags & PF_USER_WORKER

Oleg.

> And... Dave, I understand that it is very easy to criticize someone else's code ;)
> But - if I am right - the current logic doesn't look clean to me. Regardless.
>
> Oleg.