linux-kernel - Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for PF_USER

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Message-ID: <dfdf2af0-7154-415e-96f4-3e4fefbe96dc@sirena.org.uk>
Date: Fri, 15 Aug 2025 16:48:39 +0100
From: Mark Brown <broonie@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Mehta, Sohil" <sohil.mehta@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"debug@...osinc.com" <debug@...osinc.com>
Subject: Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for
 PF_USER_WORKERs

On Fri, Aug 15, 2025 at 05:43:11PM +0200, Oleg Nesterov wrote:
> On 08/15, Mark Brown wrote:

> > Sure, but OTOH at least for arm64 there's no cost to leaving the feature
> > enabled unless you actually execute userspace code so if we never return
> > to userspace writing the code to disable isn't really buying us anything.

> The fact that a kernel thread can have the pointless ARCH_SHSTK_SHSTK is
> the only reason I know why x86_task_fpu(PF_USER_WORKER) has to work.

> I'd like to make this logic consistent with PF_KTHREAD, and in the longer
> term change the x86 FPU code so that the kernel threads can run without
> without "struct fpu" attached to task_struct.

OK, that's entirely x86 specific - there's no reason we'd want to do
that for arm64.

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