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Date: Mon, 23 Oct 2017 10:08:30 +0100
From: Mark Rutland <mark.rutland@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Alexander Popov <alex.popov@...ux.com>,
	kernel-hardening@...ts.openwall.com, keescook@...omium.org,
	pageexec@...email.hu, spender@...ecurity.net,
	Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
	tycho@...ker.com, Laura Abbott <labbott@...hat.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH RFC v5 0/5] Introduce the STACKLEAK feature and a test
 for it

On Sun, Oct 22, 2017 at 03:11:57PM +0200, Peter Zijlstra wrote:
> On Sun, Oct 22, 2017 at 03:22:48AM +0300, Alexander Popov wrote:
> > Further work
> > =============
> > 
> >  - Rewrite erase_kstack() in C (if Ingo Molnar insists).
> 
> Aside from legacy, is there any sane reason that stuff is in ASM? That
> is, I too will insist it being in C unless you can provide good
> arguments on why it needs be asm. All it does is memzero() right? And I
> would think architectures already have fairly optimized implementations
> of that around.

One argument for this being in asm is that you can't control the stack
usage of a C implementation of erase_kstack(), which leaves a (small)
portion of the stack potentially not cleared depending on what exactly
the compiler does with the stack.

If it's just a call to memzero(), the asm should be very simple, and I'd
think it's better to err on the side of clearing as much of the stack as
possible.

Thanks,
Mark.

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