linux-kernel - Re: [PATCH 3/3] mm: Care about shadow stack guard gap when getting an unmapped area

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Message-ID: <dbaf5653-df46-4e17-bce1-aec7fb168197@sirena.org.uk>
Date: Tue, 3 Sep 2024 20:57:20 +0100
From: Mark Brown <broonie@...nel.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Richard Henderson <richard.henderson@...aro.org>,
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	Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH 3/3] mm: Care about shadow stack guard gap when getting
 an unmapped area

On Tue, Sep 03, 2024 at 03:41:49PM -0400, Liam R. Howlett wrote:
> * Mark Brown <broonie@...nel.org> [240902 15:09]:

> > +static inline unsigned long stack_guard_placement(vm_flags_t vm_flags)
> > +{
> > +	if (vm_flags & VM_SHADOW_STACK)
> > +		return PAGE_SIZE;

> Is PAGE_SIZE is enough?

It's what x86 currently uses so it'll be no worse off if it gets moved
to the generic code (there's a comment in the arch code explaing what's
needed there) and it's enough for arm64, we only do single record
pushes/pops or (optionally) writes to unconstrained addresses.

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