Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

[Catalin Marinas]

On Wed, Aug 09, 2023 at 04:34:38PM +0100, Mark Brown wrote:
> On Wed, Aug 09, 2023 at 03:24:14PM +0100, Catalin Marinas wrote:
> > On Mon, Aug 07, 2023 at 11:00:08PM +0100, Mark Brown wrote:
> > > +* When set PR_SHADOW_STACK_ENABLE flag allocates a Guarded Control Stack for
> > 
> > The 'for' at the end of the line above is not needed.
> > 
> > > +  and enables GCS for the thread, enabling the functionality controlled by
> 
> I find it a little clearer that it's a per thread stack here but sure.

If it reads better for you, feel free to keep it as is.

> > > +3.  Allocation of Guarded Control Stacks
> > > +----------------------------------------
> 
> > > +* When GCS is enabled for a thread a new Guarded Control Stack will be
> > > +  allocated for it of size RLIMIT_STACK / 2 or 2 gigabytes, whichever is
> > > +  smaller.
> 
> > Is this number based on the fact that a function call would only push
> > the LR to GCS while standard function prologue pushes at least two
> > registers?
> 
> It's actually based on bitrot that I'd initially chosen a smaller value
> since it's likely that functions will push at least something as you
> suggest, the patches now just use RLIMIT_STACK.  I'll fix.

A related question - it may have been discussed intensively on the x86
thread (I may read it sometime) - why not have the libc map the shadow
stack and pass the pointer/size to clone3()? It saves us from having to
guess what the right size we'd need. struct clone_args is extensible.

(I plan to get back next week to this series, I'll need to read a bit
more on the spec)

-- 
Catalin