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

[Szabolcs Nagy <Szabolcs.Nagy@arm.com>]

The 08/09/2023 16:34, 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 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.

the pcs requires 16byte aligned stack frames, with 8byte per gcs entry
there is no need for same gcs size as stack size in userspace.

you can argue about a fixed size small increment (stacksize/2 + inc)
for signal handling on alt stack and special tokens, but stack size is
overkill i think.

fwiw my current makecontext patch uses roundup(stacksize/2+160).
(threads guaranteed to have about 300bytes of data on the stack in glibc
so if gcs is stacksize/2, that accounts for the increment. this is for
the theoretical case when an empty thread just tries to overflow the
stack and then handle the fault on sigaltstack.)