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

Szabolcs Nagy
2023-08-10T13:34:54Z

The 08/10/2023 12:41, Mark Brown wrote:
> On Thu, Aug 10, 2023 at 09:55:50AM +0100, Szabolcs Nagy wrote:
> > The 08/09/2023 16:34, Mark Brown wrote:
>
> > > 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.
>
> I agree that it's going to be excessive for pretty much all
> applications, I adjusted it to match x86 as part of the general effort
> to avoid divergence and because I was a bit concerned about non-PCS
> cases (eg, JITed code) potentially running into trouble, especially with

is that even possible?

16byte alignment is not a convention but architectural:
access via unaligned sp traps (at least in userspace).

it is possible to use bl such that the stack is not involved
e.g. if there is no bl/ret pairing, but if we base the gcs
size on the stack size then i'd expect one stack frame per
bl/ret pair with 16byte alignment, or is there a programming
model possible that uses 8byte stack per bl?

> smaller stack limits. It's not an issue I have super strong opinions on
> though, as you can see I had implemented it both ways at various times.