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

[Catalin Marinas]

On Fri, Aug 18, 2023 at 08:38:02PM +0100, Mark Brown wrote:
> On Fri, Aug 18, 2023 at 06:29:54PM +0100, Catalin Marinas wrote:
> > 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
> 
> Your assumption that this is a single thread feels optimistic there.

Yeah and I unfortunately ignored all of them.

> > 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 can't recall or locate the specific reasoning there right now, perhaps
> Rick or someone else can?  I'd guess there would be compat concerns for
> things that don't go via libc which would complicate the story with
> identifying and marking things as GCS/SS safe, it's going to be more
> robust to just supply a GCS if the process is using it.  That said
> having a default doesn't preclude us using the extensibility to allow
> userspace directly to control the GCS size, I would certainly be in
> favour of adding support for that.

It would be good if someone provided a summary of the x86 decision (I'll
get to those thread but most likely in September). I think we concluded
that we can't deploy GCS entirely transparently, so we need a libc
change (apart from the ELF annotations). Since libc is opting in to GCS,
we could also update the pthread_create() etc. to allocate the shadow
together with the standard stack.

Anyway, that's my preference but maybe there were good reasons not to do
this.

-- 
Catalin