Re: [PATCH v10 05/40] arm64/gcs: Document the ABI for Guarded Control Stacks

[Mark Brown]

On Fri, Aug 16, 2024 at 12:09:01PM +0100, Catalin Marinas wrote:
> On Thu, Aug 01, 2024 at 01:06:32PM +0100, Mark Brown wrote:

> > +* EL0 GCS entries with bit 63 set are reserved for use, one such use is defined

> Maybe "reserved for specific uses". The proposed sentenced feels like
> it's missing something.

Actually we removed the usage of bit 63 so I'll just drop this.

> > +* When a new thread is created by a thread which has GCS enabled then a
> > +  new Guarded Control Stack will be allocated for the new thread with
> > +  half the size of the standard stack.

> Is the half size still the case? It also seems a bit inconsistent to
> have RLIMIT_STACK when GCS is enabled and half the stack size when a new
> thread is created.

Yes, this predates the rebase onto clone3() - I'll update.

> [...]
> > +* When a thread is freed the Guarded Control Stack initially allocated for
> > +  that thread will be freed.  Note carefully that if the stack has been
> > +  switched this may not be the stack currently in use by the thread.

> Is this true for shadow stacks explicitly allocated by the user with
> map_shadow_stack()?

It is only true for the stacks allocaeted by the kernel, if we didn't
allocate a stack we don't free it.

> > +* The signal handler will use the same GCS as the interrupted context.

> I assume this is true even with sigaltstack. Not easy to have
> alternative shadow stack without additional ABI.

Yes.

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