Re: [PATCH v10 05/40] arm64/gcs: Document the ABI for Guarded Control Stacks
From: Mark Brown
Date: Fri Aug 16 2024 - 08:02:48 EST
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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