On Tue, 2023-08-01 at 18:57 +0100, Mark Brown wrote:

quoted

You don't have to support all the flags actually, you could just
support the one mode you already have and reject all other
combinations... Then it matches between arch's, and you still have
the
guaranteed-ish end marker.

Sure, though if we're going to the trouble of checking for the flag
we
probably may as well implement it.  I guess x86 is locked in at this
point by existing userspace.  I guess I'll implement it assuming
nobody
from userspace complains, it's trivial for a kernel.

To make sure we are on the same page: What I'm saying is say we do
something like add another flag SHADOW_STACK_SET_MARKER that means add
a marker at the end (making the token off by one frame). Then you can
just reject any flags != (SHADOW_STACK_SET_MARKER |
SHADOW_STACK_SET_TOKEN) value, and leave the rest of the code as is. So
not really implementing anything new. 

Then x86 could use the same flag meanings if/when it implements end
markers. If it doesn't seem worth it, it's not a big deal on my end.
Just seemed that they were needlessly diverging.