Re: [PATCH v2 18/39] mm: Add guard pages around a shadow stack.

[Florian Weimer]

* Andrew Cooper:

> You don't actually need a hole to create a guard.  Any mapping of type
> != shstk will do.
>
> If you've got a load of threads, you can tightly pack stack / shstk /
> stack / shstk with no holes, and they each act as each other guard pages.

Can userspace read the shadow stack directly?  Writing is obviously
blocked, but reading?

GCC's stack-clash probing uses OR instructions, so it would be fine with
a readable mapping.  POSIX does not appear to require PROT_NONE mappings
for the stack guard region, either.  However, the
pthread_attr_setguardsize manual page pretty clearly says that it's got
to be unreadable and unwriteable.  Hence my question.

Thanks,
Florian