Re: [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description

[Edgecombe, Rick P]

On Wed, 2023-03-01 at 10:07 -0800, Rick Edgecombe wrote:
> > If one wants to scan the shadow stack how to detect the end (e.g.
> > fast
> > backtrace)? Is it useful to put an invalid value (-1) there?
> > (affects map_shadow_stack syscall too).
> 
> Interesting idea. I think it's probably not a breaking ABI change if
> we
> wanted to add it later.

One complication could be how to handle shadow stacks created outside
of thread creation. map_shadow_stack would typically add a token at the
end so it could be pivoted to. So then the backtracing algorithm would
have to know to skip it or something to find a special start of stack
marker.

Alternatively, the thread shadow stacks could get an already used token
pushed at the end, to try to match what an in-use map_shadow_stack
shadow stack would look like. Then the backtracing algorithm could just
look for the same token in both cases. It might get confused in exotic
cases and mistake a token in the middle of the stack for the end of the
allocation though. Hmm...