[PATCH v3 21/36] arm64/mm: Implement map_shadow_stack()

[Edgecombe, Rick P]

On Mon, 2023-07-31 at 18:06 +0100, Mark Brown wrote:
> > Someday when the x86 side is finally upstream I have a manpage for
> > map_shadow_stack. Any differences on the arm side would need to be
> > documented, but I'm not sure why there should be any differences.
> > Like,
> > why not use the same flags? Or have a new flag for token+end marker
> > that x86 can use as well?
> 
> Ah, it wasn't clear to me that this was a question rather than just
> open decisions about the eventual manpage.  Looking again I think
> what
> you're asking about is that I see that at some point in development I
> lost the SHADOW_STACK_SET_TOKEN flag which x86 has.  I suspect that
> was
> a rebasing issue as it wasn't a deliberate decision, there's no
> reason
> we couldn't have that.  Other than that and the fact that we add both
> a
> stack swap token and a top of stack marker I'm not aware of any
> differences.

The thing I was trying to get at was, we have this shared syscall that
means create shadow stack memory and prepopulate it like this flag
says. On x86 we optionally support SHADOW_STACK_SET_TOKEN which means
put a token right at the end of size. So maybe arm should have a
different flag value that includes putting the marker and then the
token, and x86 could match it someday if we get markers too.

It could be a different flag, like SHADOW_STACK_SET_TOKEN_MARKER, or it
could be SHADOW_STACK_SET_MARKER, and callers could pass
(SHADOW_STACK_SET_TOKEN | SHADOW_STACK_SET_MARKER) to get what you have
implemented here. What do you think?