Re: [PATCH RFT v9 4/8] fork: Add shadow stack support to clone3()

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Wed, 2024-08-21 at 00:34 +0100, Mark Brown wrote:
> > Why do we need both shadow_stack and shadow_stack_size? We are basically
> > asking
> > it to consume a token at a pointer and have userspace manage the shadow
> > stack
> > itself. So why does the kernel care what size it is? Couldn't we just have
> > 'shadow_stack' have that mean consume a token here.
> 
> I was doing things this way for symmetry with how we specify the normal
> stack.  That's a bit different since the kernel will actually use the
> size for the normal stack but it felt nicer to keep things looking
> consistent, it saves users wondering why they work differently.  It's
> also a bit of a help with portability given that arm64 expects to have a
> top of stack marker above the token by default while x86 doesn't support
> that.

Hmm, so then on arm the kernel would look for the token down a frame. Hmm. I
think it makes it even stranger ABI wise.

SHADOW_STACK_SET_MARKER can be optional (not on arm, but could be in the
future). Then the shadow_stack_size to token offset behavior would depend on
some historical originally supported combination of map_shadow_stack args.

BTW, just to try to reduce potential future revisions, what do you think about
the 8 byte alignment need? Did I miss the check somewhere?