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?
