On Tue, Oct 01, 2024 at 05:12:38PM +0200, Christian Brauner wrote:

quoted

Thanks for the info!

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My preference is to keep the api consistent and require a stack_size for
shadow stacks as well.

Did you catch that a token can be at a different offsets location on the stack
depending on args passed to map_shadow_stack? So userspace will need something
like the code above, but that adjusts the 'shadow_stack_size' such that the
kernel looks for the token in the right place. It will be even weirder if
someone uses clone3 to switch to a stack that has already been used, and pivoted
off of, such that a token was left in the middle of the stack. In that case
userspace would have to come up with args disconnected from the actual size of
the shadow stack such that the kernel would be cajoled into looking for the
token in the right place.

A shadow stack size is more symmetric on the surface, but I'm not sure it will
be easier for userspace to handle. So I think we should just have a pointer to
the token. But it will be a usable implementation either way.

Maybe it's best to let glibc folks decide what is better/more ergonomic for them.

I agree that it would be better to just have a pointer to the token.

My preference would be to avoid having obscure additional arguments that may end up
having misleading name or bear some hidden functionality. If kernel is not going to
use stack size as such, then users should not have to provide it.

Thanks,
Yury

PS Apologies for delayed reply