filepath-securejoin

> The new syscall implementation would still need a fallback impl. for older kernels after all. Yup, this is why I'm planning on getting a sane API available in <https://github.com/cyphar/filepath-securejoin> which projects can use so that the correct thing is done with both old and new kernels. Right now it has a (slighly) improved version of the code Docker has, but I'm rewriting it. It should be noted that there are lots of examples of interfaces which are incredibly hard to make safe without openat2 -- such as O_CREAT.

The only currently known and exploitable API is with `docker cp`, but FollowSymlinkInScope is used all over the place. Unfortunately, fixing FollowSymlinkInScope requires redesigning the API and then redesigning all the callers so they stop passing around path strings blindly and instead pass around handles (which are O_PATH fds). But, as I mentioned in TFA, the plan is to rework https://github.com/cyphar/filepath-securejoin to have a sane API that detects attacks on older kernels while using the new kernel bits (once merged).