On the off chance the author is reading the comments here, there are a couple security features they need to add for it to be more complete (I'm probably missing some things).

1) They should add a call to access_ok(VERIFY_WRITE, pointer, mem::size_of<Syscall>()) in rust_syscall_handle to ensures that the user space pointer points to valid userspace memory of the right shape for the syscall type.

2) They should sanitize (hopefully, using some zero-cost method) the Syscall type to guarantee that it is well-formed before calling handle. If the userspace constructs some normally impossible to construct Syscall, it's unclear how a rust match pattern handles it (i.e., if an enum has only 3 types and you pass in a falsely constructed variant with tag 100 is match guaranteed not to just be using a jump table and accidentally jump 100 instructions?)