It's not a cache write primitive though; it's a write-to-readable-mappings primitive. At least the way I understood it is, you need to be able to get a (read) f

It's not a cache write primitive though; it's a write-to-readable-mappings primitive. At least the way I understood it is, you need to be able to get a (read) file descriptor to the target in order to throw it into the splice() syscall. Now, there are some "funky" no-fs things that could be opened and are mmap'able/spliceable (some stuff in /proc/*, no idea what exactly though), but it's not immediately obvious to me how this is a generic container escape.