This addresses an issue I realized during Kirk's talk at BSDCan. In the existing model, it's quite hard to support third-party modules and even in-tree modules for downstreams would require reserving a portion of the identifier space. This approach adds cost to the EXTERROR_VERBOSE reporting path, but significantly decouples the kernel and libc. I'm not sure I'd want to go as far moving entirely to dynamic, but it's probably not a problem in practice.

If I wanted to make this more efficient I'd add an AT_EXTERR_CATS and AT_EXTERR_CAT_MAX or the like and use shared pages to push out an array of pointers (or maybe with a third variable indexes) into processes addresses spaces. You might still want a fallback for categories larger than AT_EXTERR_CAT_MAX to use the sysctl interface if modules were loaded so I think this is the right direction regardless.