In terms of code and doco quality, the BSDs set a standard we should all strive for.
Whoever started that systemd thing should be ashamed.
Otherwise, I see a use-after-free here. A read could occur right after the write function free'd the old buffer in sc.buf, but before assigning a new pointer value. The read function would be able to read kernel memory via the old buffer pointer, although the memory pages pointed to may already have been reused for something else.
If my assumption is correct, the solution would be to use a mutex object to make the read block while the write updates the buffer.
It's a direct outcome of the clean and highly modular codebase that has made NetBSD run "on your toaster" for so many years:
Contrast that with rump Linux kernels: maintained completely out-of-tree and, AFAIK, very little interest in merging and maintaining a complete rump environment in mainline. Rump kernels are easier in NetBSD, anyhow, because of their very carefully designed and maintained device subsystems.
For me, the community is a big part of the reason I use it. The codebase still feels as if it hits a good balance between having the features that I need for daily use and still being clean enough to see how new stuff could be added.