In a modern codebase, would there be tests for code like this? Is it too late for a plucky contributor to start adding them?
It's not in the codebase proper, but the Linux Test Project https://github.com/linux-test-project/ltp is probably a good place to start to see what's currently being tested for a syscall and to add new tests if there's a gap. https://github.com/linux-test-project/ltp/tree/master/testca... is some tests of this particular syscall.