I'd say from a kernel perspective they should be copy-on-write.
Firstly, the generalized kernel mechanism which scans for equal pages and de-dupes them [which by the way is disabled by default on Linux] probably doesn't care about if it's working on data or code; it seems like the primary use case at its introduction was for KVM, which, a kernel probably loads code pages and hence writes to them at some point, such as when it reads them from disk.
Second, someone can use mprotect(2) to make them writeable.
