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.
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.