memcpy is UB because it reads padding bytes in structs.

memmove fails as it requires pointer comparisons between separate allocations, which is UB.

Sounds like we shouldn't use memcpy on structs containing padding!

No, memcpy is fine because it's a language level function. What you _can't_ do is implement your own memcpy alternative safely.

memcpy() and memmove() are language-level functions. They are allowed to do things that cannot be implemented in user code. The C standard library's implementation of these functions have tight integration with the compiler to ensure that it can implement all the required semantics without triggering undefined behavior.

reading padding bytes is not UB.

Comparing unrelated pointers is unspecified, not undefined. It's also an implementation detail of memmove. It can use a magical total order comparison of pointers that is otherwise unavailable to users of the language.