Handling argc==0 in the kernel

"And writing &argv[1] is UB if argv is an array of length 1 (only containing the terminating NULL) as writing argv[1] is already UB."

No: &argv[1] is semantically identical to argv + 1. It is pointer arithmetic and does not cause a dereference.

However: in deference to C support on AS400 (and Unisys? anything with fancy pointer representations) it *is* undefined behaviour for arrays of length zero. It's only legal to construct a pointer value to the single element past the end of the array, whether or not the pointer is subsequently dereferenced. This is also why it's technically illegal to use a post-decremented pointer traversal that accesses the first element of an array. The one-past-the-end rule doesn't extend to one-before-the-beginning. Sigh.