Performing pointer arithmetic on a null pointer?

[Hyeonggon Yoo]

Hello, I was compiling kernel with make CC=clang-10 W=1 -s

there are some places that compiler complains about
pointer arithmetic like below.it says it's undefined behavior.

is it just OK to use UBs like this (I hope it's not),
or am I missing something?

fs/kernfs/file.c:128:15: warning: performing pointer arithmetic on
	a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
                return NULL + !*ppos;