Are kernels with KASAN || KCSAN || KMSAN enabled supposed to be bootable?
They are all intended to be used for runtime debugging, so I'd imagine so.
Then I strongly suggest putting a nonzero value here. As you write that "with every release of LLVM, both of these sanitizers eat up more and more of the stack", don't you want to have at least some canary to detect when "more and more" is guaranteed to run into problems?
FRAME_WARN is a poor canary. First, it does not necessarily indicate that a build is faulty (a single bloated stack frame won't crash the system). Second, devs are unlikely to fix a function because its stack frame is too big under some exotic tool+compiler combination. So the remaining option would be to just increase the frame size every time a new function surpasses the limit.