compile time.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 has more
+CONFIG KERNEL_KASAN_VMALLOC
+ bool "Back mappings in vmalloc space with real shadow memory"
+ depends on KERNEL_KASAN
+ By default, the shadow region for vmalloc space is the read-only
+ zero page. This means that KASAN cannot detect errors involving
+ vmalloc space.
+
+ Enabling this option will hook in to vmap/vmalloc and back those
+ mappings with real shadow memory allocated on demand. This allows
+ for KASAN to detect more sorts of errors (and to support vmapped
+ stacks), but at the cost of higher memory usage.
+
+ This option depends on HAVE_ARCH_KASAN_VMALLOC, but we can't
+ depend on that in here, so it is possible that enabling this
+ will have no effect.
+
if KERNEL_KASAN
config KERNEL_KASAN_GENERIC
def_bool y