`copy_to_kernel_nofault()` is an internal helper which should not be
visible to loadable modules – exporting it would give exploit code a
cheap oracle to probe kernel addresses. Instead, keep the helper
un-exported and compile the kunit case that exercises it only when
`mm/kasan/kasan_test.o` is linked into vmlinux.
Fixes: ca79a00bb9a8 ("kasan: migrate copy_user_test to kunit")
Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Suggested-by: Marco Elver <elver@xxxxxxxxxx>
Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>
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