ubsan: turn off kmsan inside of ubsan instrumentation

The structure initialization in the two type mismatch handling functions
causes a call to __msan_memset() to be generated inside of a UACCESS
block, which in turn leads to an objtool warning about possibly leaking
uaccess-enabled state:

lib/ubsan.o: warning: objtool: __ubsan_handle_type_mismatch+0xda: call to __msan_memset() with UACCESS enabled
lib/ubsan.o: warning: objtool: __ubsan_handle_type_mismatch_v1+0xf4: call to __msan_memset() with UACCESS enabled

Most likely __msan_memset() is safe to be called here and could be added
to the uaccess_safe_builtin[] list of safe functions, but seeing that the
ubsan file itself already has kasan, ubsan and kcsan disabled itself, it
is probably a good idea to also turn off kmsan here, in particular this
also avoids the risk of recursing between ubsan and kcsan checks in other
functions of this file.

I saw this happen while testing randconfig builds with clang-22, but did
not try older versions, or attempt to see which kernel change introduced
the warning.

Link: https://lkml.kernel.org/r/20260306150613.350029-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/Makefile

@@ -307,6 +307,7 @@ obj-$(CONFIG_UBSAN) += ubsan.o
 UBSAN_SANITIZE_ubsan.o := n
 KASAN_SANITIZE_ubsan.o := n
 KCSAN_SANITIZE_ubsan.o := n
+KMSAN_SANITIZE_ubsan.o := n
 CFLAGS_ubsan.o := -fno-stack-protector $(DISABLE_KSTACK_ERASE)
 
 obj-$(CONFIG_SBITMAP) += sbitmap.o