x86/ibt: Ensure module init/exit points have references

Since the references to the module init/exit points only have external
references, a module LTO run will consider them 'unused' and seal
them, leading to an immediate fail on module load.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.113767246@infradead.org

include/linux/cfi.h

@@ -34,8 +34,17 @@ static inline void cfi_module_remove(struct module *mod, unsigned long base_addr
 
 #else /* !CONFIG_CFI_CLANG */
 
-#define __CFI_ADDRESSABLE(fn, __attr)
+#ifdef CONFIG_X86_KERNEL_IBT
+
+#define __CFI_ADDRESSABLE(fn, __attr) \
+	const void *__cfi_jt_ ## fn __visible __attr = (void *)&fn
+
+#endif /* CONFIG_X86_KERNEL_IBT */
 
 #endif /* CONFIG_CFI_CLANG */
 
+#ifndef __CFI_ADDRESSABLE
+#define __CFI_ADDRESSABLE(fn, __attr)
+#endif
+
 #endif /* _LINUX_CFI_H */