[RFC PATCH 3/7] static_call: use helper to access non-exported key

[Peter Zijlstra]

On Tue, Nov 09, 2021 at 05:45:45PM +0100, Ard Biesheuvel wrote:
> @@ -196,13 +190,21 @@ extern long __static_call_return0(void);
>  	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
>  	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
>  
> +#define EXPORT_STATIC_CALL_GETKEY_HELPER(name)				\
> +	struct static_call_key *STATIC_CALL_GETKEY(name)(void) {	\
> +		BUG_ON(!core_kernel_text(				\
> +			(unsigned long)__builtin_return_address(0)));	\
> +		return &STATIC_CALL_KEY(name);				\
> +	}								\
> +	EXPORT_SYMBOL_GPL(STATIC_CALL_GETKEY(name))

So if I were a nevarious module, I would look up the above symbol from
kallsyms (it is exported and easily obtainable) and then simply
read the text to discover the key address and we're in business.