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Date:   Tue, 9 Nov 2021 19:53:05 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [RFC PATCH 3/7] static_call: use helper to access non-exported
 key
Message-ID: <YYrDkUsJVcOzxMPL@hirez.programming.kicks-ass.net>
References: <20211109164549.1724710-1-ardb@kernel.org>
 <20211109164549.1724710-4-ardb@kernel.org>
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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.