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Date:   Mon, 8 Nov 2021 11:08:04 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Quentin Perret <qperret@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>,
        Will Deacon <will@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v6 1/2] static_call: use non-function types to refer to
 the trampolines
Message-ID: <YYj3BMbuP/mkPAIn@hirez.programming.kicks-ass.net>
References: <20211105145917.2828911-1-ardb@kernel.org>
 <20211105145917.2828911-2-ardb@kernel.org>
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On Fri, Nov 05, 2021 at 03:59:16PM +0100, Ard Biesheuvel wrote:
> In order to prevent CFI enabled code from grabbing a jump table entry
> that jumps to the trampoline, rather than the trampoline itself, use an
> incomplete non-function type for the trampoline, and cast it to the
> right type only when invoking it.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Very grudingly:

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
>  include/linux/static_call.h       |  4 ++--
>  include/linux/static_call_types.h | 11 ++++++++---
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/static_call.h b/include/linux/static_call.h
> index 3e56a9751c06..616607393273 100644
> --- a/include/linux/static_call.h
> +++ b/include/linux/static_call.h
> @@ -151,7 +151,7 @@ extern void arch_static_call_transform(void *site, void *tramp, void *func, bool
>  
>  #define static_call_update(name, func)					\
>  ({									\
> -	typeof(&STATIC_CALL_TRAMP(name)) __F = (func);			\
> +	typeof(&STATIC_CALL_TYPE(name)) __F = (func);			\
>  	__static_call_update(&STATIC_CALL_KEY(name),			\
>  			     STATIC_CALL_TRAMP_ADDR(name), __F);	\
>  })
> @@ -306,7 +306,7 @@ static inline void __static_call_nop(void) { }
>  	void *func = READ_ONCE(STATIC_CALL_KEY(name).func);		\
>  	if (!func)							\
>  		func = &__static_call_nop;				\
> -	(typeof(STATIC_CALL_TRAMP(name))*)func;				\
> +	(typeof(&STATIC_CALL_TYPE(name)))func;				\
>  })
>  
>  #define static_call_cond(name)	(void)__static_call_cond(name)
> diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
> index 5a00b8b2cf9f..5e658ef537e4 100644
> --- a/include/linux/static_call_types.h
> +++ b/include/linux/static_call_types.h
> @@ -18,6 +18,9 @@
>  #define STATIC_CALL_TRAMP(name)		__PASTE(STATIC_CALL_TRAMP_PREFIX, name)
>  #define STATIC_CALL_TRAMP_STR(name)	__stringify(STATIC_CALL_TRAMP(name))
>  
> +#define STATIC_CALL_TYPE_PREFIX		__SCtype__
> +#define STATIC_CALL_TYPE(name)		__PASTE(STATIC_CALL_TYPE_PREFIX, name)
> +
>  /*
>   * Flags in the low bits of static_call_site::key.
>   */
> @@ -36,11 +39,13 @@ struct static_call_site {
>  
>  #define DECLARE_STATIC_CALL(name, func)					\
>  	extern struct static_call_key STATIC_CALL_KEY(name);		\
> -	extern typeof(func) STATIC_CALL_TRAMP(name);
> +	extern struct static_call_tramp STATIC_CALL_TRAMP(name);	\
> +	extern typeof(func) STATIC_CALL_TYPE(name)
>  
>  #ifdef CONFIG_HAVE_STATIC_CALL
>  
> -#define __raw_static_call(name)	(&STATIC_CALL_TRAMP(name))
> +#define __raw_static_call(name)						\
> +	((typeof(&STATIC_CALL_TYPE(name)))&STATIC_CALL_TRAMP(name))
>  
>  #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
>  
> @@ -96,7 +101,7 @@ struct static_call_key {
>  };
>  
>  #define static_call(name)						\
> -	((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_KEY(name).func))
> +	((typeof(&STATIC_CALL_TYPE(name)))(STATIC_CALL_KEY(name).func))
>  
>  #endif /* CONFIG_HAVE_STATIC_CALL */
>  
> -- 
> 2.30.2
>