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SubjectRe: [PATCH v6 1/2] static_call: use non-function types to refer to the trampolines
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
>

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