Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 30, 2021 at 11:05:20AM -0700, Sami Tolvanen wrote:
> This change introduces the DEFINE_CFI_IMMEDIATE_RETURN_STUB macro,
> which defines a stub function that immediately returns and when
> defined in the core kernel, always passes indirect call checking
> with CONFIG_CFI_CLANG. Note that this macro should only be used when
> a stub cannot be called using the correct function type.

> diff --git a/include/linux/cfi.h b/include/linux/cfi.h
> index 879744aaa6e0..19f74af8eac2 100644
> --- a/include/linux/cfi.h
> +++ b/include/linux/cfi.h
> @@ -20,6 +20,17 @@ extern void __cfi_check(uint64_t id, void *ptr, void *diag);
>  #define __CFI_ADDRESSABLE(fn, __attr) \
>  	const void *__cfi_jt_ ## fn __visible __attr = (void *)&fn
>  
> +/*
> + * Defines a stub function that returns immediately, and when defined and
> + * referenced in the core kernel, always passes CFI checking. This should
> + * be used only for stubs that cannot be called using the correct function
> + * pointer type, which should be rare.
> + */
> +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \
> +	void fn(void) { return; } \
> +	const void *__cfi_excl_ ## fn __visible \
> +		__section(".cfi_excluded_stubs") = (void *)&fn
> +
>  #ifdef CONFIG_CFI_CLANG_SHADOW
>  
>  extern void cfi_module_add(struct module *mod, unsigned long base_addr);
> @@ -35,6 +46,8 @@ static inline void cfi_module_remove(struct module *mod, unsigned long base_addr
>  #else /* !CONFIG_CFI_CLANG */
>  
>  #define __CFI_ADDRESSABLE(fn, __attr)
> +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \
> +	void fn(void) { return; }
>  
>  #endif /* CONFIG_CFI_CLANG */
>  

Why DEFINE_CFI_IMMEDIATE_RETURN_STUB() vs __no_cfi attribute that we can
stick on the relvant functions?

Because I've got at least one more variant for you :-) See
kernel/static_call.c:__static_call_return0