linux-kernel - Re: [PATCH bpf-next v10 1/3] cfi: add C CFI type macro

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Message-ID: <aHow-yVsJCO2AJsn@willie-the-truck>
Date: Fri, 18 Jul 2025 12:33:15 +0100
From: Will Deacon <will@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: bpf@...r.kernel.org, Puranjay Mohan <puranjay@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Maxwell Bland <mbland@...orola.com>, Dao Huang <huangdao1@...o.com>
Subject: Re: [PATCH bpf-next v10 1/3] cfi: add C CFI type macro

On Tue, Jul 15, 2025 at 10:57:35PM +0000, Sami Tolvanen wrote:
> From: Mark Rutland <mark.rutland@....com>
> 
> Currently x86 and riscv open-code 4 instances of the same logic to
> define a u32 variable with the KCFI typeid of a given function.
> 
> Replace the duplicate logic with a common macro.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Co-developed-by: Maxwell Bland <mbland@...orola.com>
> Signed-off-by: Maxwell Bland <mbland@...orola.com>
> Co-developed-by: Sami Tolvanen <samitolvanen@...gle.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Tested-by: Dao Huang <huangdao1@...o.com>
> ---
>  arch/riscv/kernel/cfi.c       | 35 +++--------------------------------
>  arch/x86/kernel/alternative.c | 31 +++----------------------------
>  include/linux/cfi_types.h     | 23 +++++++++++++++++++++++
>  3 files changed, 29 insertions(+), 60 deletions(-)

[...]

> diff --git a/include/linux/cfi_types.h b/include/linux/cfi_types.h
> index 6b8713675765..e5567c0fd0b3 100644
> --- a/include/linux/cfi_types.h
> +++ b/include/linux/cfi_types.h
> @@ -41,5 +41,28 @@
>  	SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)
>  #endif
>  
> +#else /* __ASSEMBLY__ */
> +
> +#ifdef CONFIG_CFI_CLANG
> +#define DEFINE_CFI_TYPE(name, func)						\
> +	/*									\
> +	 * Force a reference to the function so the compiler generates		\
> +	 * __kcfi_typeid_<func>.						\
> +	 */									\
> +	__ADDRESSABLE(func);							\
> +	/* u32 name __ro_after_init = __kcfi_typeid_<func> */			\
> +	extern u32 name;							\
> +	asm (									\
> +	"	.pushsection	.data..ro_after_init,\"aw\",@progbits	\n"	\
> +	"	.type	" #name ",@object				\n"	\
> +	"	.globl	" #name "					\n"	\
> +	"	.p2align	2, 0x0					\n"	\
> +	#name ":							\n"	\
> +	"	.4byte	__kcfi_typeid_" #func "				\n"	\
> +	"	.size	" #name ", 4					\n"	\
> +	"	.popsection						\n"	\
> +	);
> +#endif

This looks good to me. I was initially a bit worried about the portability
of the '.4byte' directive, but it seems that cfi_types.h is already using
that for the __CFI_TYPE() macro so I'm assuming it's not an issue.

In which case:

Acked-by: Will Deacon <will@...nel.org>

Thanks for cleaning it up.

Will