Re: [PATCH] x86/kcfi: Don't emit KCFI_REFERENCE() in modules

[Nathan Chancellor <nathan@kernel.org>]

On Sat, Oct 24, 2026 at 06:46:33PM -0700, Cytops via B4 Relay wrote:
> From: Cytops <cyridge@proton.me>
> 
> KCFI_REFERENCE() takes the address of a symbol so that the compiler
> emits its __kcfi_typeid_ symbol, which the SYM_TYPED_FUNC_START()
> assembly implementing it needs. It is used from arch/x86 headers for
> __clear_pages_unrolled(), copy_page(), __memset() and __memmove(), and
> all of that assembly is built into vmlinux.
> 
> Modules include the same headers, so with CONFIG_CFI=y every module
> also gets an __ADDRESSABLE() reference to each of these symbols in
> .discard.addressable. This serves no purpose there, and since
> __clear_pages_unrolled() is EXPORT_SYMBOL_GPL, modpost now rejects any
> GPL-incompatible module that includes <linux/mm.h>:
> 
>   ERROR: modpost: GPL-incompatible module repro.ko uses GPL-only
>   symbol '__clear_pages_unrolled'
> 
> even though the module never uses the symbol. This makes OpenZFS

Shouldn't .discard.addressable be discarded from the module during
linking according to scripts/module.lds.S? Is there some other
interaction going on here with LTO?

> impossible to build against CFI kernels; its configure checks fail
> in the same way and are misreported as CONFIG_DEBUG_LOCK_ALLOC.
> 
> Only emit the reference when building vmlinux.
> 
> Tested on x86_64 with clang 23.1.3, CONFIG_CFI=y and
> CONFIG_LTO_CLANG_FULL=y (the configuration from the Fixes: commit):
> vmlinux links without warnings and still contains
> __kcfi_typeid___clear_pages_unrolled, __kcfi_typeid_copy_page,
> __kcfi_typeid___memset and __kcfi_typeid___memmove. It boots in QEMU
> with no CFI failures. A minimal module with MODULE_LICENSE("Proprietary")
> including <linux/mm.h> fails modpost before this change and builds and
> loads after it. OpenZFS builds against a CFI kernel with this change,
> and its pool, encryption, checksum and send/recv operations run with
> CONFIG_CFI_PERMISSIVE=y without CFI failures.
> 
> Fixes: 9b041a4b66b3 ("x86/mm: Ensure clear_page() variants always have __kcfi_typeid_ symbols")

I suppose this is just because this is the first commit that added
KCFI_REFERENCE to a GPL-only symbol? Unlucky me :)

> Link: https://github.com/openzfs/zfs/issues/18092
> Assisted-by: LLM
> Signed-off-by: Cytops <cyridge@proton.me>

Acked-by: Nathan Chancellor <nathan@kernel.org>

> ---
> Notes for reviewers:
> 
> - The fix was found and written with an AI coding assistant (Claude
>   Opus 5.5, Claude Code). It started from "CONFIG_CFI breaks
>   OpenZFS"; the assistant traced that to the modpost failure,
>   wrote this change, and ran the tests described in the changelog.
>   I have reviewed it and take responsibility for it.
> - An alternative would be wrapping the KCFI_REFERENCE() lines in
>   arch/x86/include/asm/page_64.h in #ifndef MODULE, as suggested in
>   the OpenZFS report; doing it in the macro covers all users.
> - Only x86_64 was tested; it is the only architecture using
>   KCFI_REFERENCE(). Tested with CONFIG_LTO_CLANG_FULL=y on this tree
>   and with LTO disabled on v7.3; not tested with
>   CONFIG_LTO_CLANG_THIN=y. In-tree modules were built but not loaded.
> - checkpatch reports "Non-standard signature: Assisted-by:", which
>   conflicts with Documentation/process/coding-assistants.rst.
> ---
>  include/linux/compiler.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 0ea7150db..ea161a72d 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -258,10 +258,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  
>  #endif /* __KERNEL__ */
>  
> -#if defined(CONFIG_CFI) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
> +#if defined(CONFIG_CFI) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO) && \
> +	!defined(MODULE)
>  /*
>   * Force a reference to the external symbol so the compiler generates
> - * __kcfi_typid.
> + * __kcfi_typid. Only vmlinux needs it, as that is where the assembly
> + * using the typeid lives; in a module it would only add a reference to
> + * the symbol, which fails modpost if the symbol is GPL-only.
>   */
>  #define KCFI_REFERENCE(sym) __ADDRESSABLE(sym)
>  #else
> 
> ---
> base-commit: 6fba331225f5040e93d0947d1027e9712238291f
> change-id: 20261024-kcfi-reference-modules-04c32004af41
> 
> Best regards,
> --  
> Cytops <cyridge@proton.me>
> 
> 

-- 
Cheers,
Nathan