linux-kernel - Re: [PATCH] x86/mm: Ensure clear_page() variants always have __kcfi_typeid

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Message-ID: <CABCJKucLzfs99ssROzj4pt_1jy860vkJutqS-CCo=LW-PwnmqA@mail.gmail.com>
Date: Wed, 22 Oct 2025 16:53:00 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: Re: [PATCH] x86/mm: Ensure clear_page() variants always have
 __kcfi_typeid_ symbols

Hi Nathan,

On Mon, Oct 13, 2025 at 2:27 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> When building with CONFIG_CFI=y and CONFIG_LTO_CLANG_FULL=y, there is a
> series of errors from the various versions of clear_page() not having
> __kcfi_typeid_ symbols.
>
>   $ cat kernel/configs/repro.config
>   CONFIG_CFI=y
>   # CONFIG_LTO_NONE is not set
>   CONFIG_LTO_CLANG_FULL=y
>
>   $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 clean defconfig repro.config bzImage
>   ld.lld: error: undefined symbol: __kcfi_typeid_clear_page_rep
>   >>> referenced by ld-temp.o
>   >>>               vmlinux.o:(__cfi_clear_page_rep)
>
>   ld.lld: error: undefined symbol: __kcfi_typeid_clear_page_orig
>   >>> referenced by ld-temp.o
>   >>>               vmlinux.o:(__cfi_clear_page_orig)
>
>   ld.lld: error: undefined symbol: __kcfi_typeid_clear_page_erms
>   >>> referenced by ld-temp.o
>   >>>               vmlinux.o:(__cfi_clear_page_erms)
>
> With full LTO, it is possible for LLVM to realize that these functions
> never have their address taken (as they are only used within an
> alternative, which will make them a direct call) across the whole kernel
> and either drop or skip generating their kCFI type identification
> symbols.
>
> clear_page_{rep,orig,erms}() are defined in clear_page_64.S with
> SYM_TYPED_FUNC_START as a result of commit 2981557cb040 ("x86,kcfi: Fix
> EXPORT_SYMBOL vs kCFI"), as exported functions are free to be called
> indirectly thus need kCFI type identifiers.
>
> Use KCFI_REFERENCE with these clear_page() functions to force LLVM to
> see these functions as address-taken and generate then keep the kCFI
> type identifiers.
>
> Fixes: 2981557cb040 ("x86,kcfi: Fix EXPORT_SYMBOL vs kCFI")
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2128
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
> Alternatively, these functions could move back to SYM_FUNC_START with a
> comment that they need to be exported to be called via the alternative
> in clear_page() but they are never expected to be called indirectly.

Right, this might be a cleaner solution if we don't actually expect
these to be indirectly called.

> ---
>  arch/x86/include/asm/page_64.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
> index 015d23f3e01f..53f4089333f2 100644
> --- a/arch/x86/include/asm/page_64.h
> +++ b/arch/x86/include/asm/page_64.h
> @@ -43,6 +43,9 @@ extern unsigned long __phys_addr_symbol(unsigned long);
>  void clear_page_orig(void *page);
>  void clear_page_rep(void *page);
>  void clear_page_erms(void *page);
> +KCFI_REFERENCE(clear_page_orig);
> +KCFI_REFERENCE(clear_page_rep);
> +KCFI_REFERENCE(clear_page_erms);

In any case,

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami