Re: [PATCH] x86/cfi: Use symmetric SYM_START and SYM_END in __CFI_TYPE()

[Nathan Chancellor]

On Thu, Jun 11, 2026 at 05:57:15PM +0200, Jens Remus wrote:
> Commit ccace936eec7 ("x86: Add types to indirectly called assembly
> functions") introduced a x86-specific implementation of __CFI_TYPE()
> using an asymmetric combination of SYM_START() and SYM_FUNC_END() to
> add a symbol to the KCFI type identifier that precedes a function.
> 
> This asymmetric combination is an issue if SYM_FUNC_END() ever gets
> extended in a way that requires it to be used symmetrically with
> SYM_FUNC_START*().  For instance to emit DWARF CFI directives that
> denote the start/end of a function. [1]
> 
> Use SYM_END() with SYM_T_FUNC instead.  No functional change, as the
> generic implementation of SYM_FUNC_END(name) expands into
> SYM_END(name, SYM_T_FUNC).
> 
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Closes: https://sashiko.dev/#/patchset/20260522110427.2816637-1-jremus@xxxxxxxxxxxxx?part=3 [1]
> Signed-off-by: Jens Remus <jremus@xxxxxxxxxxxxx>

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> ---
> 
> Notes (jremus):
>     This patch applies on top of linus' tree (9716c086c8e8):
>     
>       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  master
>     
>     I verified as follows that in x86-64 builds of vmlinux using Clang with
>     kCFI enabled without and with my patch applied vmlinux.o only differs in
>     relocations targeting .rodata.str* and a few symbols in .rodata shifted
>     (both likely due to differences in string merging):
>     
>     $ objdump -d vmlinux.o > vmlinux.o.{old|new}.objdump
>     $ readelf -Wa vmlinux.o > vmlinux.o.{old|new}.readelf
>     $ diff -u vmlinux.o.old.objdump vmlinux.o.new.objdump
>     [no differences]
>     $ diff -u0 vmlinux.o.old.readelf vmlinux.o.new.readelf | \
>       grep --invert-match -E "\.rodata\.str|@@"
>     [see above]
> 
>  arch/x86/include/asm/linkage.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
> index a7294656ad90..c9769a7b6e66 100644
> --- a/arch/x86/include/asm/linkage.h
> +++ b/arch/x86/include/asm/linkage.h
> @@ -103,7 +103,7 @@
>  	.byte 0xb8 ASM_NL					\
>  	.long __kcfi_typeid_##name ASM_NL			\
>  	CFI_POST_PADDING					\
> -	SYM_FUNC_END(__cfi_##name)
> +	SYM_END(__cfi_##name, SYM_T_FUNC)
>  
>  /* UML needs to be able to override memcpy() and friends for KASAN. */
>  #ifdef CONFIG_UML
> -- 
> 2.53.0
> 

-- 
Cheers,
Nathan