Re: [PATCH] arm64: disable patchable function entry on big-endian clang builds

[Mark Rutland]

On Tue, May 05, 2020 at 04:12:36PM +0200, Arnd Bergmann wrote:
> Clang only supports the patchable_function_entry attribute on
> little-endian arm64 builds, but not on big-endian:
> 
> include/linux/kasan-checks.h:16:8: error: unknown attribute 'patchable_function_entry' ignored [-Werror,-Wunknown-attributes]
> 
> Disable that configuration with another dependency. Unfortunately
> the existing check is not enough, as $(cc-option) at this point does
> not pass the -mbig-endian flag.

Well that's unfortunate. :(

Do we know if this is deliberate and/or likely to change in future? This
practically rules out a BE distro kernel with things like PAC, which
isn't ideal.

> Fixes: 3b23e4991fb6 ("arm64: implement ftrace with regs")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

This looks fine for now, and we can add a version check in future, so:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 4b256fa6db7a..a33d6402b934 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -151,7 +151,7 @@ config ARM64
>  	select HAVE_DMA_CONTIGUOUS
>  	select HAVE_DYNAMIC_FTRACE
>  	select HAVE_DYNAMIC_FTRACE_WITH_REGS \
> -		if $(cc-option,-fpatchable-function-entry=2)
> +		if $(cc-option,-fpatchable-function-entry=2) && !(CC_IS_CLANG && CPU_BIG_ENDIAN)
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS
>  	select HAVE_FAST_GUP
>  	select HAVE_FTRACE_MCOUNT_RECORD
> -- 
> 2.26.0
>