linux-kernel - Re: [PATCH v2] kbuild: add --target to correctly cross-compile UAPI headers with Clang

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Date:   Tue, 8 Mar 2022 14:34:04 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev, Fangrui Song <maskray@...gle.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Elliot Berman <quic_eberman@...cinc.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] kbuild: add --target to correctly cross-compile UAPI
 headers with Clang

On Sat, Mar 05, 2022 at 09:56:05PM +0900, Masahiro Yamada wrote:
> When you compile-test UAPI headers (CONFIG_UAPI_HEADER_TEST=y) with
> Clang, they are currently compiled for the host target (likely x86_64)
> regardless of the given ARCH=.
> 
> In fact, some exported headers include libc headers. For example,
> include/uapi/linux/agpgart.h includes <stdlib.h> after being exported.
> The header search paths should match to the target we are compiling
> them for.
> 
> Pick up the --target triple from KBUILD_CFLAGS in the same ways as
> commit 7f58b487e9ff ("kbuild: make Clang build userprogs for target
> architecture").
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

I suspect that Android will still need to do something with USERCFLAGS
in its build wrapper to pick up the correct libc (Bionic vs. glibc) but
that is tangential to this patch, we should still do this change
regardless.

> ---
> 
> Changes in v2:
>   - Reword the commit description to mention agpgart.h instead of
>     asound.h because the latter is in the no-header-test list.
> 
>  usr/include/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/usr/include/Makefile b/usr/include/Makefile
> index ac206fb27c65..4215801e1110 100644
> --- a/usr/include/Makefile
> +++ b/usr/include/Makefile
> @@ -10,7 +10,7 @@ UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
>  
>  # In theory, we do not care -m32 or -m64 for header compile tests.
>  # It is here just because CONFIG_CC_CAN_LINK is tested with -m32 or -m64.
> -UAPI_CFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
> +UAPI_CFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
>  
>  # USERCFLAGS might contain sysroot location for CC.
>  UAPI_CFLAGS += $(USERCFLAGS)
> -- 
> 2.32.0
>