linux-kernel - Re: [PATCH] kbuild: avoid static

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Date:   Thu, 10 Dec 2020 17:12:20 +0100
From:   Marco Elver <elver@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kbuild@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...db.de>, stable@...r.kernel.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: avoid static_assert for genksyms

On Fri, Dec 04, 2020 at 12:09AM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> genksyms does not know or care about the _Static_assert() built-in,
> and sometimes falls back to ignoring the later symbols, which causes
> undefined behavior such as
> 
> WARNING: modpost: EXPORT symbol "ethtool_set_ethtool_phy_ops" [vmlinux] version generation failed, symbol will not be versioned.
> ld: net/ethtool/common.o: relocation R_AARCH64_ABS32 against `__crc_ethtool_set_ethtool_phy_ops' can not be used when making a shared object
> net/ethtool/common.o:(_ftrace_annotated_branch+0x0): dangerous relocation: unsupported relocation
> 
> Redefine static_assert for genksyms to avoid that.
> 
> Cc: stable@...r.kernel.org
> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/linux/build_bug.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
> index e3a0be2c90ad..7bb66e15b481 100644
> --- a/include/linux/build_bug.h
> +++ b/include/linux/build_bug.h
> @@ -77,4 +77,9 @@
>  #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
>  #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>  
> +#ifdef __GENKSYMS__
> +/* genksyms gets confused by _Static_assert */
> +#define _Static_assert(expr, ...)
> +#endif
> +

I had sent

	https://lkml.kernel.org/r/20201201152017.3576951-1-elver@google.com

3 days before your patch. :-)

I guess what you propose is simpler, but might still have corner cases
where we still get warnings. In particular, if some file (for whatever
reason) does not include build_bug.h and uses a raw _Static_assert(),
then we still get warnings. E.g. I see 1 user of raw _Static_assert()
(drivers/gpu/drm/amd/amdgpu/amdgv_sriovmsg.h ).

In the end I don't mind either way, as long as those warnings are fixed
in 5.11. :-)

Thanks,
-- Marco