linux-kernel - Re: [PATCH] ARM: fix __get_user_check() in case uaccess

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Message-ID: <7429d35ebf2115ee9020bd261f504858c3419486.camel@suse.de>
Date:   Mon, 30 Sep 2019 10:13:57 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-arm-kernel@...ts.infradead.org,
        Russell King <rmk+kernel@...linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Stefan Agner <stefan@...er.ch>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: fix __get_user_check() in case uaccess_* calls are
 not inlined

On Mon, 2019-09-30 at 14:59 +0900, Masahiro Yamada wrote:
> KernelCI reports that bcm2835_defconfig is no longer booting since
> commit ac7c3e4ff401 ("compiler: enable CONFIG_OPTIMIZE_INLINING
> forcibly"):
> 
>   https://lkml.org/lkml/2019/9/26/825
> 
> I also received a regression report from Nicolas Saenz Julienne:
> 
>   https://lkml.org/lkml/2019/9/27/263
> 
> This problem has cropped up on arch/arm/config/bcm2835_defconfig
> because it enables CONFIG_CC_OPTIMIZE_FOR_SIZE. The compiler tends
> to prefer not inlining functions with -Os. I was able to reproduce
> it with other boards and defconfig files by manually enabling
> CONFIG_CC_OPTIMIZE_FOR_SIZE.
> 
> The __get_user_check() specifically uses r0, r1, r2 registers.
> So, uaccess_save_and_enable() and uaccess_restore() must be inlined
> in order to avoid those registers being overwritten in the callees.
> 
> Prior to commit 9012d011660e ("compiler: allow all arches to enable
> CONFIG_OPTIMIZE_INLINING"), the 'inline' marker was always enough for
> inlining functions, except on x86.
> 
> Since that commit, all architectures can enable CONFIG_OPTIMIZE_INLINING.
> So, __always_inline is now the only guaranteed way of forcible inlining.
> 
> I want to keep as much compiler's freedom as possible about the inlining
> decision. So, I changed the function call order instead of adding
> __always_inline around.
> 
> Call uaccess_save_and_enable() before assigning the __p ("r0"), and
> uaccess_restore() after evacuating the __e ("r0").
> 
> Fixes: 9012d011660e ("compiler: allow all arches to enable
> CONFIG_OPTIMIZE_INLINING")
> Reported-by: "kernelci.org bot" <bot@...nelci.org>
> Reported-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---

Thanks!

Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>


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