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

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Date:   Mon, 30 Sep 2019 09:57:11 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Russell King <linux@...linux.org.uk>,
        Stefan Agner <stefan@...er.ch>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Olof Johansson <olof@...om.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Subject: Re: [PATCH] ARM: fix __get_user_check() in case uaccess_* calls are
 not inlined

On Mon, Sep 30, 2019 at 8:01 AM Masahiro Yamada
<yamada.masahiro@...ionext.com> 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>

Acked-by: Arnd Bergmann <arnd@...db.de>

The patch looks reasonable to me, but I think we should also revert
commit ac7c3e4ff401 ("compiler: enable CONFIG_OPTIMIZE_INLINING
forcibly") in mainline for now, as it caused this to happen all the time rather
than only for users that expliticly select CONFIG_OPTIMIZE_INLINING.

We have had other bug reports starting with that commit that run into
similar issues, and I'm sure there are more of them. I don't mind having it
in linux-next for a while long, but I think it was premature to have it merged
into mainline.

        Arnd