Re: [PATCH] ARM: fix __get_user_check() in case uaccess_* calls are not inlined

[Masahiro Yamada]

Hi Arnd,

On Mon, Sep 30, 2019 at 4:57 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Mon, Sep 30, 2019 at 8:01 AM Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > Reported-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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


Hmm, I know you are testing randconfig build tests,
but how many people are testing the kernel boot in linux-next?

People and kernelci started to report the issue immediately
after it landed in the mainline...


-- 
Best Regards
Masahiro Yamada