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

Masahiro Yamada
2019-10-01T08:29:38Z

Hi Nick,

On Tue, Oct 1, 2019 at 7:19 AM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Sun, Sep 29, 2019 at 11:00 PM Masahiro Yamada
> <yamada.masahiro@socionext.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.
>
> Yep, that part is obvious, but...
>
> > So, uaccess_save_and_enable() and uaccess_restore() must be inlined
> > in order to avoid those registers being overwritten in the callees.
>
> Right, r0, r1, r2 are caller saved, meaning that __get_user_check must
> save/restore them when making function calls. So
> uaccess_save_and_enable() and uaccess_restore() should either be made
> into macros (macros and typecheck (see include/linux/typecheck.h) are
> peanut butter and chocolate), or occur at different points in the
> function when those register variables are no longer in use.
>
> >
> > 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@kernelci.org>
> > Reported-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> > ---
> >
> > arch/arm/include/asm/uaccess.h | 8 +++++---
> > 1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
> > index 303248e5b990..559f252d7e3c 100644
> > --- a/arch/arm/include/asm/uaccess.h
> > +++ b/arch/arm/include/asm/uaccess.h
> > @@ -191,11 +191,12 @@ extern int __get_user_64t_4(void *);
> > #define __get_user_check(x, p) \
> > ({ \
> > unsigned long __limit = current_thread_info()->addr_limit - 1; \
> > + unsigned int __ua_flags = uaccess_save_and_enable(); \
> > register typeof(*(p)) __user *__p asm("r0") = (p); \
> > register __inttype(x) __r2 asm("r2"); \
> > register unsigned long __l asm("r1") = __limit; \
> > register int __e asm("r0"); \
>
> What does it mean for there to be two different local variables pinned
> to the same register? Ie. it looks like __e and __p are defined to
> exist in r0. Would having one variable and an explicit cast result in
> differing storage?

In my understanding,
__p is input (a pointer to the user-space)
__e is output (return value)

Maybe, does it use two variables for clarification?

>
> > - unsigned int __ua_flags = uaccess_save_and_enable(); \
> > + unsigned int __err; \
> > switch (sizeof(*(__p))) { \
> > case 1: \
> > if (sizeof((x)) >= 8) \
> > @@ -223,9 +224,10 @@ extern int __get_user_64t_4(void *);
> > break; \
> > default: __e = __get_user_bad(); break; \
>
> ^ I think this assignment to __e should be replaced with an assignment
> to __err? We no longer need the register at this point and could skip
> the assignment of x.

Right, but '__err = __e' is necessary for non-default cases.

--
Best Regards
Masahiro Yamada