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

[Masahiro Yamada]

Hi Nick,

On Tue, Oct 1, 2019 at 7:19 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Sun, Sep 29, 2019 at 11:00 PM 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.
>
> 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@xxxxxxxxxxxx>
> > Reported-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > ---
> >
> >  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