Messages in this thread Patch in this message | | | From | Nick Desaulniers <> | Date | Mon, 30 Sep 2019 15:19:08 -0700 | Subject | Re: [PATCH] ARM: fix __get_user_check() in case uaccess_* calls are not inlined |
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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?
> - 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.
> } \ > - uaccess_restore(__ua_flags); \ > + __err = __e; \ > x = (typeof(*(p))) __r2; \ > - __e; \ > + uaccess_restore(__ua_flags); \ > + __err; \ > }) > > #define get_user(x, p) \ > -- > 2.17.1 >
-- Thanks, ~Nick Desaulniers
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