Hi Nick, On Tue, Oct 1, 2019 at 7:19 AM Nick Desaulniers <ndesaulni...@google.com> wrote: > > On Sun, Sep 29, 2019 at 11:00 PM Masahiro Yamada > <yamada.masah...@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" <b...@kernelci.org> > > Reported-by: Nicolas Saenz Julienne <nsaenzjulie...@suse.de> > > Signed-off-by: Masahiro Yamada <yamada.masah...@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