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

[Fabrizio Castro]

> From: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-owner@xxxxxxxxxxxxxxx> On Behalf Of Masahiro Yamada
> Sent: 30 September 2019 06:59
> Subject: [PATCH] ARM: fix __get_user_check() in case uaccess_* calls are not inlined
> 
> 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>

Tested-by: Fabrizio Castro <fabrizio.castro@xxxxxxxxxxxxxx>

> ---
> 
>  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");				\
> -		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;			\
>  		}							\
> -		uaccess_restore(__ua_flags);				\
> +		__err = __e;						\
>  		x = (typeof(*(p))) __r2;				\
> -		__e;							\
> +		uaccess_restore(__ua_flags);				\
> +		__err;							\
>  	})
> 
>  #define get_user(x, p)							\
> --
> 2.17.1