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

Masahiro Yamada
2019-09-30T06:02:56Z

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@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"); \
- 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