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

[Masahiro Yamada]

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>
---

 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