[PATCH RFC v2 2/2] m68k: improve __constant_copy_to_user_asm() fault handling

[Michael Schmitz <schmitzmic@gmail.com>]

A problem similar to that reported for __put_user_asm and
__generic_copy_to_user exists in __constant_copy_to_user_asm.

Address the problem by extending the exception table to cover
two instructions past each moves instruction, and adding a
single NOP at the very end to catch faults on the final
instruction (which is not guaranteed to be a movesb!).

Tested on 68030 (Atari Falcon 030) with a transfer beginning
at a single byte at the end of a mapped page followed by
seven more bytes on an unmapped page (testcase derived from
stress-ng sysbadaddr stressor by Finn Thain and modified to
use the llseek syscall).

Cc: Finn Thain <fthain@linux-m68k.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/all/e0f23460779e6d16e2633486ac4841790ef2aca0.1713176294.git.fthain@linux-m68k.org
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
---
 arch/m68k/include/asm/uaccess.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/m68k/include/asm/uaccess.h b/arch/m68k/include/asm/uaccess.h
index 64914872a5c9..0671df8c6b59 100644
--- a/arch/m68k/include/asm/uaccess.h
+++ b/arch/m68k/include/asm/uaccess.h
@@ -286,10 +286,11 @@ __constant_copy_from_user(void *to, const void __user *from, unsigned long n)
 		"21:	"MOVES"."#s2"	%3,(%1)+\n"			\
 		"22:\n"							\
 		"	.ifnc	\""#s3"\",\"\"\n"			\
-		"	move."#s3"	(%2)+,%3\n"			\
-		"31:	"MOVES"."#s3"	%3,(%1)+\n"			\
-		"32:\n"							\
+		"31:	move."#s3"	(%2)+,%3\n"			\
+		"32:	"MOVES"."#s3"	%3,(%1)+\n"			\
+		"33:\n"							\
 		"	.endif\n"					\
+		"34:	nop\n"						\
 		"4:\n"							\
 		"\n"							\
 		"	.section __ex_table,\"a\"\n"			\
@@ -301,7 +302,9 @@ __constant_copy_from_user(void *to, const void __user *from, unsigned long n)
 		"	.ifnc	\""#s3"\",\"\"\n"			\
 		"	.long	31b,5f\n"				\
 		"	.long	32b,5f\n"				\
+		"	.long	33b,5f\n"				\
 		"	.endif\n"					\
+		"	.long	34b,5f\n"				\
 		"	.previous\n"					\
 		"\n"							\
 		"	.section .fixup,\"ax\"\n"			\
-- 
2.17.1