[PATCH] m68knommu: fix memcpy() out of bounds warning in get_user()

[gerg@linux-m68k.org]

From: Greg Ungerer <gerg@linux-m68k.org>

Newer versions of gcc are giving warnings in the non-MMU m68k version
of the get_user() macro:

    ./arch/m68k/include/asm/string.h:72:25: warning: ‘__builtin_memcpy’ forming offset [3, 4] is out of the bounds [0, 2] of object ‘__gu_val’ with type ‘short unsigned int’ [-Warray-bounds]

The warnings are generated when smaller sized variables are used as the
result of user space pointers to larger values. For example a
short/2-byte variable stores the result of a user space int (4-byte)
pointer. The warning is in the 8-byte branch of get_user() - even
though that branch is not the taken branch in the warning cases.

Refactor the 8-byte branch of get_user() so that it uses a correctly
formed union type to read and write the source and destination objects.
Keep using the memcpy() just in case the user space pointer is not
naturaly aligned.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
---
 arch/m68k/include/asm/uaccess_no.h | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/m68k/include/asm/uaccess_no.h b/arch/m68k/include/asm/uaccess_no.h
index 0134008bf539..6bc80c35726d 100644
--- a/arch/m68k/include/asm/uaccess_no.h
+++ b/arch/m68k/include/asm/uaccess_no.h
@@ -71,26 +71,29 @@ extern int __put_user_bad(void);
 #define get_user(x, ptr)					\
 ({								\
     int __gu_err = 0;						\
-    typeof(x) __gu_val = 0;					\
     switch (sizeof(*(ptr))) {					\
     case 1:							\
-	__get_user_asm(__gu_err, __gu_val, ptr, b, "=d");	\
+	__get_user_asm(__gu_err, x, ptr, b, "=d");		\
 	break;							\
     case 2:							\
-	__get_user_asm(__gu_err, __gu_val, ptr, w, "=r");	\
+	__get_user_asm(__gu_err, x, ptr, w, "=r");		\
 	break;							\
     case 4:							\
-	__get_user_asm(__gu_err, __gu_val, ptr, l, "=r");	\
+	__get_user_asm(__gu_err, x, ptr, l, "=r");		\
 	break;							\
-    case 8:							\
-	memcpy((void *) &__gu_val, ptr, sizeof (*(ptr)));	\
+    case 8: {							\
+	union {							\
+	    u64 l;						\
+	    __typeof__(*(ptr)) t;				\
+	} __gu_val;						\
+	memcpy(&__gu_val.l, ptr, sizeof(__gu_val.l));		\
+	(x) = __gu_val.t;					\
 	break;							\
+    }								\
     default:							\
-	__gu_val = 0;						\
 	__gu_err = __get_user_bad();				\
 	break;							\
     }								\
-    (x) = (typeof(*(ptr))) __gu_val;				\
     __gu_err;							\
 })
 #define __get_user(x, ptr) get_user(x, ptr)
-- 
2.17.1