s390/uaccess: get_user() should zero on failure (again)

commit d09c5373e8e4eaaa09233552cbf75dc4c4f21203 upstream.

Commit fd2d2b191fe7 ("s390: get_user() should zero on failure")
intended to fix s390's get_user() implementation which did not zero
the target operand if the read from user space faulted. Unfortunately
the patch has no effect: the corresponding inline assembly specifies
that the operand is only written to ("=") and the previous value is
discarded.

Therefore the compiler is free to and actually does omit the zero
initialization.

To fix this simply change the contraint modifier to "+", so the
compiler cannot omit the initialization anymore.

Fixes: c9ca78415ac1 ("s390/uaccess: provide inline variants of get_user/put_user")
Fixes: fd2d2b191fe7 ("s390: get_user() should zero on failure")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/s390/include/asm/uaccess.h b/arch/s390/include/asm/uaccess.h
index 5c7381c..c8d837f 100644 (file)
--- a/arch/s390/include/asm/uaccess.h
+++ b/arch/s390/include/asm/uaccess.h
@@ -150,7 +150,7 @@ unsigned long __must_check __copy_to_user(void __user *to, const void *from,
                "       jg      2b\n"                           \
                ".popsection\n"                                 \
                EX_TABLE(0b,3b) EX_TABLE(1b,3b)                 \
-               : "=d" (__rc), "=Q" (*(to))                     \
+               : "=d" (__rc), "+Q" (*(to))                     \
                : "d" (size), "Q" (*(from)),                    \
                  "d" (__reg0), "K" (-EFAULT)                   \
                : "cc");                                        \