linux-kernel - [PATCH 3.16 101/178] s390/uaccess: get_user() should zero on failure (again)

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Date:   Sun, 16 Jul 2017 14:56:46 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Martin Schwidefsky" <schwidefsky@...ibm.com>,
        "Heiko Carstens" <heiko.carstens@...ibm.com>,
        "Al Viro" <viro@...iv.linux.org.uk>
Subject: [PATCH 3.16 101/178] s390/uaccess: get_user() should zero on
 failure (again)

3.16.46-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Heiko Carstens <heiko.carstens@...ibm.com>

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@...iv.linux.org.uk>
Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/s390/include/asm/uaccess.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/s390/include/asm/uaccess.h
+++ b/arch/s390/include/asm/uaccess.h
@@ -148,7 +148,7 @@ unsigned long __must_check __copy_to_use
 		"	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");					\