linux-kernel - [tip: x86/asm] x86/uaccess: Use XORL %0,%0 in __get_user

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Date:   Thu, 03 Sep 2020 20:57:53 -0000
From:   "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Borislav Petkov <bp@...e.de>,
        "H. Peter Anvin (Intel)" <hpa@...or.com>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/asm] x86/uaccess: Use XORL %0,%0 in __get_user_asm()

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     767ec7289e83721fee205a13b459f12fb2cf922f
Gitweb:        https://git.kernel.org/tip/767ec7289e83721fee205a13b459f12fb2cf922f
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Thu, 27 Aug 2020 20:09:04 +02:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Thu, 03 Sep 2020 22:49:03 +02:00

x86/uaccess: Use XORL %0,%0 in __get_user_asm()

XORL %0,%0 is equivalent to XORQ %0,%0 as both will zero the entire
register. Use XORL %0,%0 for all operand sizes to avoid REX prefix byte
when legacy registers are used and to avoid size prefix byte when 16bit
registers are used.

Zeroing the full register is OK in this use case.

As a result, the size of the .fixup section decreases by 20 bytes.

 [ bp: Massage commit message. ]

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>
Link: https://lkml.kernel.org/r/20200827180904.96399-1-ubizjak@gmail.com
---
 arch/x86/include/asm/uaccess.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index ecefaff..2bffba2 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -343,7 +343,7 @@ do {									\
 		     "2:\n"						\
 		     ".section .fixup,\"ax\"\n"				\
 		     "3:	mov %[efault],%[errout]\n"		\
-		     "	xor"itype" %[output],%[output]\n"		\
+		     "	xorl %k[output],%k[output]\n"			\
 		     "	jmp 2b\n"					\
 		     ".previous\n"					\
 		     _ASM_EXTABLE_UA(1b, 3b)				\