linux-kernel - [PATCH 5.15 197/667] x86/delay: Fix the wrong asm constraint in delay

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Message-Id: <20220607164940.709379593@linuxfoundation.org>
Date:   Tue,  7 Jun 2022 18:57:42 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ammar Faizi <ammarfaizi2@...weeb.org>,
        Borislav Petkov <bp@...e.de>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 197/667] x86/delay: Fix the wrong asm constraint in delay_loop()

From: Ammar Faizi <ammarfaizi2@...weeb.org>

[ Upstream commit b86eb74098a92afd789da02699b4b0dd3f73b889 ]

The asm constraint does not reflect the fact that the asm statement can
modify the value of the local variable loops. Which it does.

Specifying the wrong constraint may lead to undefined behavior, it may
clobber random stuff (e.g. local variable, important temporary value in
regs, etc.). This is especially dangerous when the compiler decides to
inline the function and since it doesn't know that the value gets
modified, it might decide to use it from a register directly without
reloading it.

Change the constraint to "+a" to denote that the first argument is an
input and an output argument.

  [ bp: Fix typo, massage commit message. ]

Fixes: e01b70ef3eb3 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Signed-off-by: Ammar Faizi <ammarfaizi2@...weeb.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lore.kernel.org/r/20220329104705.65256-2-ammarfaizi2@gnuweeb.org
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/lib/delay.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index 65d15df6212d..0e65d00e2339 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -54,8 +54,8 @@ static void delay_loop(u64 __loops)
 		"	jnz 2b		\n"
 		"3:	dec %0		\n"

-		: /* we don't need output */
-		:"a" (loops)
+		: "+a" (loops)
+		:
 	);
 }

-- 
2.35.1