linux-kernel - [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay

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Message-Id: <20220301094608.118879-2-ammarfaizi2@gnuweeb.org>
Date:   Tue,  1 Mar 2022 16:46:07 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>, linux-edac@...r.kernel.org,
        linux-kernel@...r.kernel.org, gwml@...r.gnuweeb.org,
        x86@...nel.org, stable@...r.kernel.org,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Jiri Hladky <hladky.jiri@...glemail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ammar Faizi <ammarfaizi2@...weeb.org>
Subject: [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`

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

The asm constraint does not reflect that the asm statement can modify
the value of @loops. But the asm statement in delay_loop() does change
the @loops.

If by any chance the compiler inlines this function, it may clobber
random stuff (e.g. local variable, important temporary value in reg,
etc.).

Fortunately, delay_loop() is only called indirectly (so it can't
inline), and then the register it clobbers is %rax (which is by the
nature of the calling convention, it's a caller saved register), so it
didn't yield any bug.

^ That shouldn't be an excuse for using the wrong constraint anyway.

This changes "a" (as an input) to "+a" (as an input and output).

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Jiri Hladky <hladky.jiri@...glemail.com>
Fixes: e01b70ef3eb3 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Signed-off-by: Ammar Faizi <ammarfaizi2@...weeb.org>
---

Fortunately, the constraint violation that's fixed by patch 1 doesn't
yield any bug due to the nature of System V ABI. Should we backport
this?

 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)
+		:
 	);
 }

-- 
2.32.0