linux-kernel - [PATCH v1] x86/delay: Fix the wrong asm constraint in `delay

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Message-Id: <20220110212159.1567280-1-ammarfaizi2@gnuweeb.org>
Date:   Tue, 11 Jan 2022 04:21:59 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>, Ingo Molnar <mingo@...e.hu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>, x86ml <x86@...nel.org>,
        GNU/Weeb Mailing List <gwml@...weeb.org>
Subject: [PATCH v1] x86/delay: Fix the wrong asm constraint in `delay_loop()`

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).

Fixes: e01b70ef3eb3080fecc35e15f68cd274c0a48163 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Cc: Ingo Molnar <mingo@...e.hu>
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: Peter Zijlstra <peterz@...radead.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Joerg Roedel <jroedel@...e.de>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: lkml <linux-kernel@...r.kernel.org>
Cc: x86ml <x86@...nel.org>
Cc: GNU/Weeb Mailing List <gwml@...weeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@...weeb.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)
+		:
 	);
 }
 

base-commit: bf4eebf8cfa2cd50e20b7321dfb3effdcdc6e909
-- 
2.32.0