linux-kernel - Re: [PATCH v2 1/2] x86/delay: Fix the wrong asm constraint in `delay

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Message-ID: <Yh3YyU2VVK/iaLcA@kroah.com>
Date:   Tue, 1 Mar 2022 09:26:49 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        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
Subject: Re: [PATCH v2 1/2] x86/delay: Fix the wrong asm constraint in
 `delay_loop()`

On Tue, Mar 01, 2022 at 02:32:22PM +0700, Ammar Faizi wrote:
> 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>
> Fixes: e01b70ef3eb3080fecc35e15f68cd274c0a48163 ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")

You only need 12 characters here :)

> Signed-off-by: Ammar Faizi <ammarfaizi2@...weeb.org>
> ---

Why is this one not tagged for stable?