linux-kernel - Re: [PATCH v6 1/2] x86/delay: Fix the wrong asm constraint in `delay

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Message-ID: <13463eca-03a2-da0d-c274-fb576a8a051f@intel.com>
Date:   Fri, 1 Apr 2022 10:42:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Linux Edac Mailing List <linux-edac@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stable Kernel <stable@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        x86 Mailing List <x86@...nel.org>,
        David Laight <David.Laight@...LAB.COM>,
        Jiri Hladky <hladky.jiri@...glemail.com>
Subject: Re: [PATCH v6 1/2] x86/delay: Fix the wrong asm constraint in
 `delay_loop()`

On 3/29/22 03:47, 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 modify
> the @loops.
> 
> Specifiying 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.
> 
> Fix this by changing the constraint from "a" (as an input) to "+a" (as
> an input and output).

Was this found by inspection or was it causing real-world problems?