linux-kernel - Re: [PATCH v5 1/2] x86/delay: Fix the wrong asm constraint in `delay

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Date:   Mon, 28 Mar 2022 11:29:26 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Alviro Iskandar Setiawan <alviro.iskandar@...il.com>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, gwml@...r.gnuweeb.org, x86@...nel.org,
        David Laight <David.Laight@...lab.com>,
        Jiri Hladky <hladky.jiri@...glemail.com>
Subject: Re: [PATCH v5 1/2] x86/delay: Fix the wrong asm constraint in
 `delay_loop()`

On 3/28/22 4:38 AM, Borislav Petkov wrote:
> On Thu, Mar 10, 2022 at 08:53:05AM +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 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.
> 
> Add that to the commit message pls.

Will add that in the v6.

>> Cc: stable@...r.kernel.org # v2.6.27+
> 
> I don't see the need for the stable Cc. Or do you have a case where
> a corruption really does happen?

I don't find any visible issue on this. But that's undefined behavior,
different compiler may yield different result (e.g. there is no guarantee
newer compilers will produce the appropriate result due to UB). So it's not
something we should rely on.

============
Side note for inline:
Even if it is not inlined, it's still dangerous, because if the compiler is
able to see that the function to be called doesn't clobber some call-clobbered
regs, the compiler can assume the call-clobbered regs are not clobbered and it
reuses the value without reloading.

See the example from Alviro here:

   https://lore.kernel.org/lkml/CAOG64qPgTv5tQNknuG9d-=oL2EPQQ1ys7xu2FoBpNLyzv1qYzA@mail.gmail.com/

-- 
Ammar Faizi