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

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Message-ID: <3ceb39a4-f592-68b6-b5e7-a33a2b33a402@gnuweeb.org>
Date:   Mon, 4 Apr 2022 00:11:31 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.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 4/3/22 11:57 PM, Thomas Gleixner wrote:
> On Tue, Mar 29 2022 at 17:47, 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).
> 
> This analysis is plain wrong. The assembly code operates on a register
> and not on memory:



> 	asm volatile(
> 		"	test %0,%0	\n"
> 		"	jz 3f		\n"
> 		"	jmp 1f		\n"
> 
> 		".align 16		\n"
> 		"1:	jmp 2f		\n"
> 
> 		".align 16		\n"
> 		"2:	dec %0		\n"
> 		"	jnz 2b		\n"
> 		"3:	dec %0		\n"
> 
> 		: /* we don't need output */
> ---->		:"a" (loops)
> 
> This tells the compiler to use [RE]AX and initialize it from the
> variable 'loops'. It's never written back because all '%0' in the above
> assembly are substituted with [RE]AX. This also tells the compiler that
> the inline assembly clobbers [RE]AX and that's all it needs to know.

Hi Thomas,

Thanks for taking a look. I doubt about your sentence "This also tells
the compiler that the inline assembly clobbers [RE]AX".

How come it tells the compiler that the inline ASM clobbers [RE]AX?

That's an input constraint. Doesn't that mean it is read-only for the
ASM statement? That means the compiler is allowed to assume [RE]AX doesn't
change inside the ASM statement.

Those `dec`s do really change the [RE]AX. Please review this again.

Thanks!

-- 
Ammar Faizi