Re: [PATCH v6 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`

From: Thomas Gleixner <tglx@linutronix.de>
To: Ammar Faizi <ammarfaizi2@gnuweeb.org>, Borislav Petkov <bp@alien8.de>
Cc: Ammar Faizi <ammarfaizi2@gnuweeb.org>,
	Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Tony Luck <tony.luck@intel.com>,
	Yazen Ghannam <yazen.ghannam@amd.com>,
	Linux Edac Mailing List <linux-edac@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Stable Kernel <stable@vger.kernel.org>,
	GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
	x86 Mailing List <x86@kernel.org>,
	David Laight <David.Laight@ACULAB.COM>,
	Jiri Hladky <hladky.jiri@googlemail.com>
Subject: Re: [PATCH v6 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`
Date: Sun, 03 Apr 2022 18:57:40 +0200	[thread overview]
Message-ID: <87zgl2ksu3.ffs@tglx> (raw)
In-Reply-To: <20220329104705.65256-2-ammarfaizi2@gnuweeb.org>

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.

Nothing to fix here, whether the code is inlined or not.

Thanks,

        tglx