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From: Thomas Gleixner <[email protected]>
To: Ammar Faizi <[email protected]>, Borislav Petkov <[email protected]>
Cc: Ammar Faizi <[email protected]>,
	Alviro Iskandar Setiawan <[email protected]>,
	Dave Hansen <[email protected]>,
	"H. Peter Anvin" <[email protected]>, Ingo Molnar <[email protected]>,
	Tony Luck <[email protected]>,
	Yazen Ghannam <[email protected]>,
	Linux Edac Mailing List <[email protected]>,
	Linux Kernel Mailing List <[email protected]>,
	Stable Kernel <[email protected]>,
	GNU/Weeb Mailing List <[email protected]>,
	x86 Mailing List <[email protected]>,
	David Laight <[email protected]>,
	Jiri Hladky <[email protected]>
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: <[email protected]>

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

  parent reply	other threads:[~2022-04-03 16:57 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-29 10:47 [PATCH v6 0/2] Two x86 fixes Ammar Faizi
2022-03-29 10:47 ` [PATCH v6 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()` Ammar Faizi
2022-04-01 17:42   ` Dave Hansen
2022-04-02  5:15     ` Ammar Faizi
2022-04-03 16:57   ` Thomas Gleixner [this message]
2022-04-03 17:11     ` Ammar Faizi
2022-04-03 17:14     ` Thomas Gleixner
2022-04-05 20:50   ` [tip: x86/misc] x86/delay: Fix the wrong asm constraint in delay_loop() tip-bot2 for Ammar Faizi
2022-03-29 10:47 ` [PATCH v6 2/2] x86/MCE/AMD: Fix memory leak when `threshold_create_bank()` fails Ammar Faizi
2022-04-03 17:03   ` Thomas Gleixner
2022-04-03 17:43     ` Ammar Faizi
2022-04-03 17:45       ` Ammar Faizi
2022-04-03 18:46         ` Borislav Petkov
2022-04-05 20:56   ` [tip: ras/core] x86/MCE/AMD: Fix memory leak when threshold_create_bank() fails tip-bot2 for Ammar Faizi

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