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

From: Dave Hansen
Date: Fri Apr 01 2022 - 13:42:54 EST

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?