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

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

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