Re: [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`

[Ammar Faizi]

On 3/1/22 6:33 PM, Alviro Iskandar Setiawan wrote:
> hi sir, it might also be interesting to know that even if it never be
> inlined, it's still potential to break.
>
> for example this code (https://godbolt.org/z/xWMTxhTET)
>
>    __attribute__((__noinline__)) static void x(int a)
>    {
>        asm("xorl\t%%r8d, %%r8d"::"a"(a));
>    }
>
>    extern int p(void);
>
>    int f(void)
>    {
>        int ret = p();
>        x(ret);
>        return ret;
>    }
>
> translates to this asm
>
>    x:
>            movl    %edi, %eax
>            xorl    %r8d, %r8d
>            ret
>    f:
>            subq    $8, %rsp
>            call    p
>            movl    %eax, %r8d
>            movl    %eax, %edi
>            call    x
>            movl    %r8d, %eax
>            addq    $8, %rsp
>            ret
>
> See the %r8d? It should be clobbered by a function call too. But since
> no one tells the compiler that we clobber %r8d, it assumes %r8d never
> changes after that call. The compiler thinks x() is static and will
> not clobber %r8d, even the ABI says %r8d will be clobbered by a
> function call. So i think it should be backported to the stable
> kernel, it's still a fix

Thanks. I will add CC stable in the v5.

--
Ammar Faizi