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

Ammar Faizi
2022-03-03T00:07:39Z

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