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From: Ammar Faizi <[email protected]>
To: Alviro Iskandar Setiawan <[email protected]>
Cc: Borislav Petkov <[email protected]>,
	Dave Hansen <[email protected]>,
	"H. Peter Anvin" <[email protected]>, Ingo Molnar <[email protected]>,
	Thomas Gleixner <[email protected]>,
	Tony Luck <[email protected]>,
	[email protected],
	Linux Kernel Mailing List <[email protected]>,
	GNU/Weeb Mailing List <[email protected]>,
	[email protected], [email protected],
	Jiri Hladky <[email protected]>,
	Greg Kroah-Hartman <[email protected]>,
	David Laight <[email protected]>
Subject: Re: [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`
Date: Thu, 3 Mar 2022 07:06:52 +0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAOG64qPgTv5tQNknuG9d-=oL2EPQQ1ys7xu2FoBpNLyzv1qYzA@mail.gmail.com>

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

  reply	other threads:[~2022-03-03  0:06 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-01  9:46 [PATCH v4 0/2] Two x86 fixes Ammar Faizi
2022-03-01  9:46 ` [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()` Ammar Faizi
2022-03-01  9:54   ` David Laight
2022-03-03  0:14     ` Ammar Faizi
2022-03-01 11:33   ` Alviro Iskandar Setiawan
2022-03-03  0:06     ` Ammar Faizi [this message]
2022-03-03  0:35     ` David Laight
2022-03-01  9:46 ` [PATCH v4 2/2] x86/mce/amd: Fix memory leak when `threshold_create_bank()` fails Ammar Faizi
2022-03-02 17:26   ` Yazen Ghannam
2022-03-02 23:20     ` Ammar Faizi
2022-03-02 23:27       ` Ammar Faizi
2022-03-03  1:58         ` Alviro Iskandar Setiawan
2022-03-03  2:07           ` Ammar Faizi
2022-03-03  2:32             ` Ammar Faizi
2022-03-03  2:51               ` Alviro Iskandar Setiawan
2022-03-07  0:27               ` Ammar Faizi
2022-03-09 20:55                 ` Yazen Ghannam
2022-03-10  1:56                   ` Ammar Faizi

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