linux-kernel - Re: [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay

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Message-ID: <CAOG64qPgTv5tQNknuG9d-=oL2EPQQ1ys7xu2FoBpNLyzv1qYzA@mail.gmail.com>
Date:   Tue, 1 Mar 2022 18:33:54 +0700
From:   Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>, linux-edac@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "GNU/Weeb Mailing List" <gwml@...r.gnuweeb.org>, x86@...nel.org,
        stable@...r.kernel.org, Jiri Hladky <hladky.jiri@...glemail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v4 1/2] x86/delay: Fix the wrong asm constraint in `delay_loop()`

On Tue, Mar 1, 2022 at 4:46 PM Ammar Faizi wrote:
> Fortunately, the constraint violation that's fixed by patch 1 doesn't
> yield any bug due to the nature of System V ABI. Should we backport
> this?

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

-- Viro