linux-kernel - Re: linux-next: Tree for Aug 27 (objtool)

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Date:   Wed, 28 Aug 2019 11:34:33 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: linux-next: Tree for Aug 27 (objtool)

On Wed, Aug 28, 2019 at 11:13:31AM -0500, Josh Poimboeuf wrote:
> Turns out this patch does break something:
> 
>   arch/x86/xen/enlighten_pv.o: warning: objtool: xen_cpuid()+0x25: can't find jump dest instruction at .text+0x9c
> 
> I'll need to figure out a better way to whitelist that
> XEN_EMULATE_PREFIX fake instruction thing.  I'll probably just teach
> the objtool decoder about it.

Hi Masami,

Is it possible for the kernel x86 decoder to recognize the
XEN_EMULATE_PREFIX prefix?

        asm(XEN_EMULATE_PREFIX "cpuid"
                : "=a" (*ax),
                  "=b" (*bx),
                  "=c" (*cx),
                  "=d" (*dx)
                : "0" (*ax), "2" (*cx));

is disassembled to:

      33:       0f 0b                   ud2
      35:       78 65                   js     9c <xen_store_tr+0xc>
      37:       6e                      outsb  %ds:(%rsi),(%dx)
      38:       0f a2                   cpuid

which confuses objtool.  Presumably that would confuse other users of
the decoder as well.

That's a highly unlikely sequence of instructions, maybe the kernel
decoder should recognize it as a single instruction.

-- 
Josh