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

Josh Poimboeuf
2019-08-28T16:34:39Z

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