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Re: [committed, PATCH] PR binutis/18386: callw with 0x66 prefix incorrectly disassembled in 64-bit mode
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Mon, 11 May 2015 05:40:54 -0700
- Subject: Re: [committed, PATCH] PR binutis/18386: callw with 0x66 prefix incorrectly disassembled in 64-bit mode
- Authentication-results: sourceware.org; auth=none
- References: <20150509135213 dot GA720 at gmail dot com> <555076520200007800078B8F at mail dot emea dot novell dot com> <CAMe9rOpRdCftue3uFgaejAYZbi=kC91y7sXkfWg-8py3Nmy5kw at mail dot gmail dot com> <5550AEC30200007800078E84 at mail dot emea dot novell dot com>
On Mon, May 11, 2015 at 4:29 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 11.05.15 at 13:04, <email@example.com> wrote:
>> On Mon, May 11, 2015 at 12:28 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>> On 09.05.15 at 15:52, <firstname.lastname@example.org> wrote:
>>>> The operand size prefix (0x66) is ignored for 32-bit PC-relative call,
>>>> jmp and jcc in 64-bit mode.
>>> Now that's again a change that should have been discussed
>>> before committing: While it is correct for the Intel implementation
>>> of x86-64, it isn't for AMD's original (and I'd tend to say that the
>>> latter is more correct, since afaik there's nowhere that exceptions
>>> are being made for the meaning of the operand size prefix).
>> It is also correct for AMD.
> Very certainly not, based on practical observation: This code
> mov rcx, rsp
> .byte 0x66, 0xe8, 0x00, 0x00, 0x90, 0x90
> xchg rcx, rsp
> Reading symbols from /home/jbeulich/x86-64...done.
> (gdb) break x86_64
> Breakpoint 1 at 0x400520
> (gdb) r
> Starting program: /home/jbeulich/x86-64
> Failed to read a valid object file image from memory.
> Breakpoint 1, 0x0000000000400520 in x86_64 ()
> (gdb) x/5i $pc
> => 0x400520 <x86_64>: mov rcx,rsp
> 0x400523 <x86_64+3>: call 0x527
> 0x400527 <x86_64+7>: nop
> 0x400528 <x86_64+8>: nop
> 0x400529 <x86_64+9>: xchg rcx,rsp
> (gdb) c
> Program received signal SIGSEGV, Segmentation fault.
> 0x0000000000000527 in ?? ()
> with RSP also decremented by just 2.
The displacement is still 32-bit, but stack adjustment is
different. Display it as callq isn't accurate for AMD and
callw is not correct for both AMD and Intel. I will leave it
as is since I don't think we should add another instruction
BTW, thanks for letting me know. I was using 0x66 as
nop prefix for call/jmp PIC relaxation. I changed it to
0x48 now. Will 0x48 a problem on AMD?