[committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches

[H.J. Lu]

On Tue, May 12, 2015 at 9:03 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Tue, 12 May 2015, H.J. Lu wrote:
>
>> On Tue, May 12, 2015 at 8:47 AM, Michael Matz <matz@suse.de> wrote:
>> > Hi,
>> >
>> > On Tue, 12 May 2015, H.J. Lu wrote:
>> >
>> >> > Actually also that one is correctly printed I think (from a hello world
>> >> > main, where I added a jmprel16 +0):
>> >> >
>> >> > 000000000040055c <main>:
>> >> >   40055c:       55                      push   %rbp
>> >> >   40055d:       48 89 e5                mov    %rsp,%rbp
>> >> >   400560:       48 83 ec 30             sub    $0x30,%rsp
>> >> >   400564:       c6 45 d1 00             movb   $0x0,-0x2f(%rbp)
>> >> >   400568:       c6 45 d0 61             movb   $0x61,-0x30(%rbp)
>> >> >   40056c:       48 8d 45 d0             lea    -0x30(%rbp),%rax
>> >> >   400570:       48 89 c2                mov    %rax,%rdx
>> >> >   400573:       be 44 06 40 00          mov    $0x400644,%esi
>> >> >   400578:       66 e9 00 00             jmpw   57c <_init-0x3ffe8c>
>> >> >
>> >> > 000000000040057c <next>:
>> >> >   40057c:       bf 52 06 40 00          mov    $0x400652,%edi
>> >> >   ...
>> >> >
>> >> > It shows that rip is going to be truncated.
>> >> >
>> >>
>> >> This is the same issue as
>> >>
>> >> https://sourceware.org/bugzilla/show_bug.cgi?id=18386
>> >>
>> >> On Intel processors, 0x66 prefix before direct 32-bit unconditional
>> >> call/jmp is ignored.  Whatever we do is wrong on AMD or Intel
>> >> processors.
>> >
>> > Well, in that case I'd say the correct thing to do is to _not_ do any
>>
>> This is NO correct thing to do.
>
> Well, what do you suggest?  Your change is clearly wrong as well.

I won't call it wrong since it implies there is a right.  Given that

0x66 jmp/call rel32

works on Intel processors and crashes on AMD processors.
I will keep my change in unlessl someone can show a real usage of

066 jmp/call rel16

on AMD processors.


-- 
H.J.