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

[Michael Matz <matz at suse dot de>]

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.


Ciao,
Michael.