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Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- 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>, Michael Matz <matz at suse dot de>
- Date: Wed, 13 May 2015 04:35:21 -0700
- Subject: Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Authentication-results: sourceware.org; auth=none
- References: <20150511212331 dot GA1838 at intel dot com> <5551F4E70200007800079575 at mail dot emea dot novell dot com> <CAMe9rOpDbkeFbmNbQh0a1AKhAQy-cH4HJu20o_ERQaoR6sTxbQ at mail dot gmail dot com> <55520C440200007800079718 at mail dot emea dot novell dot com> <CAMe9rOpFgSyJm-oceuDkrBYnBQGv01ywCc43WySqX21NTJYi4Q at mail dot gmail dot com> <555216370200007800079773 at mail dot emea dot novell dot com> <CAMe9rOpOx=SSUZnCFimn4fBzFqNRDch8QYLn3Os_y7EfQH65Qw at mail dot gmail dot com> <5552318402000078000798A8 at mail dot emea dot novell dot com> <CAMe9rOoqcr7aE8dr6E44KUK6JLrNMNcDNFFWhcb6K+14M=Y+=w at mail dot gmail dot com> <555233B602000078000798EF at mail dot emea dot novell dot com> <CAMe9rOrOS+K0R+r1jHCNwAkgrhjftHUOGt_wTuP8wRYcPdifmQ at mail dot gmail dot com> <555235930200007800079911 at mail dot emea dot novell dot com> <alpine dot LSU dot 2 dot 20 dot 1505121736050 dot 4883 at wotan dot suse dot de> <CAMe9rOo76QirYvEH=tX7BDBws3z=g0O8c+A1wSp+19yaNXUk1w at mail dot gmail dot com> <alpine dot LSU dot 2 dot 20 dot 1505121745550 dot 27315 at wotan dot suse dot de> <CAMe9rOqM+cojMqoz9Kwb_KedgZG-14_xFaV2mk=hNEQGUkDWVw at mail dot gmail dot com> <alpine dot LSU dot 2 dot 20 dot 1505121803030 dot 27315 at wotan dot suse dot de> <CAMe9rOqECoP=-Bz4neR1LnFsnPO94axMuSpEiOzjmJzbNgA_kA at mail dot gmail dot com> <555308DB0200007800079CDA at mail dot emea dot novell dot com>
On Tue, May 12, 2015 at 11:18 PM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 12.05.15 at 18:08, <hjl.tools@gmail.com> wrote:
>> 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.
>
> What _works_ on Intel processors is secondary here. Fact is that
> the x86-64 design came from AMD, and hence Intel CPUs doing
> things differently than AMD's is - be honest - a flaw. The more
I don't think who came first is relevant here. What relevant are
1. AMD and Intel specs are different.
2. There is no real usage for AMD spec.
3. There is a bug report against Intel spec.
> that by analogy with 32-bit mode, an operand size prefix on
> branches ought to truncate rIP. Plus (other than my own testing
> says) you seem to suggest that this isn't even consistent on Intel
> CPUs, as you specifically say "unconditional" above and you also
> only changed those.
Please open a bug report against Jcc and I will look into it.
>> I will keep my change in unlessl someone can show a real usage of
>>
>> 066 jmp/call rel16
>>
>> on AMD processors.
>
> That's the wrong position, you have to show that the change is
> useful - I certainly can't see why you'd need the operand size
https://sourceware.org/bugzilla/show_bug.cgi?id=18386
> prefix when (on Intel CPUs) it has no effect whatsoever.
> Together with it not being generally usable (due to the vendor
> differences), I view the change as pointless _and_ breaking
> compatibility (i.e. both by themselves a reason to revert).
>
--
H.J.
- References:
- [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches
- Re: [committed, PATCH] Remove Disp16|Disp32 from 64-bit direct branches