Re: [PATCH] x86: x86-opcode-map.txt: explain CALLW discrepancy between Intel and AMD

[Denys Vlasenko]

On Fri, Feb 13, 2015 at 1:01 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Thu, Feb 12, 2015 at 08:06:57PM +0100, Denys Vlasenko wrote:
>> In 64-bit mode, AMD and Intel CPUs treat 0x66 prefix before branch
>> insns differently. For near branches, it affects decode too since
>> immediate offset's width is different.
>>
>> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>> CC: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
>> CC: Ingo Molnar <mingo@xxxxxxxxxx>
>> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
>> CC: linux-kernel@xxxxxxxxxxxxxxx
>> ---
>>  arch/x86/lib/x86-opcode-map.txt | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/x86/lib/x86-opcode-map.txt b/arch/x86/lib/x86-opcode-map.txt
>> index 1a2be7c..816488c 100644
>> --- a/arch/x86/lib/x86-opcode-map.txt
>> +++ b/arch/x86/lib/x86-opcode-map.txt
>> @@ -273,6 +273,9 @@ dd: ESC
>>  de: ESC
>>  df: ESC
>>  # 0xe0 - 0xef
>> +# Note: "forced64" is Intel CPU behavior: they ignore 0x66 prefix
>> +# in 64-bit mode. AMD CPUs accept 0x66 prefix, it causes RIP truncation
>> +# to 16 bits. In 32-bit mode, 0x66 is accepted by both Intel and AMD.
>
> Well, according to the SDM, Intel truncates too, see the LOOP/LOOPcc
> Operation section:
>
>         ...
>         IF BranchCond = 1
>         THEN
>         IF OperandSize = 32
>         THEN EIP â EIP + SignExtend(DEST);
>         ELSE IF OperandSize = 64
>         THEN RIP â RIP + SignExtend(DEST);
>         FI;
>         ELSE IF OperandSize = 16
>         THEN EIP â EIP AND 0000FFFFH;           <---
>
> and text talks about 0x67 but that's address size and it is used to size
> the rCX register.
>
> So something must be setting the OperandSize and text doesn't mention
> anywhere about 0x66 being ignored.
>
> Or have you been doing some empirical experiments? :-)

Yes, I did.

32-bit case: Intel CPU truncates EIP to 16 bits:

$ cat t.S
_start:         .globl  _start
1:  .byte 0x66
    loop 1b

$ gcc -nostartfiles -nostdlib -m32 t.S

$ objdump -dr a.out
a.out:     file format elf32-i386
Disassembly of section .text:
08048098 <_start>:
 8048098:    66                       data16
 8048099:    e2 fd                    loop   8048098 <_start>

$ gdb ./a.out
(gdb) run
Program received signal SIGSEGV, Segmentation fault.
0x00008098 in ?? ()


Now let's try 64-bit version - compiling without -m32:

$ gcc -nostartfiles -nostdlib t.S
$ ./a.out
(runs without SEGV)
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