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

Denys Vlasenko
2015-02-13T13:25:43Z

On Fri, Feb 13, 2015 at 1:01 PM, Borislav Petkov <bp@alien8.de> 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@redhat.com>
>> CC: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
>> CC: Ingo Molnar <mingo@kernel.org>
>> CC: Oleg Nesterov <oleg@redhat.com>
>> CC: linux-kernel@vger.kernel.org
>> ---
>> 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 ← EIP + SignExtend(DEST);
> ELSE IF OperandSize = 64
> THEN RIP ← RIP + SignExtend(DEST);
> FI;
> ELSE IF OperandSize = 16
> THEN EIP ← 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)