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

Borislav Petkov
2015-02-14T00:28:59Z

On Fri, Feb 13, 2015 at 02:25:20PM +0100, Denys Vlasenko wrote:
> > 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)
>

AMD CPU always truncates:

32-bit: a.out[13626]: segfault at 8098 ip 0000000000008098 sp 00000000ffa0ea20 error 14 in a.out[8048000+1000]

64-bit: a.out[13706]: segfault at d6 ip 00000000000000d6 sp 00007fffec14e870 error 14 in a.out[400000+1000]

Intel CPU:

32-bit: a.out[3478]: segfault at 8098 ip 0000000000008098 sp 00000000ff959da0 error 14 in a.out[8048000+1000]

64-bit:

Make the loop terminate:

_start: .globl _start
mov $1, %rcx
1: .byte 0x66
loop 1b

a.out[3523]: segfault at 0 ip 00000000004000de sp 00007ffff31674e0 error 6 in a.out[400000+1000]

segfaults because we don't have the libc glue around it, rIP is intact.

So it looks like the SDM is wrong.

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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