Re: [v3 PATCH 05/10] x86/insn-kernel: Add support to resolve 16-bit addressing encodings

From: H. Peter Anvin
Date: Wed Jan 25 2017 - 17:06:02 EST

Buchbinder <adam.buchbinder@xxxxxxxxx>,Colin Ian King <colin.king@xxxxxxxxxxxxx>,Lorenzo Stoakes <lstoakes@xxxxxxxxx>,Qiaowei Ren <>,Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>,Adrian Hunter <adrian.hunter@xxxxxxxxx>,Kees Cook <keescook@xxxxxxxxxxxx>,Thomas Garnier <thgarnie@xxxxxxxxxx>,Dmitry Vyukov <dvyukov@xxxxxxxxxx>
From: hpa@xxxxxxxxx
Message-ID: <18E8698F-6C60-4B98-AE73-C371184C5135@xxxxxxxxx>

On January 25, 2017 1:58:27 PM PST, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri
><ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
>> Tasks running in virtual-8086 mode will use 16-bit addressing form
>> encodings as described in the Intel 64 and IA-32 Architecture
>> Developer's Manual Volume 2A Section 2.1.5. 16-bit addressing
>> differ in several ways from the 32-bit/64-bit addressing form
>> the r/m part of the ModRM byte points to different registers and, in
>> cases, addresses can be indicated by the addition of the value of two
>> registers. Also, there is no support for SiB bytes. Thus, a separate
>> function is needed to parse this form of addressing.
>> Furthermore, virtual-8086 mode tasks will use real-mode addressing.
>> implies that the segment selectors do not point to a segment
>> but are used to compute logical addresses. Hence, there is a need to
>> add support to compute addresses using the segment selectors. If
>> override prefixes are present in the instructions, they take
>> Lastly, it is important to note that when a tasks is running in
>> 8086 mode and an interrupt/exception occurs, the CPU pushes to the
>> the segment selectors for ds, es, fs and gs. These are accesible via
>> struct kernel_vm86_regs rather than pt_regs.
>> Code for 16-bit addressing encodings is likely to be used only by
>> 8086 mode tasks. Thus, this code is wrapped to be built only if the
>> option CONFIG_VM86 is selected.
>That's not true. It's used in 16-bit protected mode, too. And there
>are (ugh!) six possibilities:
> - Normal 32-bit protected mode. This should already work.
> - Normal 64-bit protected mode. This should also already work. (I
>forget whether a 16-bit SS is either illegal or has no effect in this
> - Virtual 8086 mode
> - Normal 16-bit protected mode, used by DOSEMU and Wine. (16-bit CS,
>16-bit address segment)
> - 16-bit CS, 32-bit address segment. IIRC this might be used by some
>32-bit DOS programs to call BIOS.
>- 32-bit CS, 16-bit address segment. I don't know whether anything
>uses this.
>I don't know if anything you're doing cares about SS's, DS's, etc.
>size, but I suspect you'll need to handle 16-bit CS.

Only the CS bitness matters for the purpose of addressing modes; the SS bitness (which has no effect in 64-bit mode) only matters for implicit stack references unless I'm completely out to sea.
Sent from my Android device with K-9 Mail. Please excuse my brevity.