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

["H. Peter Anvin" <hpa@zytor.com>]

Buchbinder <adam.buchbinder@gmail.com>,Colin Ian King <colin.king@canonical.com>,Lorenzo Stoakes <lstoakes@gmail.com>,Qiaowei Ren <qiaowei.ren@intel.com>,Arnaldo Carvalho de Melo <acme@redhat.com>,Adrian Hunter <adrian.hunter@intel.com>,Kees Cook <keescook@chromium.org>,Thomas Garnier <thgarnie@google.com>,Dmitry Vyukov <dvyukov@google.com>
From: hpa@zytor.com
Message-ID: <18E8698F-6C60-4B98-AE73-C371184C5135@zytor.com>

On January 25, 2017 1:58:27 PM PST, Andy Lutomirski <luto@amacapital.net> wrote:
>On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri
><ricardo.neri-calderon@linux.intel.com> wrote:
>> Tasks running in virtual-8086 mode will use 16-bit addressing form
>> encodings as described in the Intel 64 and IA-32 Architecture
>Software
>> Developer's Manual Volume 2A Section 2.1.5. 16-bit addressing
>encodings
>> differ in several ways from the 32-bit/64-bit addressing form
>encodings:
>> the r/m part of the ModRM byte points to different registers and, in
>some
>> 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.
>This
>> implies that the segment selectors do not point to a segment
>descriptor
>> but are used to compute logical addresses. Hence, there is a need to
>> add support to compute addresses using the segment selectors. If
>segment-
>> override prefixes are present in the instructions, they take
>precedence.
>>
>> Lastly, it is important to note that when a tasks is running in
>virtual-
>> 8086 mode and an interrupt/exception occurs, the CPU pushes to the
>stack
>> the segment selectors for ds, es, fs and gs. These are accesible via
>the
>> struct kernel_vm86_regs rather than pt_regs.
>>
>> Code for 16-bit addressing encodings is likely to be used only by
>virtual-
>> 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
>case.)
> - 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.
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