Re: [PATCH] i386: Don't add 0x66 prefix to IRET for .code16gcc

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Apr 29, 2019 at 9:11 AM Jan Beulich <JBeulich@suse.com> wrote:
>
> >>> On 29.04.19 at 18:02, <hjl.tools@gmail.com> wrote:
> > On Mon, Apr 29, 2019 at 8:25 AM Jan Beulich <JBeulich@suse.com> wrote:
> >>
> >> >>> On 29.04.19 at 17:09, <hjl.tools@gmail.com> wrote:
> >> > On Mon, Apr 29, 2019 at 12:01 AM Jan Beulich <JBeulich@suse.com> wrote:
> >> >> >>> On 26.04.19 at 19:22, <hjl.tools@gmail.com> wrote:
> >> >> > The .code16gcc directive supports 16bit mode with 32-bit address.  Since
> >> >> > IRET (opcode 0xcf) in 16bit mode returns from an interrupt in 16bit mode,
> >> >> > we shouldn't add 0x66 prefix for IRET.
> >> >> >
> >> >> >       PR gas/24485
> >> >> >       * config/tc-i386.c (process_suffix): Don't add DATA_PREFIX_OPCODE
> >> >> >       to IRET for .code16gcc.
> >> >>
> >> >> This, at the very least, needs to be accompanied by a warning:
> >> >
> >> > This patch fixes:
> >> >[...]
> >> >> As the bug report validly says, the changed behavior is what is
> >> >> wanted only "almost always". The report even mentions the
> >> >> (supposedly uncommon) case: Code manually building a frame
> >> >> and IRETing to it will now be silently(!) broken.
> >> >
> >> > The .code16gcc directive is to support "gcc -m16".   Any other purposes
> >> > are not supported.
> >>
> >> But you realize that people may use inline assembly?
> >
> > Inline assembly with the .code16gcc directive in an interrupt
> > handler? It is a supported usage?
>
> I don't know, but I see no reasons why it would not be. Note
> that I didn't mention "in an interrupt handler" - I can see uses
> for manually created frames to IRET to elsewhere.
>
> >> >> In fact I think the better solution would be to reject ambiguous
> >> >> code by demanding a suffix in all cases in .code16gcc mode.
> >> >
> >> > This may break existing codes.
> >>
> >> Of course, but breaking things at build time (with a proper
> >> diagnostic) that's better than silently breaking things at
> >> runtime. At the very least you can't claim it would break the
> >> supposedly common case, as that was already broken (and
> >> hence your fix). So the difference between suggested
> >> and current behavior is that right now there's silent latent
> >> breakage, whereas otherwise people would be made aware
> >> of there being a problem they need to address by changing
> >> some of their code.
> >
> > Assembler has no way to know if an assembly sequence is
> > correct and it shouldn't issue a warning for "gcc -m16" just
> > because the same instruction may be incorrect.
>
> I disagree: In this case, the assembler simply can't decide
> whether adding an operand size override is correct. Instead
> of silently doing the opposite of what has been done for
> many years, it should point out that it needs programmer
> guidance.
>

So the specific case is

1. Programming in 16-bit mode with GCC using "gcc -m16".
2. Manually create a 32-bit stack frame for a function with 32-bit iret.
3. Implement such a function with .code16gcc and "iret".
4. Jump to such a function.

-- 
H.J.