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

[H.J. Lu]

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?

> >> 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.

-- 
H.J.