Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

[Stas Sergeev]

13.08.2015 19:42, Andy Lutomirski ÐÐÑÐÑ:
> On Thu, Aug 13, 2015 at 9:38 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > 13.08.2015 19:24, Andy Lutomirski ÐÐÑÐÑ:
> >
> > > On Thu, Aug 13, 2015 at 9:20 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > > > 13.08.2015 19:09, Andy Lutomirski ÐÐÑÐÑ:
> > > >
> > > > > On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > > > > > 13.08.2015 18:38, Andy Lutomirski ÐÐÑÐÑ:
> > > > > > > So...  what do we do about it?  We could revert the whole mess.  We
> > > > > > > could tell everyone to fix their DOSEMU, which violates policy and is
> > > > > > > especially annoying given how much effort we've put into keeping
> > > > > > > 16-bit mode fully functional lately.  We could add yet more heuristics
> > > > > > > and teach sigreturn to ignore the saved SS value in sigcontext if the
> > > > > > > saved CS is 64-bit and the saved SS is unusable.
> > > > > > Andy, why do you constantly ignore the proposal to make
> > > > > > new behaviour explicitly controlable? You don't have to agree
> > > > > > with it, but you could at least comment on that possibility
> > > > > > and/or mention it with the ones you listed above.
> > > > > I'm not sure what the proposal is exactly.
> > > > >
> > > > > We could add a new uc_flags flag.  If set, it means that
> > > > > sigcontext->ss is valid and should be used by sigreturn.  If clear,
> > > > > then we ignore sigcontext->ss and just restore __USER_DS.
> > > > >
> > > > > The problem is that, by itself, this won't fix old DOSEMU.  We somehow
> > > > > need to either detect that something funny is going on or just leave
> > > > > the flag clear by default.
> > > > >
> > > > > We could do this: always save SS to sigcontext->ss, but only restore
> > > > > sigcontext->ss if userspace explicitly sets the flag before sigreturn.
> > > > > If we do that, we'd need to also add my patch to preserve the actual
> > > > > HW SS selector if possible so that old DOSEMU knows what SS to program
> > > > > into its trampoline.
> > > > >
> > > > > This at least lets *new* DOSEMU set the flag and get the improved
> > > > > behavior.  I still don't know what effect it'll have on Wine and CRIU.
> > > > >
> > > > > Stas, is that what you were thinking, or were you thinking of something
> > > > > else?
> > > > Not quite.
> > > > I mean the flag that will control not only sigreturn, but
> > > > the signal delivery as well. This may probably be a sigaction()
> > > > flag or some other. If not set - ss is ignored by both signal
> > > > delivery and sigreturn(). If set - ss is saved/restored (and in
> > > > the future - also fs/gs).
> > > > Is such a flag possible?
> > > Maybe.  I think I'm more nervous about adding new flags in sigaction
> > > than I am in uc_flags.
> > Isn't uc_flags read-only for the user?
> > I look into setup_rt_frame
> > <http://lxr.free-electrons.com/ident?v=2.4.37;i=setup_rt_frame>() and see
> > ---
> >   /* Create the ucontext.  */
> > err |= __put_user(0, &frame->uc.uc_flags);
> > ---
> > so it doesn't look like the flag that user can use to _request_
> > something from the kernel. And I am talking about exactly
> > the flag to request the new behaviour, as only that can remove
> > the regression completely without patching dosemu.
> User code could rewrite it in the signal handler to request something.
But that's too late to affect the signal _delivery_ anyhow, no?
Any idea about the flag that can control both delivery and return?
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