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

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Aug 13, 2015 at 9:19 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Aug 13, 2015 at 8:43 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> I'm trying to fix it without reverting.  If that doesn't work, then we
>> revert.  Yesterday, I thought I had a reasonably clean fix, but it
>> turned out that it only solved half of the problem.
>
> The thing is, I actually think that the current situation is crazy.
>
> Especially given that we don't restore any of the other segment
> registers on x86-64 (except CS, of course)
>
> So how about this "alternate" minimal patch instead. The difference is:
>
>  - we actually leave the
>
>         regs->ss = __USER_DS;
>
>    in __setup_rt_frame, to guarantee that when we take a signal, we do
> take it with a valid SS

That by itself is enough to break DOSEMU.  I think we may be stuck
with my hack to only replace regs->ss if the old one was invalid.

>
>  - but it removes all the other games with SS (and treats it exactly
> the same as FS/GS).
>
>    So now we don't play games with the actual sigcontext, and
> hopefully dosemu is happier.

You mean that we always set ss to __USER_DS on sigreturn?  The problem
is that user code *can't* program SS when calling sigreturn because
the SYSCALL instruction zaps it.

I'll try to implement something.

If this regression were new in 4.2-rc, then I'd say revert first and
ask questions later, but the regression is in 4.1 as well :(

--Andy