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

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Aug 12, 2015 at 1:02 AM, Stas Sergeev <stsp@list.ru> wrote:
> 12.08.2015 03:38, Andy Lutomirski пишет:
>>
>> On Tue, Aug 11, 2015 at 5:17 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>
>>> Hi guys, I wonder how easily the include/uapi/* is being
>>> changed these days.
>>> The patch:
>>>
>>> http://lkml.kernel.org/r/405594361340a2ec32f8e2b115c142df0e180d8e.1426193719.git.luto@kernel.org
>>> breaks dosemu (and perhaps everyone else who used
>>> to restore the segregs by hands). And the fix involves
>>> both autoconf magic and run-time magic, so it is not even
>>> trivial.
>>> I realize this patch may be good to have in general, but
>>> breaking userspace without a single warning is a bit
>>> discouraging. Seems like the old "we don't break userspace"
>>> rule have gone.
>>
>> I didn't anticipate any breakage.  I could have been wrong.
>
> You changed include/uapi/*, which is obviously an asking
> for problems. I applied the following changes to my local
> git tree to get dosemu working again:

To be fair, I renamed a field that used to be padding.  The UAPI has
to change on occasion -- it's just not supposed to break things.

> https://github.com/stsp/dosemu2/commit/48b2a13a49a9fe1a456cd77df6b9a1feec675a01

Maybe I'm still missing something, but this seems like it should be
unnecessary.  What goes wrong without it?

The new ss field serves two purposes: it stores the old ss (dosemu
needs that on new kernels and would benefit in general) and it stores
the new post-sigreturn ss (dosemu doesn't currently have any use for
that because of the iret trampoline trick).

But maybe you're doing this to make the next patch work.

> https://github.com/stsp/dosemu2/commit/7898ac60d5e569964127d6cc48f592caecd20b81

So the problem is that dosemu was actually hacking around the old
buggy behavior and thus relying on it.  Grr.

>> We might still be able to require a new sigcontext flag to be set and
>> to forcibly return to __USER_DS if the flag is set regardless of the
>> ss value in sigcontext when sigreturn is called, if that is indeed the
>> problem with DOSEMU.  But I'm not actually sure that that's the
>> problem.
>
> Well, the flag would be an ideal solution in an ideal world,
> but in our world I don't know the current relevance of dosemu,
> and whether or not it worth a new flag to add.

It wouldn't even help here, because the breakage isn't caused by
incompatible sigcontext formats -- it's caused by dosemu's reliance on
ss being preserved across signal delivery (even if it wasn't preserved
on the way back).

>
>> In fact, DOSEMU contains this:
>>
>>    /* set up a frame to get back to DPMI via iret. The kernel does not
>> save
>>       %ss, and the SYSCALL instruction in sigreturn() destroys it.
>>
>>       IRET pops off everything in 64-bit mode even if the privilege
>>       does not change which is nice, but clobbers the high 48 bits
>>       of rsp if the DPMI client uses a 16-bit stack which is not so
>>       nice (see EMUfailure.txt). Setting %rsp to 0x100000000 so that
>>       bits 16-31 are zero works around this problem, as DPMI code
>>       can't see bits 32-63 anyway.
>>   */
>>
>> So, if DOSEMU were to realize that both sigreturnissues it's
>> complaining about are fixed in recent kernels, it could sigreturn
>> directly back to any state.
>
> Good, but have you added any flag for dosemu to even know
> it can do this? Unless I am mistaken, you didn't. So the fix you
> suggest, is not easy to detect and make portable with the older
> kernels. Any suggestions?
>

You could probe for it directly: raise a signal, change the saved ss
and see what's in ss after sigreturn.

Let me see if I can come up with a clean kernel fix.

--Andy