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

From: Andy Lutomirski
Date: Wed Aug 12 2015 - 12:19:55 EST

On Wed, Aug 12, 2015 at 1:02 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 12.08.2015 03:38, Andy Lutomirski ÐÐÑÐÑ:
>> On Tue, Aug 11, 2015 at 5:17 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>> Hi guys, I wonder how easily the include/uapi/* is being
>>> changed these days.
>>> The patch:
>>> 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.


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.


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.

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