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

[Stas Sergeev <stsp@list.ru>]

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:
https://github.com/stsp/dosemu2/commit/48b2a13a49a9fe1a456cd77df6b9a1feec675a01
https://github.com/stsp/dosemu2/commit/7898ac60d5e569964127d6cc48f592caecd20b81

> Do you know what the actual breakage is?  I'm curious how this ever
> worked for DOSEMU, given that, before this patch, it appeared to be
> impossible to return to any nonstandard SS from a 64-bit signal
> handler.
This is not the point.
What dosemu wants is to simply save the DOS SS somewhere.
After your patch, it saves the Linux SS instead, then crashes.

>    FWIW, DOSEMU seems to work for me on recent kernels.
Do you have any protected mode DOS program to test?
I'll send you one in a private e-mail just about now.

> 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.

> 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?

> I don't actually see any code in DOSEMU that generates a sigcontext
It doesn't. But it manually pops the kernel-generated
sigcontext, see dpmisel.S:DPMI_direct_transfer.
This part didn't broke though, so no need to look there in fact.

> from scratch (as opposed to copying one and modifying it), so I'm not
> entirely sure what the problem is.
Now you have the changes I did to get it working, and I'll
also mail you a simple test-case. Let me know if you need
something else.
The more important question is whether we ignore dosemu
or some actions should be taken. Since you changed uapi/*,
my initial guess was that you opt for ignoring it, but maybe
this was not the point.