From: Andy Lutomirski <luto@amacapital.net>
To: Stas Sergeev <stsp@list.ru>, X86 ML <x86@kernel.org>
Cc: Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu
Date: Wed, 12 Aug 2015 09:19:30 -0700 [thread overview]
Message-ID: <CALCETrUYe8xVpd6v1gc97EfY+7JXNC7t9GsVqHEGKVMjPADRHQ@mail.gmail.com> (raw)
In-Reply-To: <55CAFD9F.2070001@list.ru>
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
next prev parent reply other threads:[~2015-08-12 16:19 UTC|newest]
Thread overview: 121+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-12 0:17 [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu Stas Sergeev
2015-08-12 0:38 ` Andy Lutomirski
2015-08-12 8:02 ` Stas Sergeev
2015-08-12 16:19 ` Andy Lutomirski [this message]
2015-08-12 17:00 ` Stas Sergeev
2015-08-12 18:25 ` Andy Lutomirski
2015-08-12 18:55 ` Stas Sergeev
2015-08-12 19:20 ` Andy Lutomirski
2015-08-12 19:55 ` Stas Sergeev
2015-08-12 20:01 ` Andy Lutomirski
2015-08-12 20:14 ` Stas Sergeev
2015-08-12 20:28 ` Andy Lutomirski
2015-08-12 20:45 ` Stas Sergeev
2015-08-12 20:47 ` Andy Lutomirski
2015-08-12 20:55 ` Stas Sergeev
2015-08-12 21:37 ` Andy Lutomirski
2015-08-12 21:50 ` Stas Sergeev
2015-08-12 22:00 ` Andy Lutomirski
2015-08-13 8:39 ` Ingo Molnar
2015-08-13 10:14 ` Stas Sergeev
2015-08-13 12:44 ` Stas Sergeev
2015-08-13 14:58 ` Andy Lutomirski
2015-08-13 15:22 ` Stas Sergeev
2015-08-13 15:38 ` Andy Lutomirski
2015-08-13 16:03 ` Stas Sergeev
2015-08-13 16:09 ` Andy Lutomirski
2015-08-13 16:20 ` Stas Sergeev
2015-08-13 16:24 ` Andy Lutomirski
2015-08-13 16:38 ` Stas Sergeev
2015-08-13 16:42 ` Andy Lutomirski
2015-08-13 16:48 ` Stas Sergeev
2015-08-13 16:59 ` Andy Lutomirski
2015-08-13 17:13 ` Stas Sergeev
2015-08-13 17:17 ` Andy Lutomirski
2015-08-13 18:00 ` Stas Sergeev
2015-08-13 18:05 ` Andy Lutomirski
2015-08-13 18:19 ` Stas Sergeev
2015-08-13 18:25 ` Andy Lutomirski
2015-08-13 18:35 ` Stas Sergeev
2015-08-22 12:38 ` Ingo Molnar
2015-08-22 14:19 ` Stas Sergeev
2015-08-23 6:25 ` Ingo Molnar
2015-08-13 11:08 ` Stas Sergeev
2015-08-13 15:37 ` Linus Torvalds
2015-08-13 15:43 ` Andy Lutomirski
2015-08-13 16:19 ` Linus Torvalds
2015-08-13 16:23 ` Andy Lutomirski
2015-08-13 16:34 ` Linus Torvalds
2015-08-13 16:43 ` Linus Torvalds
2015-08-13 16:44 ` Andy Lutomirski
2015-08-13 17:00 ` Brian Gerst
2015-08-18 6:29 ` Stas Sergeev
2015-08-18 22:42 ` Andy Lutomirski
2015-08-18 22:47 ` Andy Lutomirski
2015-08-19 9:35 ` Stas Sergeev
2015-08-19 15:46 ` Andy Lutomirski
2015-08-19 16:30 ` Stas Sergeev
2015-09-02 5:12 ` Andy Lutomirski
2015-09-02 9:17 ` Stas Sergeev
2015-09-02 14:21 ` Andy Lutomirski
2015-09-02 15:02 ` Andy Lutomirski
2015-09-02 17:46 ` Stas Sergeev
2015-09-02 18:17 ` Andy Lutomirski
2015-09-02 18:23 ` Stas Sergeev
2015-09-02 19:06 ` Andy Lutomirski
2015-09-02 21:01 ` Stas Sergeev
2015-09-02 21:39 ` Andy Lutomirski
2015-09-02 22:25 ` Stas Sergeev
2015-09-02 22:25 ` Andy Lutomirski
2015-09-02 23:01 ` Stas Sergeev
2015-08-19 10:10 ` Stas Sergeev
2015-08-19 15:35 ` Andy Lutomirski
2015-08-14 8:10 ` Cyrill Gorcunov
2015-08-13 17:51 ` Stas Sergeev
2015-08-13 18:35 ` Linus Torvalds
2015-08-13 18:41 ` Andy Lutomirski
2015-08-13 19:05 ` Stas Sergeev
2015-08-13 19:49 ` Andy Lutomirski
2015-08-13 20:09 ` Stas Sergeev
2015-08-13 19:53 ` Linus Torvalds
2015-08-13 20:08 ` Cyrill Gorcunov
2015-08-13 20:09 ` Linus Torvalds
2015-08-13 21:42 ` Raymond Jennings
2015-08-13 21:46 ` Linus Torvalds
2015-08-13 22:01 ` Raymond Jennings
2015-08-13 22:05 ` Stas Sergeev
2015-08-13 23:05 ` Linus Torvalds
2015-08-13 23:18 ` Linus Torvalds
2015-08-13 23:35 ` Raymond Jennings
2015-08-13 23:43 ` Stas Sergeev
2015-08-14 0:02 ` Linus Torvalds
2015-08-13 22:02 ` Stas Sergeev
2015-08-13 22:11 ` Andy Lutomirski
2015-08-13 22:25 ` Stas Sergeev
2015-08-13 22:29 ` Andy Lutomirski
2015-08-13 22:51 ` Stas Sergeev
2015-08-13 23:00 ` Andy Lutomirski
2015-08-13 23:17 ` Stas Sergeev
2015-08-14 0:00 ` Stas Sergeev
2015-08-14 0:05 ` Andy Lutomirski
2015-08-14 0:17 ` Stas Sergeev
2015-08-14 0:27 ` Linus Torvalds
2015-08-14 0:50 ` Stas Sergeev
2015-08-14 1:21 ` Andy Lutomirski
2015-08-14 1:32 ` Stas Sergeev
2015-08-14 1:37 ` Andy Lutomirski
2015-08-14 2:03 ` Stas Sergeev
2015-08-18 6:19 ` Stas Sergeev
2015-08-14 0:08 ` Linus Torvalds
2015-08-14 0:24 ` Andy Lutomirski
2015-08-14 0:40 ` Linus Torvalds
2015-08-14 7:22 ` Cyrill Gorcunov
2015-08-14 10:02 ` Pavel Emelyanov
2015-08-14 10:53 ` Cyrill Gorcunov
2015-08-13 18:57 ` Stas Sergeev
2015-08-13 19:01 ` Andy Lutomirski
2015-08-13 19:13 ` Stas Sergeev
2015-08-13 19:37 ` Linus Torvalds
2015-08-13 19:59 ` Stas Sergeev
2015-08-13 20:07 ` Linus Torvalds
2015-08-18 6:40 ` Stas Sergeev
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