From: Andy Lutomirski <luto@amacapital.net>
To: Stas Sergeev <stsp@list.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Raymond Jennings <shentino@gmail.com>,
Cyrill Gorcunov <gorcunov@gmail.com>,
Pavel Emelyanov <xemul@parallels.com>,
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, 2 Sep 2015 12:06:28 -0700 [thread overview]
Message-ID: <CALCETrX5ZtdWax8gWqmDEx1+gQFQZ+QKHB04tH=4dLkLsj3ptQ@mail.gmail.com> (raw)
In-Reply-To: <55E73EB5.5040204@list.ru>
On Wed, Sep 2, 2015 at 11:23 AM, Stas Sergeev <stsp@list.ru> wrote:
> 02.09.2015 21:17, Andy Lutomirski пишет:
>> On Wed, Sep 2, 2015 at 10:46 AM, Stas Sergeev <stsp@list.ru> wrote:
>>> 02.09.2015 17:21, Andy Lutomirski пишет:
>>>>>> This should work for old DOSEMU. It's a bit gross, but it has the
>>>>>> nice benefit that everyone (even things that aren't DOSEMU) gain the
>>>>>> ability to catch signals thrown from bogus SS contexts, which probably
>>>>>> improves debugability. It's also nice to not have the SA flag.
>>>>>
>>>>> Pros:
>>>>> - No new SA flag
>>>>> - May improve debugability in some unknown scenario where people
>>>>> do not want to just use the new flag to get their things improved
>>>>>
>>>>> Cons:
>>>>> - Does not allow to cleanly use siglongjmp(), as then there is a risk
>>>>> to jump to 64bit code with bad SS
>>>>
>>>> What's the issue here? I don't understand.
>>>>
>>>> On musl, (sig)longjmp just restores rsp, rbx, rbp, and r12-r15, so it
>>>> won't be affected. AFAIK all implementations of siglongjmp are likely
>>>> to call sigprocmask or similar, and that will clobber SS. I'm not
>>>> aware of an implementation of siglongjmp that uses sigreturn.
>>> I am not saying siglongjmp() will be affected.
>>> Quite the opposite: it won't, which is bad. :)
>>> If you have always correct SS, you can use siglongjmp(). If you have
>>> broken SS at times, siglongjmp() will be an asking for troubles, as
>>> it exactly does not restore SS.
>>> dosemu could do a good use of siglongjmp() to get back to 64bit code
>>> from its sighandler.
>>
>> This seems like it would be relying unpleasantly heavily on libc internals.
> Could you please clarify?
> If kernel always passes the right SS to the sighandler, then what's
> the problem?
What's the exact siglongjmp usage you have in mind? Signal context
isn't normally involved AFAIK.
>
>>>>> - Async signals can silently "validate" SS behind your back
>>>>
>>>> True, and that's unfortunate. But async signals without SA_SAVE_SS
>>>> set with the other approach have exactly the same problem.
>>> Yes, and as such, they should be blocked.
>>> You could improve on that and on siglongjmp().
>>> And on TLS in the future.
>>
>> *I* can't do anything to siglongjmp because that's almost entirely
>> outside the kernel. :-/
> Except for passing the SS=__USER_DS to the sighandler, for which we
> discussed the new SA_hyz?
I'm still not understanding what you're looking for. If you
siglongjmp out of a signal handler, the hardware SS value is
irrelevant, at least on 64-bit binaries, because siglongjmp is just
going to replace it.
>
>>>>> Is the new SA flag such a big deal here to even bother?
>>>>
>>>> Not really, but given that the new behavior seems clearly better
>>>> behaved than the old, it would be nice to be able to have the good
>>>> behavior, or at least most of it, be the default.
>>> Surely, but how about then having the heuristics you suggest,
>>> only if the new SA_hyz is not set? And when it is set, have a
>>> properly defined and predictable behaviour. Then it seems like
>>> we'll get all the possible wishes covered.
>>
>> That could work. The result is quite similar to explicitly setting
>> UC_STRICT_RESTORE_SS.
> I am much more bothered with delivering the right SS than with
> restoring it on sigreturn().
For 64-bit delivery, ignoring backwards compatibility, delivering
signals with ss = __USER_DS would be the right solution, I think: it's
trivial and it works. Because of backwards compatibility, we need to
deliver signals with ss preserved when possible unless the program
opts out. But I don't see why new programs would care what SS is,
since it has no effect during 64-bit code execution unless you read it
directly or long jmp/long ret to non-64-bit mode.
--Andy
next prev parent reply other threads:[~2015-09-02 19:06 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
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 [this message]
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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