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

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Message-ID: <55CD4515.2030202@list.ru>
Date:	Fri, 14 Aug 2015 04:32:05 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raymond Jennings <shentino@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

14.08.2015 04:21, Andy Lutomirski пишет:
> On Thu, Aug 13, 2015 at 5:50 PM, Stas Sergeev <stsp@...t.ru> wrote:
>> 14.08.2015 03:27, Linus Torvalds пишет:
>>> On Thu, Aug 13, 2015 at 5:17 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>> For example because you can as well do:
>>>> prctl(ARCH_SET_SIGNAL_SS, 0)
>>>> which will mean "restore ss in sighandler to its current value",
>>> I really think a prctl() is the wrong thing to do.
>>>
>>> If you want a signal handler to save/restore segments, I think it
>>> should be a SA_xyz flag to sigaction() (the way we have SA_RESTART
>> Yes, I was proposing the new sigaction() flag in this thread
>> already too. But at the end, prctl() looks better to me because
>> it allows to pass the TLS value to use when restoring FS.
>> The thing is that I am trying to find the similar treatment for
>> both the SS and FS problems. If you don't think they need a
>> similar treatment, then perhaps the Andy's patch is enough.
>>
>>> etc).  And off by default because of the obvious compatibility issues.
>> Of course.
>>
>> So, what we have right now (in the latest Andy's patch) is:
>> 1. lar heuristics
>> 2. new uc_flags flag
>>
>> What it solves: dosemu's regression.
>>
>> What prctl() can give:
>> - fix to dosemu's regression
>> - fix to the TLS problem in the future
>> - no hack and heuristics
>>
>> With SA_xyz you can only solve the SS problem, so it is
>> probably not any better than the uc_flags things coded
>> up by Andy.
> I'm leaning slightly toward LAR heuristic + SA_SAVE_SS.
Stop right here, doesn't the SA_xyz allow to avoid the
lar heuristic? Why would you still need the lar heuristic then?
Just call it SA_RESTORE_SS instead of SA_SAVE_SS, and
the lar heuristic is gone.

> Unfortunately, I don't think we were clever enough to allow this to be
> probed easily -- we silently ignore unrecognized sa_flags bits.
Big deal, check the kversion. :)

Unforunately, in my eyes SA_xyz doesn't help with FS, so
whether it is better than uc_flags or not, is not what I care
about.
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