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

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: Thu, 13 Aug 2015 18:21:57 -0700	[thread overview]
Message-ID: <CALCETrVQoUjUH3F=BnhecTqnmDDsaJe3spySj=KdoEFiK90JTg@mail.gmail.com> (raw)
In-Reply-To: <55CD3B3F.9040203@list.ru>

On Thu, Aug 13, 2015 at 5:50 PM, Stas Sergeev <stsp@list.ru> wrote:
> 14.08.2015 03:27, Linus Torvalds пишет:
>>
>> On Thu, Aug 13, 2015 at 5:17 PM, Stas Sergeev <stsp@list.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.  Using a
sigaction flag is a bit less weird than using uc_flags.  It's also
kind of nice that it's more composable -- you can install a SIGUSR1
handler that's just normal code and set SA_SAVE_SS and it'll work,
whereas with uc_flags you need to explicitly twiddle uc_flags in the
handler.

Unfortunately, I don't think we were clever enough to allow this to be
probed easily -- we silently ignore unrecognized sa_flags bits.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC