Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu
To: Andy Lutomirski <luto@amacapital.net>
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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>
From: Stas Sergeev <stsp@list.ru>
Message-ID: <55CD1F79.2010508@list.ru>
Date: Fri, 14 Aug 2015 01:51:37 +0300
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14.08.2015 01:29, Andy Lutomirski пишет:
> On Thu, Aug 13, 2015 at 3:25 PM, Stas Sergeev <stsp@list.ru> wrote:
>> 14.08.2015 01:11, Andy Lutomirski пишет:
>>
>>> Now suppose you set some magic flag and jump (via sigreturn,
>>> trampoline, whatever) into DOS code.  The DOS code loads 0x7 into FS
>>> and then gets #GP.  You land in a signal handler.  As far as the
>>> kernel's concerned, the FS base register is whatever the base of LDT
>>> entry 0 is.  What else is the kernel supposed to shove in there?
>> The same as what happens when you do in userspace:
>> ---
>> asm ("mov $0,%%fs\n");
>> prctl(ARCH_SET_FS, my_tls_base);
>> ---
>>
>> This was the trick I did before gcc started to use FS in prolog,
>> now I have to do this in asm.
>> But how simpler for the kernel is to do the same?
>>
>>> I think that making this work fully in the kernel would require a
>>> full-blown FS equivalent of sigaltstack, and that seems like overkill.
>> Setting selector and base is what you call an "equivalent of sigaltstack"?
> Yes.  sigaltstack says "hey, kernel! here's my SP for signal
> handling."  I think we'd need something similar to tell the kernel
> what my_tls_base is.  Using the most recent thing passed to
> ARCH_SET_FS is no good because WRFSBASE systems might not use
> ARCH_SET_FS, and we can't break DOSEMU on Ivy Bridge and newer as soon
> as we enable WRFSBASE.
If someone uses WRFSBASE and wants things to be preserved
in a sighandler, he'll just not set the aforementioned flag. No regression.
Whoever wants to use that flag properly, will not use WRFSBASE,
and will use ARCH_SET_FS or set_thread_area().
What exactly breakage do you have in mind?
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