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: X86 ML <x86@kernel.org>, 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 13:28:57 -0700	[thread overview]
Message-ID: <CALCETrVHiRtB9VCZTBkfuy+4NGBCVJ_+5bZLrshb9+qeXee+NA@mail.gmail.com> (raw)
In-Reply-To: <55CBA909.3020306@list.ru>

On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev <stsp@list.ru> wrote:
> 12.08.2015 23:01, Andy Lutomirski пишет:
>>
>> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>
>>> 12.08.2015 22:20, Andy Lutomirski пишет:
>>>>
>>>> current kernels, it stays switched.  If we change this, it won't stay
>>>> switched.  Even ignoring old ABI, it's not really clear to me what the
>>>> right thing to do is.
>>>
>>> There can be the following cases:
>>> - switch_userspace_thread() switches fs to non-zero selector
>>> - switch_userspace_thread() switches the fs base via syscall
>>> - switch_userspace_thread() switches fs in sigcontext
>>> - switch_userspace_thread() switches fs_base in sigcontext (???)
>>> What exactly case do you have in mind?
>>> I'd say, the way x86_32 is doing things - is good, but the
>>> bases... perhaps, in ideal world, they should be a part of
>>> the sigcontext as well?
>>
>> Any of the above.  What do you want the kernel to do, and how does the
>> kernel know you want to do that?  The kernel has to pick *some*
>> semantics here.
>
> Assuming the bases are made the part of a sigcontext,
> I'd say there would be no ambiguities remained at all:
> whatever you change in a sigcontext, will be "applied" by
> the sigreturn(). Whatever you put in the registers
> (either segregs or MSRs), is valid until sigreturn(), then
> forgotten forever.
> The mess only comes in when some things are part of
> sigcontext and some are not. But if you have _all_ things
> accessable in sigcontext, then the user has a way of expressing
> his needs very clearly: he'll either touch sigcontext or direct
> values, depending on what he need.
>
> Is this right?

Maybe, except that doing this might break existing code (Wine and Java
come to mind).  I'm not really sure.

Anyway, can you give this and its parent a try:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/sigcontext&id=83a08d8c3f43c5524ffc0d88c0eff747716696f5

If they fix the problem for you, I'll improve the test cases and send
them to -stable.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC