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: <55CBA909.3020306@list.ru>
Date:	Wed, 12 Aug 2015 23:14:01 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	X86 ML <x86@...nel.org>,
	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

12.08.2015 23:01, Andy Lutomirski пишет:
> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <stsp@...t.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?
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