linux-kernel - Re: [PATCH v5 1/4] x86/signal/64: Add a comment about sigcontext->fs and gs

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Date:	Wed, 17 Feb 2016 13:03:12 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, Denys Vlasenko <dvlasenk@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Brian Gerst <brgerst@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v5 1/4] x86/signal/64: Add a comment about sigcontext->fs
 and gs

17.02.2016 10:21, Ingo Molnar пишет:
> 
> * Andy Lutomirski <luto@...nel.org> wrote:
> 
>> +	 * If the kernel ever adds explicit fs, gs, fsbase, and gsbase
>> +	 * save/restore, it will most likely need to be opt-in and use
>> +	 * different context slots.
> 
> Btw., that's not necessarily true: it could also be made opt-out, and a 
> modify_ldt() or any other cleanly identifiable legacy usage/signature that is 
> associated with DOSEMU might trigger the opt-out automatically as well.
But there are the new versions of dosemu that still use
"modify_ldt() or any other cleanly identifiable legacy usage/signature"
and yet wants a new functionality.
Please don't go for such an unreliable heuristic.

> I.e. behaviorally it would still keep the ABI and modern default behavior could 
> still be whatever we want to make it.
You can allow glibc to deal with the opt-ins.
It has symbol versioning system AFAIK exactly for that.