linux-kernel - Re: [PATCH v2] livepatch: Add some basic LivePatch documentation

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Date:	Wed, 27 Apr 2016 11:37:20 +0200 (CEST)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Balbir Singh <bsingharora@...il.com>
cc:	Petr Mladek <pmladek@...e.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linuxppc-dev@...abs.org, Torsten Duwe <duwe@...e.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nikolay Borisov <kernel@...p.com>
Subject: Re: [PATCH v2] livepatch: Add some basic LivePatch documentation

On Tue, 26 Apr 2016, Balbir Singh wrote:

> > +  + Anything inlined into __schedule() can not be patched.
> > +
> > +    The switch_to macro is inlined into __schedule(). It switches the
> > +    context between two processes in the middle of the macro. It does
> > +    not save RIP in x86_64 version (contrary to 32-bit version). Instead,
> > +    the currently used __schedule()/switch_to() handles both processes.
> > +
> > +    Now, let's have two different tasks. One calls the original
> > +    __schedule(), its registers are stored in a defined order and it
> > +    goes to sleep in the switch_to macro and some other task is restored
> > +    using the original __schedule(). Then there is the second task which
> > +    calls patched__schedule(), it goes to sleep there and the first task
> > +    is picked by the patched__schedule(). Its RSP is restored and now
> > +    the registers should be restored as well. But the order is different
> > +    in the new patched__schedule(), so...
> > +
> > +    There is a work in progress to remove this limitation.
> > +
> 
> I am afraid the example requires more clarification. I don't quite get the order is different

Different order is not inevitable but perfectly possible (even probable). 
GCC may simply generate different object code for patched__schedule() than 
it did for __schedule(). The problem is when the prologue and epilogue are 
different.

Miroslav