linux-kernel - Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

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Date:	Thu, 1 May 2014 16:01:54 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Seth Jennings <sjenning@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Slaby <jslaby@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

On Thu, May 01, 2014 at 01:45:33PM -0700, Andi Kleen wrote:
> Josh Poimboeuf <jpoimboe@...hat.com> writes:
> >
> > kpatch checks the backtraces of all tasks in stop_machine() to ensure
> > that no instances of the old function are running when the new function
> > is applied.
> 
> How does that work for tail calls? 
> 
>       call foo
> foo:
>       ...
>       jmp bar
> 
> bar:
>       ... code executing ...
> 
> When you backtrace you will see foo, but you are running in bar.

> Note that tail calls can be indirect, so they cannot be resolved
> statically.
> 
> CONFIG_DEBUG_INFO usually disables tail calls, but not supporting 
> it would seem like a large limitation, as the cost can be high.
> 
> It wouldn't surprise me if there are some similar special cases that
> can even happen with them disabled.
> 
> In theory you could read LBRs, but even that may miss some extreme
> cases.

When bar returns, would it skip foo and go straight back to foo's
caller?  If so, then it should be safe to patch foo after it jumps to
bar.

-- 
Josh
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