linux-kernel - Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions

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Date:	Mon, 23 Feb 2009 10:55:11 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for
 conversions


On Mon, 23 Feb 2009, Steven Rostedt wrote:

> 
> On Mon, 23 Feb 2009, Mathieu Desnoyers wrote:
> > > 
> > > As for RO_DATA and bugs, it is a very small window for this to happen, and 
> > > the sys-admin is the one making the change. This is not some periodical 
> > > update. The sys-admin must be the one to initiate the tracer to modify 
> > > text, ie, enabling or disabling the function tracer. Which, by the way, is 
> > > something a sys-admin should only do when the system is off line. The 
> > > overhead of all functions being traced, would not be something to be 
> > > doing on a production system, unless they need to analyze something going 
> > > wrong.
> > > 
> > 
> > The argument "not to be used on production systems" is incompatible with
> > the LTTng view, sorry. If you design your code so it's usable only in
> > debugging scenarios on development machines and not in the field, then I
> > doubt LTTng will be able to rely on it. I'm OK with that, as long as
> > nobody argue that such tracepoint could be replaced by the function
> > tracer, because tracepoints has to be enabled in the field on production
> > machines.
> 
> Please do not confuse ftrace with the function tracer. The stop_machine
> is only about the function tracer and has nothing to do with the rest of
> ftrace. This is one detail. Yes, tracing EVERY function in the kernel
> will add an overhead. There's no way around it. It's OK to do it on a
> production system, but it WILL add overhead. That's what happens when you
> trace EVERY function.
> 
> Note, I leave a lot of the other tracers on by default, and those are all
> within the noise of overhead. I'm only talking about the function tracer
> that is meant to do a lot of tracing. Does LTTng trace EVERY function?

BTW, The above is more about the answer to my statement about running on
a production system. Below, is more an answer to the above. After 
rereading what I wrote, I did not explain it very well.

-- Steve

> 
> Now, yes, if you only select a few functions, there's no noticeable 
> overhead. And yes then you would need to do the stop_machine anyway, and 
> there will be a small window where the kernel text will be writable. 
> Tracing only a small set of functions (say a few 100) is not much of an 
> overhead, and I could see that being done on a production system.
> 
> > 
> > I agree that the racy time window is not that large and is not really a
> > security concern, but it's still just annoying.
> 
> Annoying? how so?
> 
> Again, the stop_machine part has nothing to do with DEBUG_RODATA, it is 
> about the safest and easiest way to modify kernel text.
> 
> -- Steve
> 
> 
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