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

Steven Rostedt
2009-02-23T15:55:28Z

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
>
>