linux-kernel - Re: [PATCH v2][GIT PULL] tracing: Prevent unloadable modules from using trace

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Message-Id: <201010221504.51535.rusty@rustcorp.com.au>
Date:	Fri, 22 Oct 2010 15:04:51 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2][GIT PULL] tracing: Prevent unloadable modules from using trace_bprintk()

On Fri, 22 Oct 2010 02:28:38 pm Steven Rostedt wrote:
> On Fri, 2010-10-22 at 14:13 +1030, Rusty Russell wrote:
> 
> > > > 
> > > > I think disabling use in modules is lazy,
> > > 
> > > and safer.
> > 
> > Well then just delete trace_bprintk altogether.  Even safer!
> 
> Reminds me of the argument against lowering the speed limit to 55. "it's
> safer"... "Then lower it to zero, even safer!"
> 
> 
> > 
> > > > Can't you detect this on module unload and fix it up?  Or delay freeing the
> > > > module until the trace ring is emptied?
> > > 
> > > One possibility is to magically make all string formats used in
> > > trace_printk into its own section, and keep it allocated until the ring
> > > buffer is empty. Or, we can just do that with the module's entire string
> > > section, since we know whether or not that module has a trace_printk in
> > > it or not.
> > 
> > Exactly.  Set a flag in the module if it resolves trace_printk, and defer freeing
> > the module in that case.  This shouldn't be that hard...
> 
> Here's my worry.
> 
> 1) Some module with tracepoints is loaded at boot up.
> 2) The user does tracing and forgets about it (ring buffer filled)
> 3) Unloads module (don't free)
> 4) loads module with trace points
> 5) unloads module (don't free)
> etc, etc
> 
> memory leak.

Sure.  Then mark the rb count or something in the module at init time,
then compare before deciding too dangerous to free.

> Thus this is not that trivial. We probably need to have a way to lock a
> module when its tracepoint is activated, and only unlock it when the
> ring buffer is emptied.

How about the intuitive and completely obvious thing?  When a tp activated,
use the module.  When deactivated, unuse it?

> Do we want to prevent the module from being unloaded while the ring
> buffer is full (after that module has been traced?), or do we let the
> module be unloaded, but just prevent this one section from being freed?

I was thinking the latter, basically defer the module_free() call.

Cheers,
Rusty.
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