linux-kernel - Re: kmemleak not always catching stuff

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Date:   Fri, 8 Sep 2017 14:34:48 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev@...glegroups.com
Subject: Re: kmemleak not always catching stuff

On Fri, 8 Sep 2017 18:45:20 +0100
Catalin Marinas <catalin.marinas@....com> wrote:

> > Hmm, could this also be what causes the miss of catching the lingering
> > ftrace trampoline?  
> 
> Not sure (not without some additional support in kmemleak to help track
> down the source of false negatives; I'm on a long flight to LA next
> week, maybe I manage to hack something up ;)).

I'll be there too. We can talk more about this in person ;-)

I'll probably be at the VMware booth a bit, as one of our team had a
family emergency and had to cancel.

> 
> BTW, I had a quick look at the trace_selftest_ops() function (without
> pretending I understand the ftrace code) and there is one case where

I personally just pretend I understand it.

> this function can exit without freeing dyn_ops. Is this intentional?
> 
> diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
> index cb917cebae29..b17ec642793b 100644
> --- a/kernel/trace/trace_selftest.c
> +++ b/kernel/trace/trace_selftest.c
> @@ -273,7 +273,7 @@ static int trace_selftest_ops(struct trace_array *tr, int cnt)
>  		goto out_free;
>  	if (cnt > 1) {
>  		if (trace_selftest_test_global_cnt == 0)
> -			goto out;
> +			goto out_free;
>  	}
>  	if (trace_selftest_test_dyn_cnt == 0)
>  		goto out_free;
> 

You mean something like this:

http://lkml.kernel.org/r/20170905133217.770389331@goodmis.org

 :-)

> > 
> > Without the patch in the link above, there's a memory leak with the
> > ftrace trampoline with the following commands:
> > 
> > Requires: CONFIG_FUNCTION_TRACER and CONFIG_SCHED_TRACER
> > 
> >  # cd /sys/kernel/debug/tracing
> >  # mkdir instances/foo
> >  # echo wakeup > instances/foo/current_tracer
> >  # echo 0 > /proc/sys/kernel/ftrace_enabled
> >  # echo nop > instances/foo/current_tracer
> >  # rmdir instances/foo
> > 
> > What the above does, is creates a new (allocated/non-static) buffer in
> > the instance directory. Then we enable the wakeup tracer which
> > enables function tracing and also creates a dynamic ftrace trampoline
> > for it. We disable function tracing for all tracers with the proc
> > sysctl ftrace_enabled set to zero. The nop removes the wakeup tracer
> > and unregisters its function tracing handler. This is where the leak
> > happens. The unregister path sees that function tracing is disabled and
> > exits out early, without releasing the trampoline.  
> 
> Are the ftrace_ops allocated dynamically in this case (and freed when
> unregistered)? Otherwise, you may have an ops->trampoline still around
> that kmemleak finds.
> 

The ftrace_ops is allocated when the instance is created, and freed
when the instance is removed:

instance_mkdir() {
	init_tracer_fs() {
		ftrace_create_function_files() {
			allocate_ftrace_ops() {
				ops = kzalloc();
				tr->ops = ops;
			}
		}
	}
}

instance_rmdir() {
	ftrace_destroy_function_files() {
		kfree(tr->ops);
	}
}

-- Steve