lkml.org 
[lkml]   [2017]   [Sep]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: kmemleak not always catching stuff
On Fri, 8 Sep 2017 18:45:20 +0100
Catalin Marinas <catalin.marinas@arm.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

\
 
 \ /
  Last update: 2017-09-08 20:35    [W:0.042 / U:4.268 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site