linux-kernel - Re: [PATCH 1/2] tracing: fix bad use of igrab in trace

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Date:   Wed, 18 Apr 2018 11:19:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Song Liu <songliubraving@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...hat.com>,
        Howard McLauchlan <hmclauchlan@...com>,
        Josef Bacik <jbacik@...com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] tracing: fix bad use of igrab in trace_uprobe.c

On Wed, 18 Apr 2018 16:40:19 +0200
Miklos Szeredi <miklos@...redi.hu> wrote:


> > The trace_uprobe (the probe event) is created, it doesn't do anything
> > until it is enabled. This function is called when it is enabled. The
> > trace_uprobe (probe event) can not be deleted while it is enabled
> > (EBUSY).
> >
> > Are you asking what happens if the file is deleted while it has probe?
> > That I don't know about (haven't tried it out). But I would hope that
> > it keeps a reference to the inode, isn't that what the igrab is for?
> > And is now being replaced by a reference on the path, or is that the
> > problem?  
> 
> No, that's not the problem.
> 
> What I don't see is how the uprobe object relates to the trace_uprobe object.
> 
> Because after the patch the uprobe object still only has a ref to the
> inode, and that can lead to the same issue as with trace_uprobe.
> OTOH if uprobe can't survive its creating trace_uprobe, then it
> doesn't need to take a ref to the inode at all, since trace_uprobe
> already holds it.   Taking an extra ref isn't incorrect, it's just
> unnecessary and confusing.
> 
> So this needs to be cleared up in some way.

The uprobe created by the trace_uprobe creation must be deleted before
the trace_uprobe can be deleted. Basically we have this:

 # cd /sys/kernel/tracing
 # echo "uprobe creation text" > uprobe_events

The trace_uprobe is created (but not the uprobe itself). This is what
calls create_trace_uprobe().

 # echo 1 > events/uprobes/enable

This enables all the trace uprobe events, which creates the uprobes.
This is the action that calls probe_event_enable(), which creates
uprobes.

At this point, any write to uprobe_events that would destroy the trace
uprobes would return with -EBUSY, and the trace uprobes will not be
deleted.

 # echo 0 > events/uprobes/enable

This will call the probe_event_disable() which will call
uprobe_unregister() which will destroy the uprobe.

Now we can delete the trace uprobe.

Does that answer your question? A uprobe created for trace uprobes can
not survive the trace uprobe itself.

-- Steve