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

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Date:   Wed, 18 Apr 2018 10:25:04 -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 16:03:42 +0200
Miklos Szeredi <miklos@...redi.hu> wrote:

> > @@ -937,7 +928,8 @@ probe_event_enable(struct trace_uprobe *tu, struct trace_event_file *file,
> >                 goto err_flags;
> >
> >         tu->consumer.filter = filter;
> > -       ret = uprobe_register(tu->inode, tu->offset, &tu->consumer);
> > +       ret = uprobe_register(d_inode(tu->path.dentry), tu->offset,
> > +                             &tu->consumer);  
> 
> It is not entirely clear how the lifetime of uprobe relates to the
> lifetime of trace_uprobe.  Is the uprobe object never going to survive
> its creator trace_uprobe object?

Not exactly sure what you mean here.

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?

-- Steve


> 
> If that's the case, it warrants a comment.  If that's not the case,
> then the path would need to be passed to uprobe_resister() which would
> need to obtain its own reference.
> 
> >         if (ret)
> >                 goto err_buffer;
> >