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

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Date:   Wed, 18 Apr 2018 16:40:19 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Apr 18, 2018 at 4:25 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 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?

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.

Thanks,
Miklos