linux-kernel - Re: [PATCH 1/2 v3] eventfs: Remove eventfs_file and just use eventfs

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Date:   Wed, 20 Sep 2023 18:20:35 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ajay Kaher <akaher@...are.com>, chinglinyu@...gle.com,
        lkp@...el.com, namit@...are.com, oe-lkp@...ts.linux.dev,
        amakhalov@...are.com, er.ajay.kaher@...il.com,
        srivatsa@...il.mit.edu, tkundu@...are.com, vsirnapalli@...are.com
Subject: Re: [PATCH 1/2 v3] eventfs: Remove eventfs_file and just use
 eventfs_inode

On Tue, 19 Sep 2023 21:18:04 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> Hmm, actually looking at this, it's worse than what you stated. This is
> called when a directory is closed. So if you had:
> 
> 	open(dir);
> 
> 	// look at all the content of this dir to create dentries
> 
> 	// another task creates a new entry and looks at it too.
> 
> 	close(dir);
> 
> Now we iterate over all the dentries of the dir and dput it.
> 
> I think this will cause the ref counts to get out of sync. I'll have to try
> to create this scenario and see what happens.

And yes it does break :-p

Even without this patch it breaks. That is, this bug exists currently upstream.

I run the attached file (requires libtracefs)

and then run:

  # cd /sys/kernel/tracing
  # echo 99999999 > buffer_size_kb&

Wait a bit.

This will cause the ref counts to go negative.

Then do a: trace-cmd reset

Which will remove the kprobes created by the attached program, and will
crash the kernel :-p

I have an idea on how to fix it. Let my try it out.

-- Steve

View attachment "test_eventfs_dir.c" of type "text/x-c++src" (2049 bytes)