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Date:   Mon, 31 Oct 2022 14:48:50 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Takashi Iwai <tiwai@suse.de>
Cc:     regressions@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [REGRESSION 6.0.x / 6.1.x] NULL dereferencing at tracing
Message-ID: <20221031144850.5522b036@rorschach.local.home>
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References: <87h6zklb6n.wl-tiwai@suse.de>
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On Mon, 31 Oct 2022 08:11:28 +0100
Takashi Iwai <tiwai@suse.de> wrote:

> Hi Steven,
> 
> we've got a bug report indicating the NULL dereference at the recent
> tracing changes, showing at the start of KDE.  The details including
> the dmesg are found at:
>   https://bugzilla.opensuse.org/show_bug.cgi?id=1204705
> 
> It was reported at first for 6.0.3, and confirmed that the problem
> persists with 6.1-rc, too.
> 
> The culprit seems to be the commit
> f3ddb74ad0790030c9592229fb14d8c451f4e9a8
>     tracing: Wake up ring buffer waiters on closing of the file
> and reverting it seems fixing the problem.
> 
> Could you take a look?
> 
>

Can you apply this to see if it fixes it?

I'm guessing there's a path to the release of the file descriptor where
the ring buffer isn't allocated (and this expected it to be).

I'll investigate further to see if I can find that path.

-- Steve

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 199759c73519..c1c7ce4c6ddb 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -937,6 +937,9 @@ void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct rb_irq_work *rbwork;
 
+	if (!buffer)
+		return;
+
 	if (cpu == RING_BUFFER_ALL_CPUS) {
 
 		/* Wake up individual ones too. One level recursion */