[for-linus][PATCH 03/16] ring-buffer: Do not show buffer_meta for CPUs without a buffer

[Steven Rostedt <rostedt@kernel.org>]

From: David Carlier <devnexen@gmail.com>

The per_cpu/cpuN/buffer_meta files are created for every possible CPU,
but buffer->buffers[cpu] is only allocated once that CPU comes online.
Reading the file of a CPU that never came online dereferences a NULL
cpu_buffer in rbm_start().

Fail the open with -ENODEV when the CPU has no buffer, as the other
per CPU ring buffer interfaces do.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260914133506.4865-3-devnexen@gmail.com
Fixes: 950032ffcee7 ("ring-buffer: Add output of ring buffer meta page")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260913134906.6E8A01F000FF@smtp.kernel.org/
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/ring_buffer.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 04bb94c29f58..ce179619e655 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2380,6 +2380,9 @@ int ring_buffer_meta_seq_init(struct file *file, struct trace_buffer *buffer, in
 	struct seq_file *m;
 	int ret;
 
+	if (!cpumask_test_cpu(cpu, buffer->cpumask))
+		return -ENODEV;
+
 	ret = seq_open(file, &rb_meta_seq_ops);
 	if (ret)
 		return ret;
-- 
2.53.0