[PATCH] ring-buffer: Check for uninitialized cpu buffer before resizing

[Vaibhav Nagarnaik <vnagarnaik@google.com>]

With a system where, num_present_cpus < num_possible_cpus, even if all
CPUs are online, non-present CPUs don't have per_cpu buffers allocated.
If per_cpu/<cpu>/buffer_size_kb is modified for such a CPU, it can cause
a panic due to NULL dereference in ring_buffer_resize().

To fix this, resize operation is allowed only if the per-cpu buffer has
been initialized.

Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@google.com>
---
 kernel/trace/ring_buffer.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b32ed0e..b979426 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1567,6 +1567,10 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
 
 		put_online_cpus();
 	} else {
+		/* Make sure this CPU has been intitialized */
+		if (!cpumask_test_cpu(cpu_id, buffer->cpumask))
+			goto out;
+
 		cpu_buffer = buffer->buffers[cpu_id];
 
 		if (nr_pages == cpu_buffer->nr_pages)
-- 
1.7.7.3