[PATCH] trace: Fix race in trace_open and buffer resize call

[Gaurav Kohli <gkohli@codeaurora.org>]

Below race can come, if trace_open and resize of
cpu buffer is running parallely on different cpus
CPUX                                CPUY
				    ring_buffer_resize
				    atomic_read(&buffer->resize_disabled)
tracing_open
tracing_reset_online_cpus
ring_buffer_reset_cpu
rb_reset_cpu
				    rb_update_pages
				    remove/insert pages
resetting pointer
This race can cause data abort or some times infinte loop in 
rb_remove_pages and rb_insert_pages while checking pages 
for sanity.
Take ring buffer lock in trace_open to avoid resetting of cpu buffer.

Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 136ea09..55f9115 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -163,6 +163,8 @@ bool ring_buffer_empty_cpu(struct trace_buffer *buffer, int cpu);
 
 void ring_buffer_record_disable(struct trace_buffer *buffer);
 void ring_buffer_record_enable(struct trace_buffer *buffer);
+void ring_buffer_mutex_acquire(struct trace_buffer *buffer);
+void ring_buffer_mutex_release(struct trace_buffer *buffer);
 void ring_buffer_record_off(struct trace_buffer *buffer);
 void ring_buffer_record_on(struct trace_buffer *buffer);
 bool ring_buffer_record_is_on(struct trace_buffer *buffer);
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 93ef0ab..638ec8f 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3632,6 +3632,25 @@ void ring_buffer_record_enable(struct trace_buffer *buffer)
 EXPORT_SYMBOL_GPL(ring_buffer_record_enable);
 
 /**
+ * ring_buffer_mutex_acquire - prevent resetting of buffer
+ * during resize
+ */
+void ring_buffer_mutex_acquire(struct trace_buffer *buffer)
+{
+	mutex_lock(&buffer->mutex);
+}
+EXPORT_SYMBOL_GPL(ring_buffer_mutex_acquire);
+
+/**
+ * ring_buffer_mutex_release - prevent resetting of buffer
+ * during resize
+ */
+void ring_buffer_mutex_release(struct trace_buffer *buffer)
+{
+	mutex_unlock(&buffer->mutex);
+}
+EXPORT_SYMBOL_GPL(ring_buffer_mutex_release);
+/**
  * ring_buffer_record_off - stop all writes into the buffer
  * @buffer: The ring buffer to stop writes to.
  *
@@ -4918,6 +4937,8 @@ void ring_buffer_reset(struct trace_buffer *buffer)
 	struct ring_buffer_per_cpu *cpu_buffer;
 	int cpu;
 
+	/* prevent another thread from changing buffer sizes */
+	mutex_lock(&buffer->mutex);
 	for_each_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];
 
@@ -4936,6 +4957,7 @@ void ring_buffer_reset(struct trace_buffer *buffer)
 		atomic_dec(&cpu_buffer->record_disabled);
 		atomic_dec(&cpu_buffer->resize_disabled);
 	}
+	mutex_unlock(&buffer->mutex);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset);
 
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f40d850..392e9aa 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2006,6 +2006,8 @@ void tracing_reset_online_cpus(struct array_buffer *buf)
 	if (!buffer)
 		return;
 
+	ring_buffer_mutex_acquire(buffer);
+
 	ring_buffer_record_disable(buffer);
 
 	/* Make sure all commits have finished */
@@ -2016,6 +2018,8 @@ void tracing_reset_online_cpus(struct array_buffer *buf)
 	ring_buffer_reset_online_cpus(buffer);
 
 	ring_buffer_record_enable(buffer);
+
+	ring_buffer_mutex_release(buffer);
 }
 
 /* Must have trace_types_lock held */
-- 
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