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

[Gaurav Kohli]

Hi Steven,

Please let us know, if below change looks good.
Or let us know some other way to solve this.

Thanks,
Gaurav



On 9/4/2020 11:39 AM, Gaurav Kohli wrote:

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 <gko...@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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