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

[Gaurav Kohli <gkohli@xxxxxxxxxxxxxx>]

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 <gkohli@xxxxxxxxxxxxxx>
>
> 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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