Re: [PATCH v16 4/6] tracing: Allow user-space mapping of the ring-buffer

[Steven Rostedt <rostedt@goodmis.org>]

On Fri,  9 Feb 2024 16:34:46 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> +static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
> +{
> +	struct ftrace_buffer_info *info = vma->vm_file->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +	struct trace_array __maybe_unused *tr = iter->tr;
> +
> +	ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file);
> +
> +#ifdef CONFIG_TRACER_MAX_TRACE
> +	spin_lock(&tr->snapshot_trigger_lock);
> +	if (!WARN_ON(!tr->mapped))
> +		tr->mapped--;
> +	spin_unlock(&tr->snapshot_trigger_lock);
> +#endif

You can add a section with the #ifdef *MAX_TRACE and use static inline
for these (or use an existing section):

#ifdef CONFIG_TRACER_MAX_TRACE
static void put_snapshot_map(struct trace_array *tr)
{
	spin_lock(&tr->snapshot_trigger_lock);
	if (!WARN_ON(!tr->mapped))
		tr->mapped--;
	spin_unlock(&tr->snapshot_trigger_lock);	
}
[..]
#else
static inline void put_snapshot_map(struct trace_array *tr) { }
[..]
#endif

> +}
> +
> +static void tracing_buffers_mmap_open(struct vm_area_struct *vma)
> +{
> +	struct ftrace_buffer_info *info = vma->vm_file->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +
> +	WARN_ON(ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file));
> +}
> +
> +static const struct vm_operations_struct tracing_buffers_vmops = {
> +	.open		= tracing_buffers_mmap_open,
> +	.close		= tracing_buffers_mmap_close,
> +	.fault		= tracing_buffers_mmap_fault,
> +};
> +
> +static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
> +{
> +	struct ftrace_buffer_info *info = filp->private_data;
> +	struct trace_iterator *iter = &info->iter;
> +	struct trace_array __maybe_unused *tr = iter->tr;
> +	int ret = 0;
> +
> +	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC)
> +		return -EPERM;
> +
> +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTDUMP, VM_MAYWRITE);
> +	vma->vm_ops = &tracing_buffers_vmops;
> +
> +#ifdef CONFIG_TRACER_MAX_TRACE
> +	/*
> +	 * We hold mmap_lock here. lockdep would be unhappy if we would now take
> +	 * trace_types_lock. Instead use the specific snapshot_trigger_lock.
> +	 */
> +	spin_lock(&tr->snapshot_trigger_lock);
> +	if (tr->snapshot || tr->mapped == UINT_MAX) {
> +		spin_unlock(&tr->snapshot_trigger_lock);
> +		return -EBUSY;
> +	}
> +	tr->mapped++;
> +	spin_unlock(&tr->snapshot_trigger_lock);
> +
> +	/* Wait for update_max_tr() to observe iter->tr->mapped */
> +	if (tr->mapped == 1)
> +		synchronize_rcu();
> +#endif

The above could be:

static int get_snapshot_map(struct trace_array *tr)
{
	/*
	 * We hold mmap_lock here. lockdep would be unhappy if we would now take
	 * trace_types_lock. Instead use the specific snapshot_trigger_lock.
	 */
	spin_lock(&tr->snapshot_trigger_lock);
	if (tr->snapshot || tr->mapped == UINT_MAX) {
		spin_unlock(&tr->snapshot_trigger_lock);
		return -EBUSY;
	}
	tr->mapped++;
	spin_unlock(&tr->snapshot_trigger_lock);

	/* Wait for update_max_tr() to observe iter->tr->mapped */
	if (tr->mapped == 1)
		synchronize_rcu();

	return 0;
}
#else
static inline test_snapshot_map(struct trace_array *tr)
{
	return 0;
}
[..]

Then here just have:

	ret = test_snapshot_map(tr);
	if (ret < 0)
		return ret;


> +	ret = ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file);
> +#ifdef CONFIG_TRACER_MAX_TRACE
> +	if (ret) {
> +		spin_lock(&tr->snapshot_trigger_lock);
> +		iter->tr->mapped--;
> +		spin_unlock(&tr->snapshot_trigger_lock);
> +	}
> +#endif

and the above is just:

	if (ret)
		put_snapshot_map(tr);

-- Steve

> +	return ret;
> +}
> +
>  static const struct file_operations tracing_buffers_fops = {
>  	.open		= tracing_buffers_open,
>  	.read		= tracing_buffers_read,
> @@ -8681,6 +8794,7 @@ static const struct file_operations tracing_buffers_fops = {
>  	.splice_read	= tracing_buffers_splice_read,
>  	.unlocked_ioctl = tracing_buffers_ioctl,
>  	.llseek		= no_llseek,
> +	.mmap		= tracing_buffers_mmap,
>  };
>  
>  static ssize_t
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index bd312e9afe25..8a96e7a89e6b 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -336,6 +336,7 @@ struct trace_array {
>  	bool			allocated_snapshot;
>  	spinlock_t		snapshot_trigger_lock;
>  	unsigned int		snapshot;
> +	unsigned int		mapped;
>  	unsigned long		max_latency;
>  #ifdef CONFIG_FSNOTIFY
>  	struct dentry		*d_max_latency;