Allow to reset the trace remote buffer by writing to the Tracefs "trace"
file. This is similar to the regular Tracefs interface.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/linux/trace_remote.h b/include/linux/trace_remote.h
index feb3433c2128..55754ef78104 100644
--- a/include/linux/trace_remote.h
+++ b/include/linux/trace_remote.h
@@ -18,6 +18,8 @@
  *			remote to allow writing.
  * @swap_reader_page:	Called when Tracefs consumes a new page from a
  *			ring-buffer. It is expected from the remote to isolate a
+ * @reset:		Called on `echo 0 > trace`. It is expected from the
+ *			remote to reset all ring-buffer pages.
  *			new reader-page from the @cpu ring-buffer.
  */
 struct trace_remote_callbacks {
@@ -25,6 +27,7 @@ struct trace_remote_callbacks {
 	void	(*unload_trace_buffer)(struct trace_buffer_desc *desc, void *priv);
 	int	(*enable_tracing)(bool enable, void *priv);
 	int	(*swap_reader_page)(unsigned int cpu, void *priv);
+	int	(*reset)(unsigned int cpu, void *priv);
 };
 
 /**
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 3d76a4d7d5f5..abf83ff75df5 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -63,6 +63,7 @@ static int trace_remote_load(struct trace_remote *remote)
 	rb_remote->desc = remote->trace_buffer_desc;
 	rb_remote->swap_reader_page = remote->cbs->swap_reader_page;
 	rb_remote->priv = remote->priv;
+	rb_remote->reset = remote->cbs->reset;
 	remote->trace_buffer = ring_buffer_alloc_remote(rb_remote);
 	if (!remote->trace_buffer) {
 		remote->cbs->unload_trace_buffer(remote->trace_buffer_desc, remote->priv);
@@ -136,6 +137,21 @@ static int trace_remote_disable_tracing(struct trace_remote *remote)
 	return 0;
 }
 
+static void trace_remote_reset(struct trace_remote *remote, int cpu)
+{
+	lockdep_assert_held(&remote->lock);
+
+	if (!trace_remote_loaded(remote))
+		return;
+
+	if (cpu == RING_BUFFER_ALL_CPUS)
+		ring_buffer_reset(remote->trace_buffer);
+	else
+		ring_buffer_reset_cpu(remote->trace_buffer, cpu);
+
+	trace_remote_try_unload(remote);
+}
+
 static ssize_t
 tracing_on_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *ppos)
 {
@@ -411,6 +427,26 @@ static const struct file_operations trace_pipe_fops = {
 	.release	= trace_pipe_release,
 };
 
+static ssize_t trace_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *ppos)
+{
+	struct inode *inode = file_inode(filp);
+	struct trace_remote *remote = inode->i_private;
+	int cpu = RING_BUFFER_ALL_CPUS;
+
+	if (inode->i_cdev)
+		cpu = (long)inode->i_cdev - 1;
+
+	guard(mutex)(&remote->lock);
+
+	trace_remote_reset(remote, cpu);
+
+	return cnt;
+}
+
+static const struct file_operations trace_fops = {
+	.write		= trace_write,
+};
+
 static int trace_remote_init_tracefs(const char *name, struct trace_remote *remote)
 {
 	struct dentry *remote_d, *percpu_d;
@@ -441,7 +477,9 @@ static int trace_remote_init_tracefs(const char *name, struct trace_remote *remo
 	    !trace_create_file("buffer_size_kb", TRACEFS_MODE_WRITE, remote_d, remote,
 			       &buffer_size_kb_fops) ||
 	    !trace_create_file("trace_pipe", TRACEFS_MODE_READ, remote_d, remote,
-			       &trace_pipe_fops))
+			       &trace_pipe_fops) ||
+	    !trace_create_file("trace", TRACEFS_MODE_WRITE, remote_d, remote,
+			       &trace_fops))
 		goto err;
 
 	percpu_d = tracefs_create_dir("per_cpu", remote_d);
@@ -463,7 +501,9 @@ static int trace_remote_init_tracefs(const char *name, struct trace_remote *remo
 		}
 
 		if (!trace_create_cpu_file("trace_pipe", TRACEFS_MODE_READ, cpu_d, remote, cpu,
-					   &trace_pipe_fops))
+					   &trace_pipe_fops) ||
+		    !trace_create_cpu_file("trace", TRACEFS_MODE_WRITE, cpu_d, remote, cpu,
+					   &trace_fops))
 			goto err;
 	}
 
-- 
2.51.0.618.g983fd99d29-goog