diff --git a/include/kunit/string-stream.h b/include/kunit/string-stream.h
new file mode 100644
index 0000000000000..933ed5740cf07
--- /dev/null
+++ b/include/kunit/string-stream.h
@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * C++ stream style string builder used in KUnit for building messages.
+ *
+ * Copyright (C) 2018, Google LLC.
+ * Author: Brendan Higgins <brendanhiggins@google.com>
+ */
+
+#ifndef _KUNIT_STRING_STREAM_H
+#define _KUNIT_STRING_STREAM_H
+
+#include <linux/types.h>
+#include <linux/spinlock.h>
+#include <linux/kref.h>
+#include <stdarg.h>
+
+struct string_stream_fragment {
+	struct list_head node;
+	char *fragment;
+};
+
+struct string_stream {
+	size_t length;
+	struct list_head fragments;
+
+	/* length and fragments are protected by this lock */
+	spinlock_t lock;
+	struct kref refcount;
+	int (*add)(struct string_stream *this, const char *fmt, ...);
+	int (*vadd)(struct string_stream *this, const char *fmt, va_list args);
+	char *(*get_string)(struct string_stream *this);
+	void (*clear)(struct string_stream *this);
+	bool (*is_empty)(struct string_stream *this);
+};
+
+struct string_stream *new_string_stream(void);
+
+void destroy_string_stream(struct string_stream *stream);
+
+void string_stream_get(struct string_stream *stream);
+
+int string_stream_put(struct string_stream *stream);
+
+#endif /* _KUNIT_STRING_STREAM_H */
diff --git a/kunit/Makefile b/kunit/Makefile
index dd7a0299514b7..7fc613a9b383b 100644
--- a/kunit/Makefile
+++ b/kunit/Makefile
@@ -1 +1 @@
-obj-$(CONFIG_KUNIT)		+= test.o
+obj-$(CONFIG_KUNIT)		+= test.o string-stream.o
diff --git a/kunit/string-stream.c b/kunit/string-stream.c
new file mode 100644
index 0000000000000..1e7efa630cc35
--- /dev/null
+++ b/kunit/string-stream.c
@@ -0,0 +1,149 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * C++ stream style string builder used in KUnit for building messages.
+ *
+ * Copyright (C) 2018, Google LLC.
+ * Author: Brendan Higgins <brendanhiggins@google.com>
+ */
+
+#include <linux/list.h>
+#include <linux/slab.h>
+#include <kunit/string-stream.h>
+
+static int string_stream_vadd(struct string_stream *this,
+			       const char *fmt,
+			       va_list args)
+{
+	struct string_stream_fragment *fragment;
+	int len;
+	va_list args_for_counting;
+	unsigned long flags;
+
+	/* Make a copy because `vsnprintf` could change it */
+	va_copy(args_for_counting, args);
+
+	/* Need space for null byte. */
+	len = vsnprintf(NULL, 0, fmt, args_for_counting) + 1;
+
+	va_end(args_for_counting);
+
+	fragment = kmalloc(sizeof(*fragment), GFP_KERNEL);
+	if (!fragment)
+		return -ENOMEM;
+
+	fragment->fragment = kmalloc(len, GFP_KERNEL);
+	if (!fragment->fragment) {
+		kfree(fragment);
+		return -ENOMEM;
+	}
+
+	len = vsnprintf(fragment->fragment, len, fmt, args);
+	spin_lock_irqsave(&this->lock, flags);
+	this->length += len;
+	list_add_tail(&fragment->node, &this->fragments);
+	spin_unlock_irqrestore(&this->lock, flags);
+	return 0;
+}
+
+static int string_stream_add(struct string_stream *this, const char *fmt, ...)
+{
+	va_list args;
+	int result;
+
+	va_start(args, fmt);
+	result = string_stream_vadd(this, fmt, args);
+	va_end(args);
+	return result;
+}
+
+static void string_stream_clear(struct string_stream *this)
+{
+	struct string_stream_fragment *fragment, *fragment_safe;
+	unsigned long flags;
+
+	spin_lock_irqsave(&this->lock, flags);
+	list_for_each_entry_safe(fragment,
+				 fragment_safe,
+				 &this->fragments,
+				 node) {
+		list_del(&fragment->node);
+		kfree(fragment->fragment);
+		kfree(fragment);
+	}
+	this->length = 0;
+	spin_unlock_irqrestore(&this->lock, flags);
+}
+
+static char *string_stream_get_string(struct string_stream *this)
+{
+	struct string_stream_fragment *fragment;
+	size_t buf_len = this->length + 1; /* +1 for null byte. */
+	char *buf;
+	unsigned long flags;
+
+	buf = kzalloc(buf_len, GFP_KERNEL);
+	if (!buf)
+		return NULL;
+
+	spin_lock_irqsave(&this->lock, flags);
+	list_for_each_entry(fragment, &this->fragments, node)
+		strlcat(buf, fragment->fragment, buf_len);
+	spin_unlock_irqrestore(&this->lock, flags);
+
+	return buf;
+}
+
+static bool string_stream_is_empty(struct string_stream *this)
+{
+	bool is_empty;
+	unsigned long flags;
+
+	spin_lock_irqsave(&this->lock, flags);
+	is_empty = list_empty(&this->fragments);
+	spin_unlock_irqrestore(&this->lock, flags);
+
+	return is_empty;
+}
+
+void destroy_string_stream(struct string_stream *stream)
+{
+	stream->clear(stream);
+	kfree(stream);
+}
+
+static void string_stream_destroy(struct kref *kref)
+{
+	struct string_stream *stream = container_of(kref,
+						    struct string_stream,
+						    refcount);
+	destroy_string_stream(stream);
+}
+
+struct string_stream *new_string_stream(void)
+{
+	struct string_stream *stream = kzalloc(sizeof(*stream), GFP_KERNEL);
+
+	if (!stream)
+		return NULL;
+
+	INIT_LIST_HEAD(&stream->fragments);
+	spin_lock_init(&stream->lock);
+	kref_init(&stream->refcount);
+	stream->add = string_stream_add;
+	stream->vadd = string_stream_vadd;
+	stream->get_string = string_stream_get_string;
+	stream->clear = string_stream_clear;
+	stream->is_empty = string_stream_is_empty;
+	return stream;
+}
+
+void string_stream_get(struct string_stream *stream)
+{
+	kref_get(&stream->refcount);
+}
+
+int string_stream_put(struct string_stream *stream)
+{
+	return kref_put(&stream->refcount, &string_stream_destroy);
+}
+
