diff --git a/lib/kunit/string-stream-test.c b/lib/kunit/string-stream-test.c
index 110f3a993250..2b761ba01835 100644
--- a/lib/kunit/string-stream-test.c
+++ b/lib/kunit/string-stream-test.c
@@ -11,38 +11,238 @@
 
 #include "string-stream.h"
 
-static void string_stream_test_empty_on_creation(struct kunit *test)
+static char *get_concatenated_string(struct kunit *test, struct string_stream *stream)
 {
-	struct string_stream *stream = alloc_string_stream(test, GFP_KERNEL);
+	char *str = string_stream_get_string(stream);
+
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, str);
+
+	return str;
+}
+
+/* string_stream object is initialized correctly. */
+static void string_stream_init_test(struct kunit *test)
+{
+	struct string_stream *stream;
+
+	stream = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream);
+
+	KUNIT_EXPECT_EQ(test, stream->length, 0);
+	KUNIT_EXPECT_TRUE(test, list_empty(&stream->fragments));
+	KUNIT_EXPECT_PTR_EQ(test, stream->test, test);
+	KUNIT_EXPECT_EQ(test, stream->gfp, GFP_KERNEL);
 
 	KUNIT_EXPECT_TRUE(test, string_stream_is_empty(stream));
 }
 
-static void string_stream_test_not_empty_after_add(struct kunit *test)
+/*
+ * Add a series of lines to a string_stream. Check that all lines
+ * appear in the correct order and no characters are dropped.
+ */
+static void string_stream_line_add_test(struct kunit *test)
 {
-	struct string_stream *stream = alloc_string_stream(test, GFP_KERNEL);
+	struct string_stream *stream;
+	char line[60];
+	char *concat_string, *pos, *string_end;
+	size_t len, total_len;
+	int num_lines, i;
 
-	string_stream_add(stream, "Foo");
+	stream = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream);
 
-	KUNIT_EXPECT_FALSE(test, string_stream_is_empty(stream));
+	/* Add series of sequence numbered lines */
+	total_len = 0;
+	for (i = 0; i < 100; ++i) {
+		len = snprintf(line, sizeof(line),
+			"The quick brown fox jumps over the lazy penguin %d\n", i);
+
+		/* Sanity-check that our test string isn't truncated */
+		KUNIT_ASSERT_LT(test, len, sizeof(line));
+
+		string_stream_add(stream, line);
+		total_len += len;
+	}
+	num_lines = i;
+
+	concat_string = get_concatenated_string(test, stream);
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, concat_string);
+	KUNIT_EXPECT_EQ(test, strlen(concat_string), total_len);
+
+	/*
+	 * Split the concatenated string at the newlines and check that
+	 * all the original added strings are present.
+	 */
+	pos = concat_string;
+	for (i = 0; i < num_lines; ++i) {
+		string_end = strchr(pos, '\n');
+		KUNIT_EXPECT_NOT_NULL(test, string_end);
+
+		/* Convert to NULL-terminated string */
+		*string_end = '\0';
+
+		snprintf(line, sizeof(line),
+			 "The quick brown fox jumps over the lazy penguin %d", i);
+		KUNIT_EXPECT_STREQ(test, pos, line);
+
+		pos = string_end + 1;
+	}
+
+	/* There shouldn't be any more data after this */
+	KUNIT_EXPECT_EQ(test, strlen(pos), 0);
 }
 
-static void string_stream_test_get_string(struct kunit *test)
+/* Add a series of lines of variable length to a string_stream. */
+static void string_stream_variable_length_line_test(struct kunit *test)
 {
-	struct string_stream *stream = alloc_string_stream(test, GFP_KERNEL);
-	char *output;
+	static const char line[] =
+		"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+		" 0123456789!$%^&*()_-+={}[]:;@'~#<>,.?/|";
+	struct string_stream *stream;
+	struct rnd_state rnd;
+	char *concat_string, *pos, *string_end;
+	size_t offset, total_len;
+	int num_lines, i;
 
-	string_stream_add(stream, "Foo");
-	string_stream_add(stream, " %s", "bar");
+	stream = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream);
 
-	output = string_stream_get_string(stream);
-	KUNIT_ASSERT_STREQ(test, output, "Foo bar");
+	/*
+	 * Log many lines of varying lengths until we have created
+	 * many fragments.
+	 * The "randomness" must be repeatable.
+	 */
+	prandom_seed_state(&rnd, 3141592653589793238ULL);
+	total_len = 0;
+	for (i = 0; i < 100; ++i) {
+		offset = prandom_u32_state(&rnd) % (sizeof(line) - 1);
+		string_stream_add(stream, "%s\n", &line[offset]);
+		total_len += sizeof(line) - offset;
+	}
+	num_lines = i;
+
+	concat_string = get_concatenated_string(test, stream);
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, concat_string);
+	KUNIT_EXPECT_EQ(test, strlen(concat_string), total_len);
+
+	/*
+	 * Split the concatenated string at the newlines and check that
+	 * all the original added strings are present.
+	 */
+	prandom_seed_state(&rnd, 3141592653589793238ULL);
+	pos = concat_string;
+	for (i = 0; i < num_lines; ++i) {
+		string_end = strchr(pos, '\n');
+		KUNIT_EXPECT_NOT_NULL(test, string_end);
+
+		/* Convert to NULL-terminated string */
+		*string_end = '\0';
+
+		offset = prandom_u32_state(&rnd) % (sizeof(line) - 1);
+		KUNIT_EXPECT_STREQ(test, pos, &line[offset]);
+
+		pos = string_end + 1;
+	}
+
+	/* There shouldn't be any more data after this */
+	KUNIT_EXPECT_EQ(test, strlen(pos), 0);
+}
+
+/* Appending the content of one string stream to another. */
+static void string_stream_append_test(struct kunit *test)
+{
+	static const char * const strings_1[] = {
+		"one", "two", "three", "four", "five", "six",
+		"seven", "eight", "nine", "ten",
+	};
+	static const char * const strings_2[] = {
+		"Apple", "Pear", "Orange", "Banana", "Grape", "Apricot",
+	};
+	struct string_stream *stream_1, *stream_2;
+	const char *original_content, *stream_2_content;
+	char *combined_content;
+	size_t combined_length;
+	int i;
+
+	stream_1 = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream_1);
+
+	stream_2 = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream_2);
+
+	/* Append content of empty stream to empty stream */
+	string_stream_append(stream_1, stream_2);
+	KUNIT_EXPECT_EQ(test, strlen(get_concatenated_string(test, stream_1)), 0);
+
+	/* Add some data to stream_1 */
+	for (i = 0; i < ARRAY_SIZE(strings_1); ++i)
+		string_stream_add(stream_1, "%s\n", strings_1[i]);
+
+	original_content = get_concatenated_string(test, stream_1);
+
+	/* Append content of empty stream to non-empty stream */
+	string_stream_append(stream_1, stream_2);
+	KUNIT_EXPECT_STREQ(test, get_concatenated_string(test, stream_1), original_content);
+
+	/* Add some data to stream_2 */
+	for (i = 0; i < ARRAY_SIZE(strings_2); ++i)
+		string_stream_add(stream_2, "%s\n", strings_2[i]);
+
+	/* Append content of non-empty stream to non-empty stream */
+	string_stream_append(stream_1, stream_2);
+
+	/*
+	 * End result should be the original content of stream_1 plus
+	 * the content of stream_2.
+	 */
+	stream_2_content = get_concatenated_string(test, stream_2);
+	combined_length = strlen(original_content) + strlen(stream_2_content);
+	combined_length++; /* for terminating \0 */
+	combined_content = kunit_kmalloc(test, combined_length, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, combined_content);
+	snprintf(combined_content, combined_length, "%s%s", original_content, stream_2_content);
+
+	KUNIT_EXPECT_STREQ(test, get_concatenated_string(test, stream_1), combined_content);
+
+	/* Append content of non-empty stream to empty stream */
+	string_stream_destroy(stream_1);
+
+	stream_1 = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream_1);
+
+	string_stream_append(stream_1, stream_2);
+	KUNIT_EXPECT_STREQ(test, get_concatenated_string(test, stream_1), stream_2_content);
+}
+
+/* Adding an empty string should not create a fragment. */
+static void string_stream_append_empty_string_test(struct kunit *test)
+{
+	struct string_stream *stream;
+	int original_frag_count;
+
+	stream = alloc_string_stream(test, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, stream);
+
+	/* Formatted empty string */
+	string_stream_add(stream, "%s", "");
+	KUNIT_EXPECT_TRUE(test, string_stream_is_empty(stream));
+	KUNIT_EXPECT_TRUE(test, list_empty(&stream->fragments));
+
+	/* Adding an empty string to a non-empty stream */
+	string_stream_add(stream, "Add this line");
+	original_frag_count = list_count_nodes(&stream->fragments);
+
+	string_stream_add(stream, "%s", "");
+	KUNIT_EXPECT_EQ(test, list_count_nodes(&stream->fragments), original_frag_count);
+	KUNIT_EXPECT_STREQ(test, get_concatenated_string(test, stream), "Add this line");
 }
 
 static struct kunit_case string_stream_test_cases[] = {
-	KUNIT_CASE(string_stream_test_empty_on_creation),
-	KUNIT_CASE(string_stream_test_not_empty_after_add),
-	KUNIT_CASE(string_stream_test_get_string),
+	KUNIT_CASE(string_stream_init_test),
+	KUNIT_CASE(string_stream_line_add_test),
+	KUNIT_CASE(string_stream_variable_length_line_test),
+	KUNIT_CASE(string_stream_append_test),
+	KUNIT_CASE(string_stream_append_empty_string_test),
 	{}
 };
 
