[PATCH v2 08/10] drivers/auxdisplay: add a KFuzzTest for parse_xy()

[Ethan Graham <ethan.w.s.graham@gmail.com>]

From: Ethan Graham <ethangraham@google.com>

Add a KFuzzTest fuzzer for the parse_xy() function, located in a new
file under /drivers/auxdisplay/tests.

To validate the correctness and effectiveness of this KFuzzTest target,
a bug was injected into parse_xy() like so:

drivers/auxdisplay/charlcd.c:179
- s = p;
+ s = p + 1;

Although a simple off-by-one bug, it requires a specific input sequence
in order to trigger it, thus demonstrating the power of pairing
KFuzzTest with a coverage-guided fuzzer like syzkaller.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 drivers/auxdisplay/charlcd.c             |  8 ++++++++
 drivers/auxdisplay/tests/charlcd_kfuzz.c | 20 ++++++++++++++++++++
 2 files changed, 28 insertions(+)
 create mode 100644 drivers/auxdisplay/tests/charlcd_kfuzz.c

diff --git a/drivers/auxdisplay/charlcd.c b/drivers/auxdisplay/charlcd.c
index 09020bb8ad15..e079b5a9c93c 100644
--- a/drivers/auxdisplay/charlcd.c
+++ b/drivers/auxdisplay/charlcd.c
@@ -682,3 +682,11 @@ EXPORT_SYMBOL_GPL(charlcd_unregister);
 
 MODULE_DESCRIPTION("Character LCD core support");
 MODULE_LICENSE("GPL");
+
+/*
+ * When CONFIG_KFUZZTEST is enabled, we include this _kfuzz.c file to ensure
+ * that KFuzzTest targets are built.
+ */
+#ifdef CONFIG_KFUZZTEST
+#include "tests/charlcd_kfuzz.c"
+#endif /* CONFIG_KFUZZTEST */
diff --git a/drivers/auxdisplay/tests/charlcd_kfuzz.c b/drivers/auxdisplay/tests/charlcd_kfuzz.c
new file mode 100644
index 000000000000..28ce7069c65c
--- /dev/null
+++ b/drivers/auxdisplay/tests/charlcd_kfuzz.c
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * charlcd KFuzzTest target
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+
+struct parse_xy_arg {
+	const char *s;
+};
+
+FUZZ_TEST(test_parse_xy, struct parse_xy_arg)
+{
+	unsigned long x, y;
+
+	KFUZZTEST_EXPECT_NOT_NULL(parse_xy_arg, s);
+	KFUZZTEST_ANNOTATE_STRING(parse_xy_arg, s);
+	parse_xy(arg->s, &x, &y);
+}
-- 
2.51.0.470.ga7dc726c21-goog