[PATCH 09/10] 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>
Signed-off-by: Ethan Graham <ethan.w.s.graham@gmail.com>
Acked-by: Alexander Potapenko <glider@google.com>

---
PR v3:
- Remove conditional inclusion of charlcd_kfuzz.c from charlcd.c, as
  requested by Andy Shevchenko.
- Update auxdisplay Makefile to conditionally build charlcd_kfuzz.c when
  CONFIG_KFUZZTEST=y, as suggested by Lukas Wunner and Andy Shevchenko.
- Foward declare parse_xy in charlcd_kfuzz.c.
---
---
 drivers/auxdisplay/Makefile              |  3 +++
 drivers/auxdisplay/tests/charlcd_kfuzz.c | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 drivers/auxdisplay/tests/charlcd_kfuzz.c

diff --git a/drivers/auxdisplay/Makefile b/drivers/auxdisplay/Makefile
index f5c13ed1cd4f..af00b0a173de 100644
--- a/drivers/auxdisplay/Makefile
+++ b/drivers/auxdisplay/Makefile
@@ -6,6 +6,9 @@
 obj-$(CONFIG_ARM_CHARLCD)	+= arm-charlcd.o
 obj-$(CONFIG_CFAG12864B)	+= cfag12864b.o cfag12864bfb.o
 obj-$(CONFIG_CHARLCD)		+= charlcd.o
+ifeq ($(CONFIG_KFUZZTEST),y)
+CFLAGS_charlcd.o += -include $(src)/tests/charlcd_kfuzz.c
+endif
 obj-$(CONFIG_HD44780_COMMON)	+= hd44780_common.o
 obj-$(CONFIG_HD44780)		+= hd44780.o
 obj-$(CONFIG_HT16K33)		+= ht16k33.o
diff --git a/drivers/auxdisplay/tests/charlcd_kfuzz.c b/drivers/auxdisplay/tests/charlcd_kfuzz.c
new file mode 100644
index 000000000000..3adf510f4356
--- /dev/null
+++ b/drivers/auxdisplay/tests/charlcd_kfuzz.c
@@ -0,0 +1,22 @@
+// 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;
+};
+
+static bool parse_xy(const char *s, unsigned long *x, unsigned long *y);
+
+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