[PATCH v4 4/6] kfuzztest: add KFuzzTest sample fuzz targets

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

Add two simple fuzz target samples to demonstrate the KFuzzTest API and
provide basic self-tests for the framework.

These examples showcase how a developer can define a fuzz target using
the FUZZ_TEST_SIMPLE() macro. It also serves as a runtime sanity check,
ensuring that the framework correctly passes the input buffer and that
KASAN correctly detects out-of-bounds memory accesses (in this case, a
buffer underflow) on the allocated test data.

This target can be fuzzed naively by writing random data into the
debugfs 'input_simple' file and verifying that the KASAN report is
triggered.

Signed-off-by: Ethan Graham <ethan.w.s.graham@gmail.com>
Acked-by: Alexander Potapenko <glider@google.com>

---
PR v4:
- Remove the `test_underflow_on_nested_buffer` sample target which
  relied on the now removed `FUZZ_TEST` macro.
- Update the sample comment to demonstrate naive fuzzing (using `head`)
  instead of the removed bridge tool.
- Fix stale comments referencing internal layout structures.
PR v3:
- Use the FUZZ_TEST_SIMPLE macro in the `underflow_on_buffer` sample
  fuzz target instead of FUZZ_TEST.
PR v2:
- Fix build issues pointed out by the kernel test robot <lkp@intel.com>.
---
---
 samples/Kconfig                         |  7 ++++
 samples/Makefile                        |  1 +
 samples/kfuzztest/Makefile              |  3 ++
 samples/kfuzztest/underflow_on_buffer.c | 52 +++++++++++++++++++++++++
 4 files changed, 63 insertions(+)
 create mode 100644 samples/kfuzztest/Makefile
 create mode 100644 samples/kfuzztest/underflow_on_buffer.c

diff --git a/samples/Kconfig b/samples/Kconfig
index 6e072a5f1ed8..303a9831d404 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -320,6 +320,13 @@ config SAMPLE_HUNG_TASK
 	  Reading these files with multiple processes triggers hung task
 	  detection by holding locks for a long time (256 seconds).
 
+config SAMPLE_KFUZZTEST
+	bool "Build KFuzzTest sample targets"
+	depends on KFUZZTEST
+	help
+	  Build KFuzzTest sample targets that serve as selftests for raw input
+	  delivery and KASAN out-of-bounds detection.
+
 source "samples/rust/Kconfig"
 
 source "samples/damon/Kconfig"
diff --git a/samples/Makefile b/samples/Makefile
index 07641e177bd8..3a0e7f744f44 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -44,4 +44,5 @@ obj-$(CONFIG_SAMPLE_DAMON_WSSE)		+= damon/
 obj-$(CONFIG_SAMPLE_DAMON_PRCL)		+= damon/
 obj-$(CONFIG_SAMPLE_DAMON_MTIER)	+= damon/
 obj-$(CONFIG_SAMPLE_HUNG_TASK)		+= hung_task/
+obj-$(CONFIG_SAMPLE_KFUZZTEST)		+= kfuzztest/
 obj-$(CONFIG_SAMPLE_TSM_MR)		+= tsm-mr/
diff --git a/samples/kfuzztest/Makefile b/samples/kfuzztest/Makefile
new file mode 100644
index 000000000000..2dc5d424824d
--- /dev/null
+++ b/samples/kfuzztest/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_SAMPLE_KFUZZTEST) += underflow_on_buffer.o
diff --git a/samples/kfuzztest/underflow_on_buffer.c b/samples/kfuzztest/underflow_on_buffer.c
new file mode 100644
index 000000000000..5568c5e6be7a
--- /dev/null
+++ b/samples/kfuzztest/underflow_on_buffer.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This file contains a KFuzzTest example target that ensures that a buffer
+ * underflow on a region triggers a KASAN OOB access report.
+ *
+ * Copyright 2025 Google LLC
+ */
+
+/**
+ * test_underflow_on_buffer - a sample fuzz target
+ *
+ * This sample fuzz target serves to illustrate the usage of the
+ * FUZZ_TEST_SIMPLE macro, as well as provide a sort of self-test that KFuzzTest
+ * functions correctly for trivial fuzz targets. In KASAN builds, fuzzing this
+ * harness should trigger a report for every input (provided that its length is
+ * greater than 0 and less than KFUZZTEST_MAX_INPUT_SIZE).
+ *
+ * This harness can be invoked (naively) like so:
+ * head -c 128 /dev/urandom > \
+ *	/sys/kernel/debug/kfuzztest/test_underflow_on_buffer/input_simple
+ */
+#include <linux/kfuzztest.h>
+
+static void underflow_on_buffer(char *buf, size_t buflen)
+{
+	size_t i;
+
+	/*
+	 * Print the address range of `buf` to allow correlation with the
+	 * subsequent KASAN report.
+	 */
+	pr_info("buf = [%px, %px)", buf, buf + buflen);
+
+	/* First ensure that all bytes in `buf` are accessible. */
+	for (i = 0; i < buflen; i++)
+		READ_ONCE(buf[i]);
+	/*
+	 * Provoke a buffer underflow on the first byte preceding `buf`,
+	 * triggering a KASAN report.
+	 */
+	READ_ONCE(*((char *)buf - 1));
+}
+
+/**
+ * Define the fuzz target. This wrapper ensures that the `underflow_on_buffer`
+ * function is invoked with the data provided from userspace.
+ */
+FUZZ_TEST_SIMPLE(test_underflow_on_buffer)
+{
+	underflow_on_buffer(data, datalen);
+	return 0;
+}
-- 
2.51.0