Hello,

Currently there are almost no automated selftests for fanotify notification
events in tools/testing/selftests/ (only mount namespace
related tests exist).

This patch adds a very basic selftest that exercises three fundamental
fanotify events:
    - FAN_CREATE (file creation)
    - FAN_MODIFY (file content modification via write())
    - FAN_DELETE (file removal)

The test
    - creates a test file, appends data, and removes it
    - verifies that corresponding events are received and the masks contain
      the expected bits (0x100, 0x2, 0x200)

Test TAP output:
    ok 1 FAN_CREATE detected
    ok 2 FAN_MODIFY detected
    ok 3 FAN_DELETE detected
    # PASSED: 1 / 1 tests passed.

This is intentionally kept minimal as a starting point.

Future work ideas (not in this patch):
    - Test permission events
    - Test rename/move events
    - Verify file names
    - Run under different filesystems

Any feedback on the direction, style, or additional test cases
would be greatly appreciated.

Thanks,
Jinseok.

Signed-off-by: Jinseok Kim <always.starving0@gmail.com>
---
 tools/testing/selftests/filesystems/Makefile  |   7 +
 .../selftests/filesystems/fanotify_basic.c    | 122 ++++++++++++++++++
 2 files changed, 129 insertions(+)
 create mode 100644 tools/testing/selftests/filesystems/Makefile
 create mode 100644 tools/testing/selftests/filesystems/fanotify_basic.c

diff --git a/tools/testing/selftests/filesystems/Makefile b/tools/testing/selftests/filesystems/Makefile
new file mode 100644
index 0000000..c0e0242
--- /dev/null
+++ b/tools/testing/selftests/filesystems/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS += $(KHDR_INCLUDES)
+TEST_GEN_PROGS := devpts_pts file_stressor anon_inode_test kernfs_test fclog fanotify_basic
+TEST_GEN_PROGS_EXTENDED := dnotify_test
+
+include ../lib.mk
diff --git a/tools/testing/selftests/filesystems/fanotify_basic.c b/tools/testing/selftests/filesystems/fanotify_basic.c
new file mode 100644
index 0000000..4a4fbb4
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fanotify_basic.c
@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <fcntl.h>
+#include <sys/fanotify.h>
+#include <linux/fanotify.h>
+#include "../kselftest_harness.h"
+#include "wrappers.h"
+
+static void create_file(const char *filename)
+{
+	int fd;
+	int ret;
+
+	fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0644);
+	if (fd == -1)
+		ksft_exit_fail_msg("(create)open failed: %s\n", strerror(errno));
+	ret = write(fd, "create_file", 11);
+	if (ret == -1)
+		ksft_exit_fail_msg("(create) writing failed: %s\n", strerror(errno));
+	close(fd);
+}
+
+static void modify_file(const char *filename)
+{
+	int fd;
+	int ret;
+
+	fd = open(filename, O_RDWR);
+	if (fd == -1)
+		ksft_exit_fail_msg("(modify)open failed :%s\n", strerror(errno));
+	if (lseek(fd, 0, SEEK_END) < 0)
+		ksft_exit_fail_msg("(modify)lseek failed");
+	ret = write(fd, "modify_file", 11);
+	if (ret == -1)
+		ksft_exit_fail_msg("(modify)write failed :%s\n", strerror(errno));
+	if (fsync(fd) == -1)
+		ksft_exit_fail_msg("(modify)fsync failed: %s\n", strerror(errno));
+
+	close(fd);
+}
+
+TEST(fanotify_cud_test)
+{
+	int fan_fd;
+	char buf[4096];
+	int ret;
+	ssize_t len;
+	struct fanotify_event_metadata *meta;
+
+	fan_fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_FID, O_RDONLY);
+	ASSERT_GE(fan_fd, 0)
+	TH_LOG("fanotify_init failed: %s", strerror(errno));
+
+	ret = fanotify_mark(fan_fd, FAN_MARK_ADD,
+				  FAN_EVENT_ON_CHILD | FAN_CREATE |
+				  FAN_MODIFY | FAN_DELETE,
+				  AT_FDCWD, "/tmp");
+	ASSERT_GE(ret, 0)
+	TH_LOG("fanotify_mark failed: %s", strerror(errno));
+
+	// FAN_CREATE Test
+	create_file("/tmp/fanotify_test");
+	len = read(fan_fd, buf, sizeof(buf));
+	ASSERT_GT(len, 0)
+	TH_LOG("No event after create_file");
+
+	meta = (void *)buf;
+	if (FAN_EVENT_OK(meta, len)) {
+		TH_LOG("Event after create: mask = 0x%llx, pid=%d",
+		       (unsigned long long)meta->mask, meta->pid);
+		if (meta->mask & FAN_CREATE)
+			ksft_test_result_pass("FAN_CREATE detected\n");
+		else
+			TH_LOG("FAN_CREATE missing");
+	} else
+		ksft_test_result_fail("Invalid event metadata after create\n");
+
+	// FAN_MODIFY Test
+	modify_file("/tmp/fanotify_test");
+	len = read(fan_fd, buf, sizeof(buf));
+	ASSERT_GT(len, 0)
+		TH_LOG("No event after modify_file");
+
+	meta = (void *)buf;
+	if (FAN_EVENT_OK(meta, len)) {
+		TH_LOG("Event after modify: mask = 0x%llx, pid=%d",
+		       (unsigned long long)meta->mask, meta->pid);
+		if (meta->mask & FAN_MODIFY)
+			ksft_test_result_pass("FAN_MODIFY detected\n");
+		else
+			ksft_test_result_fail("FAN_MODIFY missing\n");
+	} else
+		ksft_test_result_fail("Invalid event metadata after modify\n");
+
+	// FAN_DELETE
+	ASSERT_EQ(unlink("/tmp/fanotify_test"), 0)
+		TH_LOG("unlink failed: %s", strerror(errno));
+
+	len = read(fan_fd, buf, sizeof(buf));
+	ASSERT_GT(len, 0)
+		TH_LOG("No event after unlink");
+
+	meta = (void *)buf;
+	if (FAN_EVENT_OK(meta, len)) {
+		TH_LOG("Event after delete: mask = 0x%llx, pid=%d",
+		       (unsigned long long)meta->mask, meta->pid);
+		if (meta->mask & FAN_DELETE)
+			ksft_test_result_pass("FAN_DELETE detected\n");
+		else
+			ksft_test_result_fail("FAN_DELETE missing\n");
+	} else
+		ksft_test_result_fail("Invalid event metadata after delete\n");
+
+	// Clean up
+	if (fan_fd >= 0) {
+		fanotify_mark(fan_fd, FAN_MARK_REMOVE, 0, AT_FDCWD, ".");
+		close(fan_fd);
+	}
+}
+
+TEST_HARNESS_MAIN
--
2.43.0