Add a simple test case for OverlayFS

test/apps/overlayfs/Makefile

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: MPL-2.0
+
+include ../test_common.mk
+
+EXTRA_C_FLAGS :=

test/apps/overlayfs/ovl_test.c

@@ -0,0 +1,189 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <sys/mount.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#define OVERLAYDIR "/overlay"
+#define LOWERDIR1 OVERLAYDIR "/lower1"
+#define LOWERDIR2 OVERLAYDIR "/lower2"
+#define UPPERDIR OVERLAYDIR "/upper"
+#define WORKDIR OVERLAYDIR "/work"
+#define MERGEDDIR OVERLAYDIR "/merged"
+
+void clean_up();
+
+void handle_error(const char *msg)
+{
+	perror(msg);
+	// Perform cleanup before exit to ensure no leftover directories or mount points
+	clean_up();
+	exit(EXIT_FAILURE);
+}
+
+void create_dir(const char *path)
+{
+	printf("Creating directory: %s\n", path);
+	if (mkdir(path, 0755) == -1 && errno != EEXIST) {
+		handle_error("mkdir");
+	}
+}
+
+void write_to_file(const char *path, const char *data)
+{
+	printf("Writing to file: %s\n", path);
+	int fd = open(path, O_WRONLY | O_CREAT, 0644);
+	if (fd == -1) {
+		handle_error("open");
+	}
+	if (write(fd, data, strlen(data)) == -1) {
+		close(fd);
+		handle_error("write");
+	}
+	close(fd);
+}
+
+void read_file(const char *path, char *buffer, size_t size)
+{
+	int fd = open(path, O_RDONLY);
+	if (fd == -1) {
+		handle_error("open");
+	}
+	ssize_t bytesRead = read(fd, buffer, size);
+	if (bytesRead == -1) {
+		close(fd);
+		handle_error("read");
+	}
+	buffer[bytesRead] = '\0'; // Null-terminate the string
+	close(fd);
+}
+
+void write_data_at_offset(const char *path, const char *data, off_t offset)
+{
+	int fd = open(path, O_WRONLY);
+	if (fd == -1) {
+		handle_error("open");
+	}
+	if (lseek(fd, offset, SEEK_SET) == -1) {
+		close(fd);
+		handle_error("lseek");
+	}
+	if (write(fd, data, strlen(data)) == -1) {
+		close(fd);
+		handle_error("write");
+	}
+	close(fd);
+}
+
+void assert_eq(const char *result, const char *expected, const char *msg)
+{
+	if (strcmp(result, expected) != 0) {
+		fprintf(stderr, "Assertion failed: %s\nExpected: %s\nGot: %s\n",
+			msg, expected, result);
+		handle_error("assert_eq");
+	}
+}
+
+void mount_overlayfs()
+{
+	char options[512];
+	snprintf(options, sizeof(options),
+		 "lowerdir=%s:%s,upperdir=%s,workdir=%s", LOWERDIR1, LOWERDIR2,
+		 UPPERDIR, WORKDIR);
+	printf("Mount options: %s\n", options);
+
+	if (mount("overlay", MERGEDDIR, "overlay", 0, options) == -1) {
+		handle_error("mount overlay");
+	}
+}
+
+void unlink_if_exists(const char *path)
+{
+	if (unlink(path) == -1 && errno != ENOENT) {
+		handle_error("unlink");
+	}
+}
+
+void clean_up()
+{
+	umount(MERGEDDIR);
+	umount(OVERLAYDIR);
+	rmdir(MERGEDDIR);
+	rmdir(WORKDIR);
+	rmdir(UPPERDIR);
+	rmdir(LOWERDIR1);
+	rmdir(LOWERDIR2);
+	rmdir(OVERLAYDIR);
+}
+
+// TODO: Enrich this test
+int main()
+{
+	create_dir(OVERLAYDIR);
+	// Mount tmpfs to /overlay first (Linux only)
+	// if (mount("tmpfs", OVERLAYDIR, "tmpfs", 0, NULL) == -1) {
+	//     handle_error("mount tmpfs");
+	// }
+
+	// Create necessary directories now that tmpfs is mounted
+	create_dir(LOWERDIR1);
+	create_dir(LOWERDIR2);
+	create_dir(UPPERDIR);
+	create_dir(WORKDIR);
+	create_dir(MERGEDDIR);
+
+	// Create files and directories in lower directories
+	create_dir(LOWERDIR1 "/d1");
+	write_to_file(LOWERDIR1 "/d1/f11", "file in lower1");
+
+	write_to_file(LOWERDIR2 "/f2", "88");
+	create_dir(LOWERDIR2 "/d1");
+	write_to_file(LOWERDIR2 "/d1/f11", "another file in lower2 d1");
+	write_to_file(LOWERDIR2 "/d1/f12", "another file in lower2 d1 f12");
+
+	printf("Mounting OverlayFS\n");
+	// Mount OverlayFS
+	mount_overlayfs();
+
+	// Read and verify the contents of the merged directory
+	char buffer[1024];
+
+	printf("Reading /overlay/merged/f2:\n");
+	read_file(MERGEDDIR "/f2", buffer, sizeof(buffer));
+	assert_eq(buffer, "88", "Content of /overlay/merged/f2 should be '88'");
+
+	// Write and read test - Copy up test
+	printf("Writing '99' to /overlay/merged/f2 at offset 2\n");
+	write_data_at_offset(MERGEDDIR "/f2", "99", 2);
+
+	read_file(MERGEDDIR "/f2", buffer, sizeof(buffer));
+	assert_eq(
+		buffer, "8899",
+		"Content of /overlay/merged/f2 should be '8899' after writing '99' at offset 2");
+
+	// Whiteout test
+	printf("Unlinking /overlay/merged/f1 if exists\n");
+	unlink_if_exists(MERGEDDIR "/f1");
+
+	// Create a new f1
+	write_to_file(MERGEDDIR "/f1", "new content for f1");
+
+	// Read and verify the new f1
+	printf("Reading /overlay/merged/f1:\n");
+	read_file(MERGEDDIR "/f1", buffer, sizeof(buffer));
+	assert_eq(
+		buffer, "new content for f1",
+		"Content of /overlay/merged/f1 should be 'new content for f1'");
+
+	// Clean up before exit
+	printf("Cleaning up\n");
+	clean_up();
+
+	return 0;
+}