vfs: Add KUnit tests for fdtable

This adds a KUnit test suite for fdtable to verify correct allocation,
max_fds initialization, and dynamic object size of the fd array under
__counted_by_ptr when CONFIG_CC_HAS_COUNTED_BY_PTR is enabled.

Signed-off-by: Bill Wendling <morbo@google.com>
Link: https://patch.msgid.link/20260827041732.188707-2-morbo@google.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/Kconfig

@@ -423,4 +423,12 @@ source "fs/unicode/Kconfig"
 config IO_WQ
 	bool
 
+config FDTABLE_KUNIT_TEST
+	bool "KUnit test for fdtable" if !KUNIT_ALL_TESTS
+	depends on KUNIT=y
+	default KUNIT_ALL_TESTS
+	help
+	  This builds the fdtable KUnit tests, which tests various aspects
+	  of the fdtable structure and allocation.
+
 endmenu

fs/file.c

@@ -1529,3 +1529,7 @@ int iterate_fd(struct files_struct *files, unsigned n,
 	return res;
 }
 EXPORT_SYMBOL(iterate_fd);
+
+#ifdef CONFIG_FDTABLE_KUNIT_TEST
+#include "tests/fdtable_kunit.c"
+#endif

fs/tests/.kunitconfig

@@ -0,0 +1,2 @@
+CONFIG_KUNIT=y
+CONFIG_FDTABLE_KUNIT_TEST=y

fs/tests/fdtable_kunit.c

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <kunit/test.h>
+#include <linux/fdtable.h>
+#include <linux/file.h>
+
+static void test_alloc_fdtable(struct kunit *test)
+{
+	struct fdtable *fdt;
+	unsigned int slots = 64;
+
+	fdt = alloc_fdtable(slots);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fdt);
+
+	/* Check that max_fds is set correctly and is >= slots */
+	KUNIT_EXPECT_GE(test, fdt->max_fds, slots);
+
+	/* Check that fd is allocated */
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fdt->fd);
+
+	/*
+	 * Check dynamic object size of fdt->fd if compiler supports
+	 * __counted_by_ptr.
+	 */
+#ifdef CONFIG_CC_HAS_COUNTED_BY_PTR
+	KUNIT_EXPECT_EQ(test, __builtin_dynamic_object_size(fdt->fd, 0),
+			fdt->max_fds * sizeof(struct file *));
+#endif
+
+	__free_fdtable(fdt);
+}
+
+static void test_dup_fd(struct kunit *test)
+{
+	struct files_struct *newf;
+	struct fdtable *fdt;
+
+	newf = dup_fd(&init_files, NULL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, newf);
+
+	fdt = rcu_dereference_raw(newf->fdt);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fdt);
+
+	/* Check that max_fds is set correctly and is >= NR_OPEN_DEFAULT */
+	KUNIT_EXPECT_GE(test, fdt->max_fds, NR_OPEN_DEFAULT);
+
+	/* Check that fd is allocated */
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, fdt->fd);
+
+	/*
+	 * Check dynamic object size of fdt->fd if compiler supports
+	 * __counted_by_ptr.
+	 */
+#ifdef CONFIG_CC_HAS_COUNTED_BY_PTR
+	KUNIT_EXPECT_EQ(test, __builtin_dynamic_object_size(fdt->fd, 0),
+			fdt->max_fds * sizeof(struct file *));
+#endif
+
+	put_files_struct(newf);
+}
+
+static struct kunit_case fdtable_test_cases[] = {
+	KUNIT_CASE(test_alloc_fdtable),
+	KUNIT_CASE(test_dup_fd),
+	{}
+};
+
+static struct kunit_suite fdtable_test_suite = {
+	.name = "fdtable",
+	.test_cases = fdtable_test_cases,
+};
+
+kunit_test_suite(fdtable_test_suite);