initramfs_test: use test init/exit hooks to override init fs

Most initramfs kunit tests interact with initramfs via
unpack_to_rootfs() and subsequently init_stat(), init_unlink(), etc.

It's cleaner and less error-prone to override current->fs with
userspace_init_fs for all initramfs_test_suite tests.

Link: https://patch.msgid.link/[email protected] [1]
Link: https://patch.msgid.link/[email protected] # folded into [1]
Fixes: 32750c77e8 ("fs: start all kthreads in nullfs")
Reported-by: Mark Brown <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]
Signed-off-by: David Disseldorp <[email protected]>
Co-developed-by: David Disseldorp <[email protected]>
Signed-off-by: Christian Brauner (Amutable) <[email protected]>

drivers/char/misc_minor_kunit.c

@@ -5,6 +5,7 @@
 #include <linux/miscdevice.h>
 #include <linux/fs.h>
 #include <linux/file.h>
+#include <linux/fs_struct.h>
 #include <linux/init_syscalls.h>
 
 /* static minor (LCD_MINOR) */
@@ -160,18 +161,22 @@ static void __init miscdev_test_can_open(struct kunit *test, struct miscdevice *
 	char *devname;
 
 	devname = kasprintf(GFP_KERNEL, "/dev/%s", misc->name);
-	ret = init_mknod(devname, S_IFCHR | 0600,
-			 new_encode_dev(MKDEV(MISC_MAJOR, misc->minor)));
-	if (ret != 0)
-		KUNIT_FAIL(test, "failed to create node\n");
 
-	filp = filp_open(devname, O_RDONLY, 0);
-	if (IS_ERR(filp))
-		KUNIT_FAIL(test, "failed to open misc device: %ld\n", PTR_ERR(filp));
-	else
-		fput(filp);
+	/* Tests run in a nullfs kthread; borrow the init fs to resolve /dev. */
+	scoped_with_init_fs() {
+		ret = init_mknod(devname, S_IFCHR | 0600,
+				 new_encode_dev(MKDEV(MISC_MAJOR, misc->minor)));
+		if (ret != 0)
+			KUNIT_FAIL(test, "failed to create node\n");
 
-	init_unlink(devname);
+		filp = filp_open(devname, O_RDONLY, 0);
+		if (IS_ERR(filp))
+			KUNIT_FAIL(test, "failed to open misc device: %ld\n", PTR_ERR(filp));
+		else
+			fput(filp);
+
+		init_unlink(devname);
+	}
 	kfree(devname);
 }
 

init/initramfs_test.c

@@ -3,6 +3,7 @@
 #include <linux/fcntl.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/fs_struct.h>
 #include <linux/init.h>
 #include <linux/init_syscalls.h>
 #include <linux/initrd.h>
@@ -562,7 +563,7 @@ static struct kunit_case __refdata initramfs_test_cases[] = {
 	{},
 };
 
-static int __init initramfs_test_init(struct kunit_suite *suite)
+static int __init initramfs_suite_init(struct kunit_suite *suite)
 {
 	/*
 	 * unpack_to_rootfs() uses module-static state (victim, byte_count,
@@ -574,9 +575,23 @@ static int __init initramfs_test_init(struct kunit_suite *suite)
 	return 0;
 }
 
+/* Tests run in a nullfs kthread; always use the init fs for path resolution. */
+static int __init initramfs_test_init(struct kunit *test)
+{
+	test->priv = __override_init_fs();
+	return 0;
+}
+
+static void __init initramfs_test_exit(struct kunit *test)
+{
+	__revert_init_fs(test->priv);
+}
+
 static struct kunit_suite __refdata initramfs_test_suite = {
 	.name = "initramfs",
-	.suite_init = initramfs_test_init,
+	.suite_init = initramfs_suite_init,
+	.init = initramfs_test_init,
+	.exit = initramfs_test_exit,
 	.test_cases = initramfs_test_cases,
 };
 kunit_test_init_section_suites(&initramfs_test_suite);