ext4: alloc test super block from sget

This fix the oops in ext4 unit test which is cuased by NULL sb.s_user_ns
as following:
<4>[ 14.344565] map_id_range_down (kernel/user_namespace.c:318)
<4>[ 14.345378] make_kuid (kernel/user_namespace.c:415)
<4>[ 14.345998] inode_init_always (include/linux/fs.h:1375 fs/inode.c:174)
<4>[ 14.346696] alloc_inode (fs/inode.c:268)
<4>[ 14.347353] new_inode_pseudo (fs/inode.c:1007)
<4>[ 14.348016] new_inode (fs/inode.c:1033)
<4>[ 14.348644] ext4_mb_init (fs/ext4/mballoc.c:3404 fs/ext4/mballoc.c:3719)
<4>[ 14.349312] mbt_kunit_init (fs/ext4/mballoc-test.c:57
fs/ext4/mballoc-test.c:314)
<4>[ 14.349983] kunit_try_run_case (lib/kunit/test.c:388 lib/kunit/test.c:443)
<4>[ 14.350696] kunit_generic_run_threadfn_adapter (lib/kunit/try-catch.c:30)
<4>[ 14.351530] kthread (kernel/kthread.c:388)
<4>[ 14.352168] ret_from_fork (arch/arm64/kernel/entry.S:861)
<0>[ 14.353385] Code: 52808004 b8236ae7 72be5e44 b90004c4 (38e368a1)

Alloc test super block from sget to properly initialize test super block
to fix the issue.
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240304163543.6700-2-shikemeng@huaweicloud.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc-test.c

@@ -21,16 +21,28 @@ struct mbt_ctx {
 };
 
 struct mbt_ext4_super_block {
-	struct super_block sb;
+	struct ext4_super_block es;
+	struct ext4_sb_info sbi;
 	struct mbt_ctx mbt_ctx;
 };
 
-#define MBT_CTX(_sb) (&(container_of((_sb), struct mbt_ext4_super_block, sb)->mbt_ctx))
+#define MBT_SB(_sb) (container_of((_sb)->s_fs_info, struct mbt_ext4_super_block, sbi))
+#define MBT_CTX(_sb) (&MBT_SB(_sb)->mbt_ctx)
 #define MBT_GRP_CTX(_sb, _group) (&MBT_CTX(_sb)->grp_ctx[_group])
 
 static const struct super_operations mbt_sops = {
 };
 
+static void mbt_kill_sb(struct super_block *sb)
+{
+	generic_shutdown_super(sb);
+}
+
+static struct file_system_type mbt_fs_type = {
+	.name			= "mballoc test",
+	.kill_sb		= mbt_kill_sb,
+};
+
 static int mbt_mb_init(struct super_block *sb)
 {
 	int ret;
@@ -72,43 +84,54 @@ static void mbt_mb_release(struct super_block *sb)
 	kfree(sb->s_bdev);
 }
 
+static int mbt_set(struct super_block *sb, void *data)
+{
+	return 0;
+}
+
 static struct super_block *mbt_ext4_alloc_super_block(void)
 {
-	struct ext4_super_block *es = kzalloc(sizeof(*es), GFP_KERNEL);
-	struct ext4_sb_info *sbi = kzalloc(sizeof(*sbi), GFP_KERNEL);
-	struct mbt_ext4_super_block *fsb = kzalloc(sizeof(*fsb), GFP_KERNEL);
+	struct mbt_ext4_super_block *fsb;
+	struct super_block *sb;
+	struct ext4_sb_info *sbi;
 
-	if (fsb == NULL || sbi == NULL || es == NULL)
+	fsb = kzalloc(sizeof(*fsb), GFP_KERNEL);
+	if (fsb == NULL)
+		return NULL;
+
+	sb = sget(&mbt_fs_type, NULL, mbt_set, 0, NULL);
+	if (IS_ERR(sb))
 		goto out;
 
+	sbi = &fsb->sbi;
+
 	sbi->s_blockgroup_lock =
 		kzalloc(sizeof(struct blockgroup_lock), GFP_KERNEL);
 	if (!sbi->s_blockgroup_lock)
-		goto out;
+		goto out_deactivate;
 
 	bgl_lock_init(sbi->s_blockgroup_lock);
 
-	sbi->s_es = es;
-	fsb->sb.s_fs_info = sbi;
+	sbi->s_es = &fsb->es;
+	sb->s_fs_info = sbi;
 
-	return &fsb->sb;
+	up_write(&sb->s_umount);
+	return sb;
 
+out_deactivate:
+	deactivate_locked_super(sb);
 out:
 	kfree(fsb);
-	kfree(sbi);
-	kfree(es);
 	return NULL;
 }
 
 static void mbt_ext4_free_super_block(struct super_block *sb)
 {
-	struct mbt_ext4_super_block *fsb =
-		container_of(sb, struct mbt_ext4_super_block, sb);
+	struct mbt_ext4_super_block *fsb = MBT_SB(sb);
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 
 	kfree(sbi->s_blockgroup_lock);
-	kfree(sbi->s_es);
-	kfree(sbi);
+	deactivate_super(sb);
 	kfree(fsb);
 }