fsck.ubifs: Check whether the TNC is empty

This is the 11/18 step of fsck. Check whether the TNC is empty, turn to
rebuild_fs if it is not found. Can we recreate a new root dir to avoid
empty TNC? The answer is no, lpt fixing should be done before creating
new entry, but lpt fixing needs a committing before new dirty data
generated to ensure that bud data won't be overwritten(bud LEB could
become freeable after replaying journal, corrected lpt may treat it as
a free one to hold new data, see details in space checking & correcting
step). Then we have to create the new root dir after fixing lpt and a
committing, znode without children(empty TNC) maybe written on disk at
the moment of committing, which corrupts the UBIFS image. So we choose
to rebuild the filesystem if the TNC is empty, this case is equivalent
to corrupted TNC.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

1 files changed, 25 insertions, 0 deletions

diff --git a/ubifs-utils/fsck.ubifs/check_files.c b/ubifs-utils/fsck.ubifs/check_files.c
index 2be9619..b9f31a7 100644
--- a/ubifs-utils/fsck.ubifs/check_files.c
+++ b/ubifs-utils/fsck.ubifs/check_files.c
@@ -499,3 +499,28 @@ int handle_dentry_tree(struct ubifs_info *c)
 
 	return 0;
 }
+
+/**
+ * tnc_is_empty - Check whether the TNC is empty.
+ * @c: UBIFS file-system description object
+ *
+ * Returns %true if the TNC is empty, otherwise %false is returned.
+ */
+bool tnc_is_empty(struct ubifs_info *c)
+{
+	/*
+	 * Check whether the TNC is empty, turn to rebuild_fs if it is empty.
+	 * Can we recreate a new root dir to avoid empty TNC? The answer is no,
+	 * lpt fixing should be done before creating new entry, but lpt fixing
+	 * needs a committing before new dirty data generated to ensure that
+	 * bud data won't be overwritten(bud LEB could become freeable after
+	 * replaying journal, corrected lpt may treat it as a free one to hold
+	 * new data, see details in space checking & correcting step). Then we
+	 * have to create the new root dir after fixing lpt and a committing,
+	 * znode without children(empty TNC) maybe written on disk at the
+	 * moment of committing, which corrupts the UBIFS image. So we choose
+	 * to rebuild the filesystem if the TNC is empty, this case is
+	 * equivalent to corrupted TNC.
+	 */
+	return c->zroot.znode->child_cnt == 0;
+}