[RFC PATCH mtd-utils 088/110] fsck.ubifs: Check whether the TNC is empty

[Zhihao Cheng]

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 childs(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@xxxxxxxxxx>
---
 ubifs-utils/fsck.ubifs/check_files.c | 25 +++++++++++++++++++++++++
 ubifs-utils/fsck.ubifs/fsck.ubifs.c  |  7 +++++++
 ubifs-utils/fsck.ubifs/fsck.ubifs.h  |  4 +++-
 ubifs-utils/fsck.ubifs/problem.c     |  1 +
 4 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/ubifs-utils/fsck.ubifs/check_files.c b/ubifs-utils/fsck.ubifs/check_files.c
index 2be96193..4190bf91 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 childs(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;
+}
diff --git a/ubifs-utils/fsck.ubifs/fsck.ubifs.c b/ubifs-utils/fsck.ubifs/fsck.ubifs.c
index 85a5baf0..14f77fc2 100644
--- a/ubifs-utils/fsck.ubifs/fsck.ubifs.c
+++ b/ubifs-utils/fsck.ubifs/fsck.ubifs.c
@@ -467,6 +467,12 @@ static int do_fsck(void)
 		goto free_disconnected_files;
 	}
 
+	log_out(c, "Check whether the TNC is empty");
+	if (tnc_is_empty(c) && fix_problem(c, EMPTY_TNC, NULL)) {
+		err = -EINVAL;
+		FSCK(c)->try_rebuild = true;
+	}
+
 free_disconnected_files:
 	destroy_file_list(c, &FSCK(c)->disconnected_files);
 free_used_lebs:
@@ -512,6 +518,7 @@ int main(int argc, char *argv[])
 	 * Step 8: Check and handle invalid files
 	 * Step 9: Check and handle unreachable files
 	 * Step 10: Check and correct files
+	 * Step 11: Check whether the TNC is empty
 	 */
 	err = do_fsck();
 	if (err && FSCK(c)->try_rebuild) {
diff --git a/ubifs-utils/fsck.ubifs/fsck.ubifs.h b/ubifs-utils/fsck.ubifs/fsck.ubifs.h
index d1423c01..663d5dcf 100644
--- a/ubifs-utils/fsck.ubifs/fsck.ubifs.h
+++ b/ubifs-utils/fsck.ubifs/fsck.ubifs.h
@@ -43,7 +43,8 @@ enum { SB_CORRUPTED = 0, MST_CORRUPTED, LOG_CORRUPTED, BUD_CORRUPTED,
        FILE_HAS_0_NLINK_INODE, FILE_HAS_INCONSIST_TYPE, FILE_HAS_TOO_MANY_DENT,
        FILE_SHOULDNT_HAVE_DATA, FILE_HAS_NO_DENT, XATTR_HAS_NO_HOST,
        XATTR_HAS_WRONG_HOST, FILE_HAS_NO_ENCRYPT, FILE_IS_DISCONNECTED,
-       FILE_ROOT_HAS_DENT, DENTRY_IS_UNREACHABLE, FILE_IS_INCONSISTENT };
+       FILE_ROOT_HAS_DENT, DENTRY_IS_UNREACHABLE, FILE_IS_INCONSISTENT,
+       EMPTY_TNC };
 
 enum { HAS_DATA_CORRUPTED = 1, HAS_TNC_CORRUPTED = 2 };
 
@@ -319,5 +320,6 @@ int traverse_tnc_and_construct_files(struct ubifs_info *c);
 void update_files_size(struct ubifs_info *c);
 int handle_invalid_files(struct ubifs_info *c);
 int handle_dentry_tree(struct ubifs_info *c);
+bool tnc_is_empty(struct ubifs_info *c);
 
 #endif
diff --git a/ubifs-utils/fsck.ubifs/problem.c b/ubifs-utils/fsck.ubifs/problem.c
index e8f08606..795f05fa 100644
--- a/ubifs-utils/fsck.ubifs/problem.c
+++ b/ubifs-utils/fsck.ubifs/problem.c
@@ -60,6 +60,7 @@ static const struct fsck_problem problem_table[] = {
 	{PROBLEM_FIXABLE | PROBLEM_MUST_FIX | PROBLEM_DROP_DATA, "Root dir should not have a dentry"},	// FILE_ROOT_HAS_DENT
 	{PROBLEM_FIXABLE | PROBLEM_MUST_FIX | PROBLEM_DROP_DATA, "Dentry is unreachable"},	// DENTRY_IS_UNREACHABLE
 	{PROBLEM_FIXABLE | PROBLEM_MUST_FIX, "File is inconsistent"},	// FILE_IS_INCONSISTENT
+	{PROBLEM_FIXABLE | PROBLEM_MUST_FIX | PROBLEM_DROP_DATA | PROBLEM_NEED_REBUILD, "TNC is empty"},	// EMPTY_TNC
 };
 
 static const char *get_question(const struct fsck_problem *problem,
-- 
2.13.6