Hi,

Thank you for your report including a test pointing out
the issue.

Attached is a patch expected to fix it.

Please test and report.

Jean-Pierre

Chris Roehrig wrote on 7/30/20 1:01 AM:
I'm trying to get my Linux-based NTFS backup drive to pass a CHKDSK and came 
upon this curious situation where CHKDSK finds errors.

It seems to be some issue with how ntfs-3g modifies a directory index when 
renaming many files.

The CHKDSK error always seems to be of the form:
Stage 2: Examining file name linkage ...
The first free byte, 0xc0, and bytes available, 0x150, for root index $I30 in 
file 0x40 are not equal.

I've attached a python script (mkbaddir.py) that creates two (apparently) 
identical directories, one of which reliably causes this CHKDSK error; the 
other doesn't.

How to demonstrate:
        - Format an NTFS partition or thumbdrive using Windows or mkfs.ntfs.
        - Mount the partition on a Linux system.
                I used Mint 20 with ntfs-3g 2017.3.23AR.3 integrated FUSE 28 and
                python 3.8.2.
        - Chdir to the new NTFS partition and run the script:
                /tmp/mkbaddir.py                # creates 'baddir' in current 
dir.
                /tmp/mkbaddir.py -G     # creates 'gooddir' in current dir.
                diff -r baddir gooddir          # no difference
                du -sB1 baddir gooddir  # same size (128K)
        - Boot into Windows (10 v1903) and run (from a terminal) chkdsk X:  
(where X: is the NTFS drive).
                - This will say:
                "Errors found.  CHKDSK cannot continue in read-only mode."
        - Delete baddir (I used cygwin's rm -rf), and run chkdsk X: again.
                - This will now have no errors.

My guess at what's happening:
The script creates a directory of 410 empty files and then renames them with 
slightly larger names, which as I understand leaves a bunch of unused nodes in 
the b-tree.  The -G option just renames the 410 known files; without the -G 
option, it uses os.walk() to traverse the directory which I'm guessing leaves 
the b-tree in a slightly different state with even more unused nodes.

The 410 was chosen by trial-and-error so that some internal threshhold is just 
exceeded by the baddir but not by the gooddir.   With more than 410 (using the 
-c option; say -c 500), both baddir and gooddir will cause CHKDSK errors.

If I run the script on Windows/cygwin (Python 3.6.9) to create the folders, it 
does not give any CHKDSK errors even with many more files.

So there seems to be some issue with how ntfs-3g modifies the b-tree when 
renaming many files that is causing CHKDSK to complain.


I encountered this issue when trying to get my Linux-based NTFS backup drive to 
consistently pass a CHKDSK.  I use a script to first rename POSIX names to 
valid windows names, replacing '?' with '@@3F', etc so I can reverse the 
renaming afterwards.  I have some website mirror folders with many files of the 
form:
        details.asp?id=xxxxx&key=val
which gave rise to this issue.   (In the mkbaddir script I use only 
alphanumeric names to be clear this is not an illegal char issue).



--- libntfs-3g/index.c.ref	2017-03-23 10:42:44.000000000 +0100
+++ libntfs-3g/index.c	2020-08-01 12:16:39.760324300 +0200
@@ -1563,19 +1563,37 @@
 			   INDEX_ENTRY *ie, INDEX_BLOCK *ib)
 {
 	INDEX_ENTRY *ie_roam;
+	ATTR_RECORD *a;
+	int space;
+	int value_length;
+	BOOL at_end;
 	int ret = STATUS_ERROR;
-	
+
 	ntfs_log_trace("Entering\n");
 	
+	at_end = ih->index_length == ih->allocated_size;
 	ie_roam = ntfs_ie_dup_novcn(ie);
 	if (!ie_roam)
 		return STATUS_ERROR;
 
 	ntfs_ie_delete(ih, ie);
 
-	if (ntfs_icx_parent_vcn(icx) == VCN_INDEX_ROOT_PARENT)
+	if (ntfs_icx_parent_vcn(icx) == VCN_INDEX_ROOT_PARENT) {
+		/*
+		 * Recover the space which may have been freed
+		 * while deleting an entry from root index
+		 */
+		space = le32_to_cpu(ih->allocated_size)
+				- le32_to_cpu(ih->index_length);
+		if (at_end && (space > 0) && !(space & 7)) {
+			a = icx->actx->attr;
+			ih->allocated_size = ih->index_length;
+			value_length = le32_to_cpu(a->value_length);
+			value_length -= space;
+			a->value_length = cpu_to_le32(value_length);
+		}
 		ntfs_inode_mark_dirty(icx->actx->ntfs_ino);
-	else
+	} else
 		if (ntfs_ib_write(icx, ib))
 			goto out;
 	
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