ExFAT Implementation Notes

Note: Need to add links to this, and probably some exFAT stuff in general to the wiki when this page is done.

Introduction

This document contains information on the implementation of the exFAT file system in The Sleuth Kit (TSK). Like FAT, exFAT has significant differences with the UNIX file system TSK was originally designed to support, and this document will identify how they were handled. A basic understanding of exFAT is assumed.

Disk Unit Addressing

Like FAT, exFAT saves file content in clusters, which are groupings of consecutive sectors (512-bytes each). It has the same problems as FAT in using these clusters as the addressable units, so like FAT the solution is to use sectors as the addressable unit instead. See FAT_Implementation_Notes#Disk_Unit_Addressing for more information.

Metadata Addressing

Again, exFAT shares the same problems discussed in FAT_Implementation_Notes#Metadata_Addressing, and the same solution is used. Note that normal exFAT files have at least three directory entry structures (more the longer the filename is), so the metadata addresses will increase by three or more for files in the same directory.

mmls and Partition Codes

ExFAT shares the same partition code as NTFS, so the string displayed by mmls for type 0x07 is now "NTFS / exFAT (0x07)". See http://www.win.tue.nl/~aeb/partitions/partition_types-1.html

fsstat and FAT Chains

Like FAT, exFAT does have FAT chains. However, they are only used to keep track of clusters for fragmented files. Printing a list of FAT Chains in fsstat could give the incorrect impression that those were the only allocated sectors, so it was decided that this part of the output should be hidden for exFAT.

Time Information

ExFAT stores the local time in its timestamp fields, so the notes in FAT_Implementation_Notes#Notes_on_Timezones also apply

here. However, exFAT does add some additional issues.

Last Accessed Time

In testing, it does not appear that the last accessed time is ever updated from a file being accessed. It does change when a file is modified.

Timezones

Timezone information for the three timestamp fields are also stored along with the DOS-format timestamps. However, these three fields are not found in the original specification, and there seem to be differences in the way the timezone is stored between exFAT implementation (see the next section). The timezone fields are empty in some older versions of Windows, such as Vista SP1. At present, TSK does not store this time zone information.

Mac OS Differences

Through analysis, it was found that exFAT images created on Mac OS store time information differently than the Windows versions.

This is the File Directory Entry from a file created with Windows 7 (UTC-5:00), at 11:27 AM local time on 2013-12-31:

000230E0 85 02 8B B6 20 00 00 00 6E 5B 9F 43 73 5B 9F 43
000230F0 73 5B 9F 43 18 00 EC EC EC 00 00 00 00 00 00 00

• Create time: 0x5B6E => 11:27:28
• Create time: 0x439F => 2013-12-31
• Create timezone: 0xEC = -20; Timezone is in 15-minute intervals, so this represents UTC-5:00

All that data is correct. Now here is the File Directory Entry from a file created on a Mac (UTC-8:00), at 10:11 local time on 2013-12-31:

000231E0 85 03 D4 40 30 00 00 00 5D 11 21 44 5D 11 21 44
000231F0 5D 11 21 44 A9 AD A0 A0 A0 00 00 00 00 00 00 00

• Create time: 0x115D => 2:10:58
• Create date: 0x4421 => 2014-01-01
• Create timezone: 0xA0

Limited testing suggests that the high order bit of timezone 0xA0 indicates that it is negative, and the rest is used as above, which gives us -(0x20) = -32, which would be UTC-8:00. This is correct, but clearly a different calculation than the Windows version.

Additionally, what should be local time is off by 16 hours. When set to GMT-5, the local time is off by 10 hours. It almost seems like they start with the UTC time and then go the wrong way with the offset. More testing is required.