Extended Attributes – Storage and Preservation

filesfilesystemsxattr

I have a small question about extended file attributes. Assume I label my files with metadata in extended attributes (e.g. to account for the integrity – but this does not matter for my question). The questions that arise now:

  • Where are these attributes stored? Surely not in the inode I guess, but in what location – or better: structure?
  • How are these attributes connected to a file? Is there a link from the attribute structture to the inode or so?
  • What happens when copying/moving around files? I just tested it, when moving a file, the file remains its attributes. When copying it, the copy does not have attributes. So I assume whe burning it to CD or emailing the file, it will also lose its attributes?

Best Answer

The answer to you question is filesystem specific. For ext3, for example, have a look at fs/ext3/xattr.c, it contains the following description:

  16 /*
  17  * Extended attributes are stored directly in inodes (on file systems with
  18  * inodes bigger than 128 bytes) and on additional disk blocks. The i_file_acl
  19 
 * field contains the block number if an inode uses an additional block. All
  20  * attributes must fit in the inode and one additional block. Blocks that
  21  * contain the identical set of attributes may be shared among several inodes.
  22  * Identical blocks are detected by keeping a cache of blocks that have
  23  * recently been accessed.
  24  *
  25  * The attributes in inodes and on blocks have a different header; the entries
  26  * are stored in the same format:
  27  *
  28  *   +------------------+
  29  *   | header           |
  30  *   | entry 1          | |
  31  *   | entry 2          | | growing downwards
  32  *   | entry 3          | v
  33  *   | four null bytes  |
  34  *   | . . .            |
  35  *   | value 1          | ^
  36  *   | value 3          | | growing upwards
  37  *   | value 2          | |
  38  *   +------------------+
  39  *
  40  * The header is followed by multiple entry descriptors. In disk blocks, the
  41  * entry descriptors are kept sorted. In inodes, they are unsorted. The
  42  * attribute values are aligned to the end of the block in no specific order.
  43  *
  44  * Locking strategy
  45  * ----------------
  46  * EXT3_I(inode)->i_file_acl is protected by EXT3_I(inode)->xattr_sem.
  47  * EA blocks are only changed if they are exclusive to an inode, so
  48  * holding xattr_sem also means that nothing but the EA block's reference
  49  * count can change. Multiple writers to the same block are synchronized
  50  * by the buffer lock.
  51  */

Regarding the "how are attributes connected" question, the link is in the other way round, the inode has a link to the extended attributes, see EXT3_XATTR_NEXT and ext3_xattr_list_entries in xattr.h and xattr.c respectively.

To recap, the attributes are linked to the inode and are fs dependent, so yes, you will lose the attributes when burning a CD rom or emailing a file.

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