Linux Files – How to Get a Dump of All Extended Attributes for a File

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.

You don’t mention it in your text, but your code block shows that you are doing your test in /tmp.  Even if your root filesystem (HDD or SSD) is ext4, you might have /tmp mounted as a separate filesystem, probably of type tmpfs, and that does not support extended attributes.  You can check whether this is the case by executing any of the following commands:

• mount | grep tmp
• df /tmp
• grep /tmp /etc/fstab

The fact that the getfattr succeeds is a little surprising, but not very.  I guess some developer thought it would be harmless to report that a file has no extended attributes when the filesystem doesn’t support extended attributes.  After all, it’s true — the file has no extended attributes.