如何存储和保留扩展属性？

Question

我有一个关于扩展文件属性的小问题。假设我用扩展属性中的元数据标记我的文件（例如，考虑完整性 - 但这对我的问题无关紧要）。现在出现的问题：

• 这些属性存储在哪里？我猜肯定不是在 inode 中，而是在什么位置 - 或者更好：结构？
• 这些属性如何连接到文件？是否有从属性结构到 inode 的链接？
• 复制/移动文件时会发生什么？我刚刚测试过，移动文件时，文件保持其属性。复制时，副本没有属性。所以我假设当它刻录到 CD 或通过电子邮件发送文件时，它也会失去它的属性？

Answer 1

您问题的答案是特定于文件系统的。例如，对于 ext3，请查看fs/ext3/xattr.c，它包含以下描述：

  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  */

关于“如何连接属性”的问题，该链接是倒过来，这个inode有一个链接到扩展属性，看到EXT3_XATTR_NEXT和ext3_xattr_list_entries分别在xattr.h和xattr.c。

概括地说，这些属性与 inode 相关联并且依赖于 fs，所以是的，在刻录 CD rom 或通过电子邮件发送文件时，您将丢失这些属性。