Documentation/filesystems/ext4/attributes.rst

Source file repositories/reference/linux-study-clean/Documentation/filesystems/ext4/attributes.rst

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Documentation/filesystems/ext4/attributes.rst
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Support Tooling And Documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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.. SPDX-License-Identifier: GPL-2.0

Extended Attributes
-------------------

Extended attributes (xattrs) are typically stored in a separate data
block on the disk and referenced from inodes via ``inode.i_file_acl*``.
The first use of extended attributes seems to have been for storing file
ACLs and other security data (selinux). With the ``user_xattr`` mount
option it is possible for users to store extended attributes so long as
all attribute names begin with “user”; this restriction seems to have
disappeared as of Linux 3.0.

There are two places where extended attributes can be found. The first
place is between the end of each inode entry and the beginning of the
next inode entry. For example, if inode.i_extra_isize = 28 and
sb.inode_size = 256, then there are 256 - (128 + 28) = 100 bytes
available for in-inode extended attribute storage. The second place
where extended attributes can be found is in the block pointed to by
``inode.i_file_acl``. As of Linux 3.11, it is not possible for this
block to contain a pointer to a second extended attribute block (or even
the remaining blocks of a cluster). In theory it is possible for each
attribute's value to be stored in a separate data block, though as of
Linux 3.11 the code does not permit this.

Keys are generally assumed to be ASCIIZ strings, whereas values can be
strings or binary data.

Extended attributes, when stored after the inode, have a header
``ext4_xattr_ibody_header`` that is 4 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - __le32
     - h_magic
     - Magic number for identification, 0xEA020000. This value is set by the
       Linux driver, though e2fsprogs doesn't seem to check it(?)

The beginning of an extended attribute block is in
``struct ext4_xattr_header``, which is 32 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - __le32
     - h_magic
     - Magic number for identification, 0xEA020000.
   * - 0x4
     - __le32
     - h_refcount
     - Reference count.
   * - 0x8
     - __le32
     - h_blocks
     - Number of disk blocks used.
   * - 0xC
     - __le32

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