[PATCH 21/22] docs: add XFS symlink structures to the DS&A book

Darrick J. Wong
2018-10-04T04:20:47Z

.../filesystems/xfs-data-structures/dynamic.rst | 1
.../xfs-data-structures/symbolic_links.rst | 140 ++++++++++++++++++++
2 files changed, 141 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/symbolic_links.rst

diff --git a/Documentation/filesystems/xfs-data-structures/dynamic.rst b/Documentation/filesystems/xfs-data-structures/dynamic.rst
index 16755381d0f8..68837d0f477e 100644
--- a/Documentation/filesystems/xfs-data-structures/dynamic.rst
+++ b/Documentation/filesystems/xfs-data-structures/dynamic.rst
@@ -7,3 +7,4 @@ Dynamic Allocated Structures
.. include:: data_extents.rst
.. include:: directories.rst
.. include:: extended_attributes.rst
+.. include:: symbolic_links.rst
diff --git a/Documentation/filesystems/xfs-data-structures/symbolic_links.rst b/Documentation/filesystems/xfs-data-structures/symbolic_links.rst
new file mode 100644
index 000000000000..9206fd44b108
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/symbolic_links.rst
@@ -0,0 +1,140 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Symbolic Links
+--------------
+
+Symbolic links to a file can be stored in one of two formats: "local" and
+"extents". The length of the symlink contents is always specified by the
+inode’s di\_size value.
+
+Short Form Symbolic Links
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Symbolic links are stored with the "local" di\_format if the symbolic link
+can fit within the inode’s data fork. The link data is an array of characters
+(di\_symlink array in the data fork union).
+
+.. figure:: images/61.png
+ :alt: Symbolic link short form layout
+
+ Symbolic link short form layout
+
+xfs\_db Short Form Symbolic Link Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A short symbolic link to a file is created:
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0120777
+ core.version = 1
+ core.format = 1 (local)
+ ...
+ core.size = 12
+ core.nblocks = 0
+ core.extsize = 0
+ core.nextents = 0
+ ...
+ u.symlink = "small_target"
+
+Raw on-disk data with the link contents highlighted:
+
+::
+
+ xfs_db> type text
+ xfs_db> p
+ 00: 49 4e a1 ff 01 01 00 01 00 00 00 00 00 00 00 00 IN..............
+ 10: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 01 ................
+ 20: 44 be e1 c7 03 c4 d4 18 44 be el c7 03 c4 d4 18 D.......D.......
+ 30: 44 be e1 c7 03 c4 d4 18 00 00 00 00 00 00 00 Oc D...............
+ 40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 50: 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 ................
+ 60: ff ff ff ff 73 6d 61 6c 6c 5f 74 61 72 67 65 74 ....small.target
+ 70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
+
+Extent Symbolic Links
+~~~~~~~~~~~~~~~~~~~~~
+
+If the length of the symbolic link exceeds the space available in the inode’s
+data fork, the link is moved to a new filesystem block and the inode’s
+di\_format is changed to "extents". The location of the block(s) is
+specified by the data fork’s di\_bmx[] array. In the significant majority of
+cases, this will be in one filesystem block as a symlink cannot be longer than
+1024 characters.
+
+On a v5 filesystem, the first block of each extent starts with the following
+header structure:
+
+.. code:: c
+
+ struct xfs_dsymlink_hdr {
+ __be32 sl_magic;
+ __be32 sl_offset;
+ __be32 sl_bytes;
+ __be32 sl_crc;
+ uuid_t sl_uuid;
+ __be64 sl_owner;
+ __be64 sl_blkno;
+ __be64 sl_lsn;
+ };
+
+**sl\_magic**
+ Specifies the magic number for the symlink block: "XSLM" (0x58534c4d).
+
+**sl\_offset**
+ Offset of the symbolic link target data, in bytes.
+
+**sl\_bytes**
+ Number of bytes used to contain the link target data.
+
+**sl\_crc**
+ Checksum of the symlink block.
+
+**sl\_uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**sl\_owner**
+ The inode number that this symlink block belongs to.
+
+**sl\_blkno**
+ Disk block number of this symlink.
+
+**sl\_lsn**
+ Log sequence number of the last write to this block.
+
+Filesystems formatted prior to v5 do not have this header in the remote block.
+Symlink data begins immediately at offset zero.
+
+.. figure:: images/62.png
+ :alt: Symbolic link extent layout
+
+ Symbolic link extent layout
+
+xfs\_db Symbolic Link Extent Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A longer link is created (greater than 156 bytes):
+
+::
+
+ xfs_db> inode <inode#>
+ xfs_db> p
+ core.magic = 0x494e
+ core.mode = 0120777
+ core.version = 1
+ core.format = 2 (extents)
+ ...
+ core.size = 182
+ core.nblocks = 1
+ core.extsize = 0
+ core.nextents = 1
+ ...
+ u.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,37530,1,0]
+ xfs_db> dblock 0
+ xfs_db> type symlink
+ xfs_db> p
+ "symlink contents..."