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diff --git a/Documentation/filesystems/ext4/ondisk/attributes.rst b/Documentation/filesystems/ext4/ondisk/attributes.rst deleted file mode 100644 index 0b01b67b81fe..000000000000 --- a/Documentation/filesystems/ext4/ondisk/attributes.rst +++ /dev/null @@ -1,191 +0,0 @@ -.. 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: 1 1 1 77 - :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: 1 1 1 77 - :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 - - h\_hash - - Hash value of all attributes. - * - 0x10 - - \_\_le32 - - h\_checksum - - Checksum of the extended attribute block. - * - 0x14 - - \_\_u32 - - h\_reserved[2] - - Zero. - -The checksum is calculated against the FS UUID, the 64-bit block number -of the extended attribute block, and the entire block (header + -entries). - -Following the ``struct ext4_xattr_header`` or -``struct ext4_xattr_ibody_header`` is an array of -``struct ext4_xattr_entry``; each of these entries is at least 16 bytes -long. When stored in an external block, the ``struct ext4_xattr_entry`` -entries must be stored in sorted order. The sort order is -``e_name_index``, then ``e_name_len``, and finally ``e_name``. -Attributes stored inside an inode do not need be stored in sorted order. - -.. list-table:: - :widths: 1 1 1 77 - :header-rows: 1 - - * - Offset - - Type - - Name - - Description - * - 0x0 - - \_\_u8 - - e\_name\_len - - Length of name. - * - 0x1 - - \_\_u8 - - e\_name\_index - - Attribute name index. There is a discussion of this below. - * - 0x2 - - \_\_le16 - - e\_value\_offs - - Location of this attribute's value on the disk block where it is stored. - Multiple attributes can share the same value. For an inode attribute - this value is relative to the start of the first entry; for a block this - value is relative to the start of the block (i.e. the header). - * - 0x4 - - \_\_le32 - - e\_value\_inum - - The inode where the value is stored. Zero indicates the value is in the - same block as this entry. This field is only used if the - INCOMPAT\_EA\_INODE feature is enabled. - * - 0x8 - - \_\_le32 - - e\_value\_size - - Length of attribute value. - * - 0xC - - \_\_le32 - - e\_hash - - Hash value of attribute name and attribute value. The kernel doesn't - update the hash for in-inode attributes, so for that case this value - must be zero, because e2fsck validates any non-zero hash regardless of - where the xattr lives. - * - 0x10 - - char - - e\_name[e\_name\_len] - - Attribute name. Does not include trailing NULL. - -Attribute values can follow the end of the entry table. There appears to -be a requirement that they be aligned to 4-byte boundaries. The values -are stored starting at the end of the block and grow towards the -xattr\_header/xattr\_entry table. When the two collide, the overflow is -put into a separate disk block. If the disk block fills up, the -filesystem returns -ENOSPC. - -The first four fields of the ``ext4_xattr_entry`` are set to zero to -mark the end of the key list. - -Attribute Name Indices -~~~~~~~~~~~~~~~~~~~~~~ - -Logically speaking, extended attributes are a series of key=value pairs. -The keys are assumed to be NULL-terminated strings. To reduce the amount -of on-disk space that the keys consume, the beginning of the key string -is matched against the attribute name index. If a match is found, the -attribute name index field is set, and matching string is removed from -the key name. Here is a map of name index values to key prefixes: - -.. list-table:: - :widths: 1 79 - :header-rows: 1 - - * - Name Index - - Key Prefix - * - 0 - - (no prefix) - * - 1 - - “user.” - * - 2 - - “system.posix\_acl\_access” - * - 3 - - “system.posix\_acl\_default” - * - 4 - - “trusted.” - * - 6 - - “security.” - * - 7 - - “system.” (inline\_data only?) - * - 8 - - “system.richacl” (SuSE kernels only?) - -For example, if the attribute key is “user.fubar”, the attribute name -index is set to 1 and the “fubar” name is recorded on disk. - -POSIX ACLs -~~~~~~~~~~ - -POSIX ACLs are stored in a reduced version of the Linux kernel (and -libacl's) internal ACL format. The key difference is that the version -number is different (1) and the ``e_id`` field is only stored for named -user and group ACLs. |