1ATTR(5) File Formats Manual ATTR(5)
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6 attr - Extended attributes
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9 Extended attributes are name:value pairs associated permanently with
10 files and directories, similar to the environment strings associated
11 with a process. An attribute may be defined or undefined. If it is
12 defined, its value may be empty or non-empty.
13 Extended attributes are extensions to the normal attributes which are
15 associated with all inodes in the system (i.e. the stat(2) data). They
16 are often used to provide additional functionality to a filesystem -
17 for example, additional security features such as Access Control Lists
18 (ACLs) may be implemented using extended attributes.
19 Users with search access to a file or directory may retrieve a list of
21 attribute names defined for that file or directory.
22 Extended attributes are accessed as atomic objects. Reading retrieves
24 the whole value of an attribute and stores it in a buffer. Writing
25 replaces any previous value with the new value.
26 Space consumed for extended attributes is counted towards the disk quo‐
28 tas of the file owner and file group.
29 Currently, support for extended attributes is implemented on Linux by
31 the ext2, ext3, ext4, XFS, JFS and reiserfs filesystems.
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34 Attribute names are zero-terminated strings. The attribute name is
35 always specified in the fully qualified namespace.attribute form, eg.
36 user.mime_type, trusted.md5sum, system.posix_acl_access, or secu‐
37 rity.selinux.
38 The namespace mechanism is used to define different classes of extended
40 attributes. These different classes exist for several reasons, e.g.
41 the permissions and capabilities required for manipulating extended
42 attributes of one namespace may differ to another.
43 Currently the security, system, trusted, and user extended attribute
45 classes are defined as described below. Additional classes may be added
46 in the future.
47 Extended security attributes
49 The security attribute namespace is used by kernel security modules,
50 such as Security Enhanced Linux. Read and write access permissions to
51 security attributes depend on the policy implemented for each security
52 attribute by the security module. When no security module is loaded,
53 all processes have read access to extended security attributes, and
54 write access is limited to processes that have the CAP_SYS_ADMIN capa‐
55 bility.
56 Extended system attributes
58 Extended system attributes are used by the kernel to store system
59 objects such as Access Control Lists and Capabilities. Read and write
60 access permissions to system attributes depend on the policy imple‐
61 mented for each system attribute implemented by filesystems in the ker‐
62 nel.
63 Trusted extended attributes
65 Trusted extended attributes are visible and accessible only to pro‐
66 cesses that have the CAP_SYS_ADMIN capability (the super user usually
67 has this capability). Attributes in this class are used to implement
68 mechanisms in user space (i.e., outside the kernel) which keep informa‐
69 tion in extended attributes to which ordinary processes should not have
70 access.
71 Extended user attributes
73 Extended user attributes may be assigned to files and directories for
74 storing arbitrary additional information such as the mime type, charac‐
75 ter set or encoding of a file. The access permissions for user
76 attributes are defined by the file permission bits.
77 The file permission bits of regular files and directories are inter‐
79 preted differently from the file permission bits of special files and
80 symbolic links. For regular files and directories the file permission
81 bits define access to the file's contents, while for device special
82 files they define access to the device described by the special file.
83 The file permissions of symbolic links are not used in access checks.
84 These differences would allow users to consume filesystem resources in
85 a way not controllable by disk quotas for group or world writable spe‐
86 cial files and directories.
87 For this reason, extended user attributes are only allowed for regular
89 files and directories, and access to extended user attributes is
90 restricted to the owner and to users with appropriate capabilities for
91 directories with the sticky bit set (see the chmod(1) manual page for
92 an explanation of Sticky Directories).
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95 The kernel and the filesystem may place limits on the maximum number
96 and size of extended attributes that can be associated with a file.
97 Some file systems, such as ext2/3 and reiserfs, require the filesystem
98 to be mounted with the user_xattr mount option in order for extended
99 user attributes to be used.
100 In the current ext2, ext3 and ext4 filesystem implementations, each
102 extended attribute must fit on a single filesystem block (1024, 2048 or
103 4096 bytes, depending on the block size specified when the filesystem
104 was created).
105 In the XFS and reiserfs filesystem implementations, there is no practi‐
107 cal limit on the number or size of extended attributes associated with
108 a file, and the algorithms used to store extended attribute information
109 on disk are scalable.
110 In the JFS filesystem implementation, names can be up to 255 bytes and
112 values up to 65,535 bytes.
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115 Since the filesystems on which extended attributes are stored might
116 also be used on architectures with a different byte order and machine
117 word size, care should be taken to store attribute values in an archi‐
118 tecture independent format.
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121 Andreas Gruenbacher, <a.gruenbacher@bestbits.at> and the SGI XFS devel‐
122 opment team, <linux-xfs@oss.sgi.com>.
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125 getfattr(1), setfattr(1).
126 ATTR(5)