1XATTR(7) Linux Programmer's Manual XATTR(7)
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6 xattr - 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).
16 They are often used to provide additional functionality to a filesys‐
17 tem—for example, additional security features such as Access Control
18 Lists (ACLs) may be implemented using extended attributes.
19 Users with search access to a file or directory may use listxattr(2) to
21 retrieve a list of attribute names defined for that file or directory.
22 Extended attributes are accessed as atomic objects. Reading (getx‐
24 attr(2)) retrieves the whole value of an attribute and stores it in a
25 buffer. Writing (setxattr(2)) replaces any previous value with the new
26 value.
27 Space consumed for extended attributes may be counted towards the disk
29 quotas of the file owner and file group.
30 Extended attribute namespaces
32 Attribute names are null-terminated strings. The attribute name is al‐
33 ways specified in the fully qualified namespace.attribute form, for ex‐
34 ample, user.mime_type, trusted.md5sum, system.posix_acl_access, or se‐
35 curity.selinux.
36 The namespace mechanism is used to define different classes of extended
38 attributes. These different classes exist for several reasons; for ex‐
39 ample, the permissions and capabilities required for manipulating ex‐
40 tended attributes of one namespace may differ to another.
41 Currently, the security, system, trusted, and user extended attribute
43 classes are defined as described below. Additional classes may be
44 added in the future.
45 Extended security attributes
47 The security attribute namespace is used by kernel security modules,
48 such as Security Enhanced Linux, and also to implement file capabili‐
49 ties (see capabilities(7)). Read and write access permissions to secu‐
50 rity attributes depend on the policy implemented for each security at‐
51 tribute by the security module. When no security module is loaded, all
52 processes have read access to extended security attributes, and write
53 access is limited to processes that have the CAP_SYS_ADMIN capability.
54 System extended attributes
56 System extended attributes are used by the kernel to store system ob‐
57 jects such as Access Control Lists. Read and write access permissions
58 to system attributes depend on the policy implemented for each system
59 attribute implemented by filesystems in the kernel.
60 Trusted extended attributes
62 Trusted extended attributes are visible and accessible only to pro‐
63 cesses that have the CAP_SYS_ADMIN capability. Attributes in this
64 class are used to implement mechanisms in user space (i.e., outside the
65 kernel) which keep information in extended attributes to which ordinary
66 processes should not have access.
67 User extended attributes
69 User extended attributes may be assigned to files and directories for
70 storing arbitrary additional information such as the mime type, charac‐
71 ter set or encoding of a file. The access permissions for user at‐
72 tributes are defined by the file permission bits: read permission is
73 required to retrieve the attribute value, and writer permission is re‐
74 quired to change it.
75 The file permission bits of regular files and directories are inter‐
77 preted differently from the file permission bits of special files and
78 symbolic links. For regular files and directories the file permission
79 bits define access to the file's contents, while for device special
80 files they define access to the device described by the special file.
81 The file permissions of symbolic links are not used in access checks.
82 These differences would allow users to consume filesystem resources in
83 a way not controllable by disk quotas for group or world writable spe‐
84 cial files and directories.
85 For this reason, user extended attributes are allowed only for regular
87 files and directories, and access to user extended attributes is re‐
88 stricted to the owner and to users with appropriate capabilities for
89 directories with the sticky bit set (see the chmod(1) manual page for
90 an explanation of the sticky bit).
91 Filesystem differences
93 The kernel and the filesystem may place limits on the maximum number
94 and size of extended attributes that can be associated with a file.
95 The VFS imposes limitations that an attribute names is limited to 255
96 bytes and an attribute value is limited to 64 kB. The list of attri‐
97 bute names that can be returned is also limited to 64 kB (see BUGS in
98 listxattr(2)).
99 Some filesystems, such as Reiserfs (and, historically, ext2 and ext3),
101 require the filesystem to be mounted with the user_xattr mount option
102 in order for user extended attributes to be used.
103 In the current ext2, ext3, and ext4 filesystem implementations, the to‐
105 tal bytes used by the names and values of all of a file's extended at‐
106 tributes must fit in a single filesystem block (1024, 2048 or 4096
107 bytes, depending on the block size specified when the filesystem was
108 created).
109 In the Btrfs, XFS, and Reiserfs filesystem implementations, there is no
111 practical limit on the number of extended attributes associated with a
112 file, and the algorithms used to store extended attribute information
113 on disk are scalable.
114 In the JFS, XFS, and Reiserfs filesystem implementations, the limit on
116 bytes used in an EA value is the ceiling imposed by the VFS.
117 In the Btrfs filesystem implementation, the total bytes used for the
119 name, value, and implementation overhead bytes is limited to the
120 filesystem nodesize value (16 kB by default).
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123 Extended attributes are not specified in POSIX.1, but some other sys‐
124 tems (e.g., the BSDs and Solaris) provide a similar feature.
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127 Since the filesystems on which extended attributes are stored might
128 also be used on architectures with a different byte order and machine
129 word size, care should be taken to store attribute values in an archi‐
130 tecture-independent format.
131 This page was formerly named attr(5).
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135 attr(1), getfattr(1), setfattr(1), getxattr(2), ioctl_iflags(2), listx‐
136 attr(2), removexattr(2), setxattr(2), acl(5), capabilities(7),
137 selinux(8)
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140 This page is part of release 5.12 of the Linux man-pages project. A
141 description of the project, information about reporting bugs, and the
142 latest version of this page, can be found at
143 https://www.kernel.org/doc/man-pages/.
144Linux 2020-06-09 XATTR(7)