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XATTR(7) Linux Programmer's Manual XATTR(7)
xattr - Extended attributes
Extended attributes are name:value pairs associated permanently with
files and directories, similar to the environment strings associated
with a process. An attribute may be defined or undefined. If it is
defined, its value may be empty or non-empty.
Extended attributes are extensions to the normal attributes which are
associated with all inodes in the system (i.e., the stat(2) data).
They are often used to provide additional functionality to a
filesystem—for example, additional security features such as Access
Control Lists (ACLs) may be implemented using extended attributes.
Users with search access to a file or directory may use listxattr(2)
to retrieve a list of attribute names defined for that file or
directory.
Extended attributes are accessed as atomic objects. Reading
(getxattr(2)) retrieves the whole value of an attribute and stores it
in a buffer. Writing (setxattr(2)) replaces any previous value with
the new value.
Space consumed for extended attributes may be counted towards the
disk quotas of the file owner and file group.
Extended attribute namespaces
Attribute names are null-terminated strings. The attribute name is
always specified in the fully qualified namespace.attribute form, for
example, user.mime_type, trusted.md5sum, system.posix_acl_access, or
security.selinux.
The namespace mechanism is used to define different classes of
extended attributes. These different classes exist for several
reasons; for example, the permissions and capabilities required for
manipulating extended attributes of one namespace may differ to
another.
Currently, the security, system, trusted, and user extended attribute
classes are defined as described below. Additional classes may be
added in the future.
Extended security attributes
The security attribute namespace is used by kernel security modules,
such as Security Enhanced Linux, and also to implement file
capabilities (see capabilities(7)). Read and write access
permissions to security attributes depend on the policy implemented
for each security attribute by the security module. When no security
module is loaded, all processes have read access to extended security
attributes, and write access is limited to processes that have the
CAP_SYS_ADMIN capability.
System extended attributes
System extended attributes are used by the kernel to store system
objects such as Access Control Lists. Read and write access
permissions to system attributes depend on the policy implemented for
each system attribute implemented by filesystems in the kernel.
Trusted extended attributes
Trusted extended attributes are visible and accessible only to
processes that have the CAP_SYS_ADMIN capability. Attributes in this
class are used to implement mechanisms in user space (i.e., outside
the kernel) which keep information in extended attributes to which
ordinary processes should not have access.
User extended attributes
User extended attributes may be assigned to files and directories for
storing arbitrary additional information such as the mime type,
character set or encoding of a file. The access permissions for user
attributes are defined by the file permission bits: read permission
is required to retrieve the attribute value, and writer permission is
required to change it.
The file permission bits of regular files and directories are
interpreted differently from the file permission bits of special
files and symbolic links. For regular files and directories the file
permission bits define access to the file's contents, while for
device special files they define access to the device described by
the special file. The file permissions of symbolic links are not
used in access checks. These differences would allow users to
consume filesystem resources in a way not controllable by disk quotas
for group or world writable special files and directories.
For this reason, user extended attributes are allowed only for
regular files and directories, and access to user extended attributes
is restricted to the owner and to users with appropriate capabilities
for directories with the sticky bit set (see the chmod(1) manual page
for an explanation of the sticky bit).
Filesystem differences
The kernel and the filesystem may place limits on the maximum number
and size of extended attributes that can be associated with a file.
The VFS imposes limitations that an attribute names is limited to 255
bytes and an attribute value is limited to 64 kB. The list of
attribute names that can be returned is also limited to 64 kB (see
BUGS in listxattr(2)).
Some filesystems, such as Reiserfs (and, historically, ext2 and
ext3), require the filesystem to be mounted with the user_xattr mount
option in order for user extended attributes to be used.
In the current ext2, ext3, and ext4 filesystem implementations, the
total bytes used by the names and values of all of a file's extended
attributes must fit in a single filesystem block (1024, 2048 or 4096
bytes, depending on the block size specified when the filesystem was
created).
In the Btrfs, XFS, and Reiserfs filesystem implementations, there is
no practical limit on the number of extended attributes associated
with a file, and the algorithms used to store extended attribute
information on disk are scalable.
In the JFS, XFS, and Reiserfs filesystem implementations, the limit
on bytes used in an EA value is the ceiling imposed by the VFS.
In the Btrfs filesystem implementation, the total bytes used for the
name, value, and implementation overhead bytes is limited to the
filesystem nodesize value (16 kB by default).
Extended attributes are not specified in POSIX.1, but some other
systems (e.g., the BSDs and Solaris) provide a similar feature.
Since the filesystems on which extended attributes are stored might
also be used on architectures with a different byte order and machine
word size, care should be taken to store attribute values in an
architecture-independent format.
This page was formerly named attr(5).
attr(1), getfattr(1), setfattr(1), getxattr(2), ioctl_iflags(2),
listxattr(2), removexattr(2), setxattr(2), acl(5), capabilities(7),
selinux(8)
This page is part of release 5.07 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-06-09 XATTR(7)
Pages that refer to this page: attr(1), chacl(1), getfattr(1), setfattr(1), attr(5), ext2(5), ext3(5), ext4(5), tmpfiles.d(5), xattr(7)
Copyright and license for this manual page
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