xattr(7) Miscellaneous Information Manual xattr(7)
NAME
xattr - Extended attributes
DESCRIPTION
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 filesys-
tem—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 (getx-
attr(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 al-
ways specified in the fully qualified namespace.attribute form, for ex-
ample, user.mime_type, trusted.md5sum, system.posix_acl_access, or se-
curity.selinux.
The namespace mechanism is used to define different classes of extended
attributes. These different classes exist for several reasons; for ex-
ample, the permissions and capabilities required for manipulating ex-
tended 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 capabili-
ties (see capabilities(7)). Read and write access permissions to secu-
rity attributes depend on the policy implemented for each security at-
tribute 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 ob-
jects 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 pro-
cesses 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, charac-
ter set or encoding of a file. The access permissions for user at-
tributes are defined by the file permission bits: read permission is
required to retrieve the attribute value, and writer permission is re-
quired to change it.
The file permission bits of regular files and directories are inter-
preted 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 spe-
cial files and directories.
For this reason, user extended attributes are allowed only for regular
files and directories, and access to user extended attributes is re-
stricted 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-imposed limits on attribute names and values are 255 bytes and
64 kB, respectively. 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 to-
tal bytes used by the names and values of all of a file's extended at-
tributes 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).
STANDARDS
Extended attributes are not specified in POSIX.1, but some other sys-
tems (e.g., the BSDs and Solaris) provide a similar feature.
NOTES
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 archi-
tecture-independent format.
This page was formerly named attr(5).
SEE ALSO
attr(1), getfattr(1), setfattr(1), getxattr(2), ioctl_iflags(2), listx-
attr(2), removexattr(2), setxattr(2), acl(5), capabilities(7),
selinux(8)
Linux man-pages 6.03 2023-02-05 xattr(7)
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