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-.\" Extended attributes manual page
-.\"
-.\" Copyright (C) 2000, 2002, 2007 Andreas Gruenbacher <agruen@suse.de>
-.\" Copyright (C) 2001, 2002, 2004, 2007 Silicon Graphics, Inc.
-.\" All rights reserved.
-.\"
-.\" SPDX-License-Identifier: GPL-2.0-or-later
-.\"
-.TH xattr 7 2023-10-31 "Linux man-pages 6.7"
-.SH NAME
-xattr \- Extended attributes
-.SH 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.
-.P
-Extended attributes are extensions to the normal attributes which are
-associated with all inodes in the system (i.e., the
-.BR stat (2)
-data).
-They are often used to provide additional functionality
-to a filesystem\[em]for example, additional security features such as
-Access Control Lists (ACLs) may be implemented using extended attributes.
-.P
-Users with search access to a file or directory may use
-.BR listxattr (2)
-to retrieve a list of attribute names defined for that file or directory.
-.P
-Extended attributes are accessed as atomic objects.
-Reading
-.RB ( getxattr (2))
-retrieves the whole value of an attribute and stores it in a buffer.
-Writing
-.RB ( setxattr (2))
-replaces any previous value with the new value.
-.P
-Space consumed for extended attributes may be counted towards the disk quotas
-of the file owner and file group.
-.SS Extended attribute namespaces
-Attribute names are null-terminated strings.
-The attribute name is always specified in the fully qualified
-.I namespace.attribute
-form, for example,
-.IR user.mime_type ,
-.IR trusted.md5sum ,
-.IR system.posix_acl_access ,
-or
-.IR security.selinux .
-.P
-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.
-.P
-Currently, the
-.IR security ,
-.IR system ,
-.IR trusted ,
-and
-.I user
-extended attribute classes are defined as described below.
-Additional classes may be added in the future.
-.SS 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
-.BR 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
-.B CAP_SYS_ADMIN
-capability.
-.SS 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.
-.SS Trusted extended attributes
-Trusted extended attributes are visible and accessible only to processes that
-have the
-.B 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.
-.SS 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.
-.P
-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.
-.P
-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
-.BR chmod (1)
-manual page for an explanation of the sticky bit).
-.SS 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
-.BR listxattr (2)).
-.P
-Some filesystems, such as Reiserfs (and, historically, ext2 and ext3),
-require the filesystem to be mounted with the
-.B user_xattr
-mount option in order for user extended attributes to be used.
-.P
-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).
-.P
-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.
-.P
-In the JFS, XFS, and Reiserfs filesystem implementations,
-the limit on bytes used in an EA value is the ceiling imposed by the VFS.
-.P
-In the Btrfs filesystem implementation,
-the total bytes used for the name, value, and implementation overhead bytes
-is limited to the filesystem
-.I nodesize
-value (16\ kB by default).
-.SH STANDARDS
-Extended attributes are not specified in POSIX.1, but some other systems
-(e.g., the BSDs and Solaris) provide a similar feature.
-.SH 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
-architecture-independent format.
-.P
-This page was formerly named
-.BR attr (5).
-.\" .SH AUTHORS
-.\" Andreas Gruenbacher,
-.\" .RI < a.gruenbacher@bestbits.at >
-.\" and the SGI XFS development team,
-.\" .RI < linux-xfs@oss.sgi.com >.
-.SH SEE ALSO
-.BR attr (1),
-.BR getfattr (1),
-.BR setfattr (1),
-.BR getxattr (2),
-.BR ioctl_iflags (2),
-.BR listxattr (2),
-.BR removexattr (2),
-.BR setxattr (2),
-.BR acl (5),
-.BR capabilities (7),
-.BR selinux (8)