man-pages/man7/xattr.7

.\" Extended attributes manual page
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.\" Copyright (C) 2000, 2002, 2007  Andreas Gruenbacher <agruen@suse.de>
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.TH XATTR 7 2020-06-09 "Linux" "Linux Programmer's Manual"
.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.
.PP
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\(emfor example, additional security features such as
Access Control Lists (ACLs) may be implemented using extended attributes.
.PP
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.
.PP
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.
.PP
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
.IR namespace.attribute
form, for example,
.IR user.mime_type ,
.IR trusted.md5sum ,
.IR system.posix_acl_access ,
or
.IR security.selinux .
.PP
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.
.PP
Currently, the
.IR security ,
.IR system ,
.IR trusted ,
and
.IR 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.
.PP
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.
.PP
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 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
.BR listxattr (2)).
.PP
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.
.PP
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).
.PP
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.
.PP
In the JFS, XFS, and Reiserfs filesystem implementations,
the limit on bytes used in an EA value is the ceiling imposed by the VFS.
.PP
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 CONFORMING TO
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.
.PP
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)