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Versions: (draft-naik-nfsv4-xattrs) 00 01 02
03 04 05 06 07 RFC 8276
NFSv4 Working Group M. Naik
Internet Draft Nutanix
Intended Status: Standards Track M. Eshel
Expires: September 8, 2016 IBM Almaden
March 7, 2016
File System Extended Attributes in NFSv4
draft-ietf-nfsv4-xattrs-02
Abstract
This document proposes extensions to the NFSv4 protocol which allow
file extended attributes (hereinafter also referred to as xattrs) to
be manipulated using NFSv4. An xattr is a file system feature that
allows opaque metadata, not interpreted by the file system, to be
associated with files and directories. Such support is present in
many modern local file systems. New file attributes are proposed to
allow clients to query the server for xattr support, and new
operations to get and set xattrs on file system objects are provided.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Copyright and License Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. File System Support . . . . . . . . . . . . . . . . . . . . . 6
4. Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Differences from Named Attributes . . . . . . . . . . . . . . 7
6. XDR Description . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Code Components Licensing Notice . . . . . . . . . . . . . 8
7. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 10
7.1. New definitions . . . . . . . . . . . . . . . . . . . . . 10
7.2. New Attribute . . . . . . . . . . . . . . . . . . . . . . 10
7.2.1. xattr_support . . . . . . . . . . . . . . . . . . . . 11
7.3. New Error Definitions . . . . . . . . . . . . . . . . . . 11
7.3.1. NFS4ERR_NOXATTR (Error Code 10095) . . . . . . . . . . 11
7.3.2. NFS4ERR_XATTR2BIG (Error Code 10096) . . . . . . . . . 11
7.4. New Operations . . . . . . . . . . . . . . . . . . . . . . 11
7.4.1. GETXATTR - Get an extended attribute of a file . . . . 12
7.4.2. SETXATTR - Set an extended attribute of a file . . . . 14
7.4.3. LISTXATTRS - List extended attributes of a file . . . 15
7.4.4. REMOVEXATTR - Remove an extended attribute of a file . 17
7.4.5. Valid Errors . . . . . . . . . . . . . . . . . . . . . 18
7.5. Modifications to Existing Operations . . . . . . . . . . . 19
7.6. Numeric Values Assigned to Protocol Extensions . . . . . . 21
7.7. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.8. Xattrs and File Locking . . . . . . . . . . . . . . . . . 24
7.9. pNFS Considerations . . . . . . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
Extended attributes, also called xattrs, are a means to associate
opaque metadata with file system objects, typically organized in
key/value pairs. They are especially useful when they add
information that is not, or cannot be, present in the associated
object itself. User-space applications can arbitrarily create,
interrogate, and modify the key/value pairs.
Extended attributes are file system-agnostic; applications use an
interface not specific to any file system to manipulate them.
Applications do not need to be concerned about how the key/value
pairs are stored internally within the underlying file system. All
major operating systems provide various flavors of extended
attributes. Many user space tools allow xattrs to be included in
regular attributes that need to be preserved when objects are
updated, moved or copied.
Extended attributes have previously been considered unsuitable for
portable use because some aspects of their handling are not precisely
defined and they are not formally documented by any standard (such as
POSIX). Nevertheless, it appears that xattrs are widely deployed and
their support in modern disk-based file systems is nearly universal.
There is no clear specification of how xattrs could be mapped to any
existing file attributes defined in the NFSv4 protocol ([RFC7530],
[RFC5661], [NFSv42]). As a result, most NFSv4 client implementations
ignore application-specified xattrs. This state of affairs results
in data loss if one copies, over the NFS protocol, a file with xattrs
from one file system to another that also supports xattrs.
There is thus a need to provide a means by which such data loss can
be avoided. This will involve exposing xattrs within the NFSv4
protocol, despite the lack of completely compatible file system
implementations.
This document discusses (in Section 5) the reasons that NFSv4 named
attributes as currently standardized in [RFC7530], are unsuitable for
representing xattrs. Instead, it proposes a separate protocol
mechanism to support xattrs. As a consequence, xattrs and named
attributes will both be optional features with servers free to
support either, both, or neither.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
2. Use Cases
Applications can store tracking information in extended attributes.
Examples include storing metadata identifying the application that
created the file, a tag to indicate when the file was last verified
by a data integrity scrubber, or a tag to hold a checksum/crypto hash
of the file contents along with the date of that signature. Xattrs
can also be used for decorations or annotations. For example, a file
downloaded from a web server can be tagged with the URL, which can be
convenient if its source has to be determined in the future.
Likewise, an email attachment, when saved, can be tagged with the
message-id of the email, making it possible to trace the original
message.
Applications may need to behave differently when handling files of
varying types. For example, file managers, such as GNOME's, offer
unique icons, different click behavior, and special lists of
operations to perform depending on the file format. This can be
achieved by looking at the file extension (Windows), or interpret the
type by inspecting it (Unix MIME type). Some file managers generate
this information on the fly; others generate the information once and
then cache it. Those that cache the information tend to put it in a
custom database. The file manager must work to keep this database in
sync with the files, which can change without the file manager's
knowledge. A better approach is to dispense with the custom database
and store such metadata in extended attributes. This is easier to
maintain, provides faster access, and is readily accessible by
applications [Love].
Swift, the OpenStack distributed object store, uses xattrs to store
an object's metadata along with all the data together in one file.
Swift-on-File [Swift] transfers the responsibility of maintaining
object durability and availability to the underlying file system.
Today, this requires a native file system client to mount the
volumes. Xattr support in NFS would open up the possibility of
storing and consuming data from other storage systems, and facilitate
the migration of data between different backend storage systems.
Baloo, the file indexing and search framework for KDE, has moved to
storing metadata such as tags, ratings and comments, in file system
xattrs instead of a custom database for simplicity. Starting with
KDE Plasma 5.1, NFS is no longer supported due to its lack of xattr
support [KDE].
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3. File System Support
Extended attributes are supported by most modern file systems.
In Linux, ext3, ext4, JFS, XFS, Btrfs, among other file systems,
support extended attributes. The getfattr and setfattr utilities can
be used to retrieve and set xattrs. The names of the extended
attributes must be prefixed by the name of the category and a dot;
hence these categories are generally qualified as name spaces.
Currently, four namespaces exist: user, trusted, security and system
[Linux]. Recommendations on how they should be used have been
published [freedesktop].
FreeBSD supports extended attributes in two universal namespaces -
user and system, although individual file systems are allowed to
implement additional namespaces [FreeBSD].
Solaris 9 and later allows files to have extended attributes, but
implements them as "forks", logically represented as files within a
hidden directory that is associated with the target file [fsattr].
In the NTFS file system, extended attributes are one of several
supported "file streams" [NTFS].
Xattrs can be retrieved and set through system calls or shell
commands and are generally supported by user-space tools that
preserve other file attributes. For example, the "rsync" remote copy
program will correctly preserve user extended attributes between
Linux/ext4 and OSX/hfs by stripping off the Linux-specific "user."
prefix.
4. Namespaces
Operating systems may define multiple "namespaces" in which xattrs
can be set. Namespaces are more than organizational classes; the
operating system may enforce different access policies and allow
different capabilities depending on the namespace. Linux, for
example, defines "security", "system", "trusted" and "user"
namespaces, the first three being specific to Linux [freedesktop].
Implementations generally agree on the semantics of a "user"
namespace, that allows applications to store arbitrary user attribute
data with file system objects. Access to this namespace is
controlled via the normal file system attributes. As such, getting
and setting xattrs from the user namespace can be considered
interoperable across platforms and vendor implementations.
Attributes from other namespaces are typically platform-specific.
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This document provides for namespaces supporting user-managed
metadata only, thus avoiding the need to specify the semantics
applicable to particular system-interpreted xattrs. The values of
xattrs are considered application data just as the contents of named
attributes, files, and symbolic links are. Servers have a
responsibility to store whatever value the client specifies and to
return it on demand. xattr keys and values MUST NOT be interpreted by
the NFS clients and servers, as such behavior would lead to non-
interoperable implementations. If there is a need to specify
attributes that servers need to be act upon, the appropriate
semantics need to be specified by adding a new attribute for the
purpose as provided by [RFC7530] and [NFSv4-vers].
5. Differences from Named Attributes
[RFC7530] defines named attributes as opaque byte streams that are
associated with a directory or file and referred to by a string name.
Named attributes are intended to be used by client applications as a
method to associate application-specific data with a regular file or
directory. In that sense, xattrs are similar in concept and use to
named attributes, but there are subtle differences.
File systems typically define operations to get and set individual
xatrrs as being atomic, although collectively they may be
independent. Xattrs generally have size limits ranging from a few
bytes to several kilobytes; the maximum supported size is not
universally defined and is usually restricted by the file system.
Similar to ACLs, the amount of xattr data exchanged between the
client and server for get/set operations can be considered to fit in
a single COMPOUND request, bounded by the channel's negotiated
maximum size for requests. Named attributes, on the other hand, are
unbounded data streams and do not impose atomicity requirements.
Individual named attributes are analogous to files, and caching of
the data for these needs to be handled just as data caching is for
ordinary files following close-to-open semantics. Xattrs, on the
other hand, impose caching requirements like other file attributes.
Named attributes and xattrs have different semantics and belong to
disjoint namespaces. As a result, mapping one to another is, at
best, a compromise.
While it should be possible to write guidance about how a client can
use the named attribute mechanism to act like xattrs, such as carving
out some namespace and specifying locking primitives to enforce
atomicity constraints on individual get/set operations, this is
problematic. A client application trying to use xattrs through named
attributes with a server that supported xattrs directly would get a
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lower level of service, and could fail to cooperate on a local
application running on the server unless the server file system
defined its own interoperability constraints. File systems that
already implement xattrs and named attributes natively would need
additional guidance such as reserving named attribute namespace
specifically for implementation purposes.
6. XDR Description
This document contains the external data representation (XDR)
[RFC4506] description of the extended attributes. The XDR
description is embedded in this document in a way that makes it
simple for the reader to extract into a ready-to-compile form. The
reader can feed this document into the following shell script to
produce the machine readable XDR description of extended attributes:
<CODE BEGINS>
#! /bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'
<CODE ENDS>
That is, if the above script is stored in a file called "extract.sh",
and this document is in a file called "spec.txt", then the reader can
do:
sh extract.sh < spec.txt > xattr_prot.x
The effect of the script is to remove leading white space from each
line, plus a sentinel sequence of "///".
The embedded XDR file header follows. Subsequent XDR descriptions,
with the sentinel sequence are embedded throughout the document.
Note that the XDR code contained in this document depends on types
from the proposed NFSv4.2 nfs4_prot.x file [NFSv42-dot-x]. This
includes both nfs types that end with a 4, such as verifier4, count4,
etc., as well as more generic types such as opaque and bool.
6.1. Code Components Licensing Notice
Both the XDR description and the scripts used for extracting the XDR
description are Code Components as described in Section 4 of "Legal
Provisions Relating to IETF Documents" [LEGAL]. These Code
Components are licensed according to the terms of that document.
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<CODE BEGINS>
/// /*
/// * Copyright (c) 2012 IETF Trust and the persons identified
/// * as authors of the code. All rights reserved.
/// *
/// * Redistribution and use in source and binary forms, with
/// * or without modification, are permitted provided that the
/// * following conditions are met:
/// *
/// * o Redistributions of source code must retain the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer.
/// *
/// * o Redistributions in binary form must reproduce the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer in the documentation and/or other
/// * materials provided with the distribution.
/// *
/// * o Neither the name of Internet Society, IETF or IETF
/// * Trust, nor the names of specific contributors, may be
/// * used to endorse or promote products derived from this
/// * software without specific prior written permission.
/// *
/// * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
/// * AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
/// * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
/// * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
/// * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
/// * EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
/// * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
/// * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
/// * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
/// * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
/// * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
/// * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
/// * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
/// * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
/// * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
/// *
/// * This code was derived from RFCTBD10.
/// * Please reproduce this note if possible.
/// */
/// /*
/// * xattr_prot.x
/// */
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/// /*
/// * The following include statements are for example only.
/// * The actual XDR definition files are generated separately
/// * and independently and are likely to have a different name.
/// * %#include <nfsv42.x>
/// * %#include <rpc_prot.x>
/// */
<CODE ENDS>
7. Protocol Extensions
This section documents extensions to the NFSv4 protocol operations to
allow xattrs to be queried and modified by clients. A new attribute
is added to allow clients to check if the server supports xattrs.
New operations are defined to allow xattr keys and values to be
queried and set. In addition, new bitmask constants are added to the
ACE access mask field to validate permissions to query and modify
xattrs.
These changes follow applicable guidelines for valid NFSv4 protocol
extension, whether the extensions occur in a minor version (as
specified in [RFC5661]) or as an extension to an existing minor
version (as specified in [NFSv4-vers]).
7.1. New definitions
<CODE BEGINS>
/// typedef component4 xattrkey4;
/// typedef opaque xattrvalue4<>;
<CODE ENDS>
Each xattr is a key/value pair. xattrkey4 is a string denoting the
xattr key name, and an attrvalue4 which is a variable-length string
that identifies the value of the xattr. The handling of xattrkey4
with regard to internationalization-related issues is the same as
that for NFSv4 file names and named attribute names, as described in
[RFC7530]. Any regular file or directory may have a set of extended
attributes, each consisting of a key and associated value. The NFS
client or server MUST NOT interpret the contents of xattrkey4 or
xattrvalue4.
7.2. New Attribute
The following RECOMMENDED per-fs read-only attribute is proposed for
use. A client can query the server to determine if xattrs are
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supported by setting the xattr_support bit in the GETATTR request.
7.2.1. xattr_support
True, if the object's file system supports extended attributes.
Since xattr_support is not a REQUIRED attribute, server need not
support it. However, a client may reasonably assume that a server
(or file system) that does not support the xattr_support attribute
does not provide xattr support and act on that basis.
Note that the protocol does not enforce any limits on the number of
keys, the length of a key or the size of a value, or the total size
of xattrs that are allowed for a file. The server file system MAY
impose additional limits. In addition, a single xattr key or value
exchanged between the client and server for get/set operations is
limited by the channel's negotiated maximum size for requests and
responses.
7.3. New Error Definitions
<CODE BEGINS>
/// /* Following lines are to be added to enum nfsstat4 */
/// /*
/// NFS4ERR_NOXATTR = 10095 /* xattr does not exist */
/// NFS4ERR_XATTR2BIG = 10096 /* xattr value is too big */
/// */
<CODE ENDS>
7.3.1. NFS4ERR_NOXATTR (Error Code 10095)
The specified xattr does not exist or the server is unable to
retrieve it.
7.3.2. NFS4ERR_XATTR2BIG (Error Code 10096)
The size of the xattr value as part of a SETXATTR operation is bigger
than that supported by the underlying file system.
7.4. New Operations
Individual xattrs generally represent separate items of metadata.
For various reasons, combining them into a single attribute results
in clumsy implementations with significant functional deficits. In
consequence, adding a new attribute to represent the set of xattrs
for an object is not an appropriate way to provide support for
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xattrs.
For example, obtaining the value of a single xattr using the bitmap
would require a client implementation to read all the xattrs of the
file and find a match for the one requested. Similarly, replacing or
deleting a single xattr while keeping the others intact would require
a client to read the xattrs first, replacing the existing list with a
modified list that excludes the one to be deleted, and writing out
the remaining xattrs. Such a read-modify-write cycle is subject to
updates being lost in the case of simultaneous updates by multiple
clients. In addition, two clients might simultaneously add the same
xattr key to the same file with each concluding that it did the
initial creation for the common xattr key, when the semantic model
implies that only one could have done so.
Applications need to perform the following operations on a given
file's extended attributes [Love]:
o Given a file, return a list of all of the file's assigned extended
attribute keys.
o Given a file and a key, return the corresponding value.
o Given a file, a key, and a value, assign that value to the key.
o Given a file and a key, remove that extended attribute from the
file.
This section introduces four new RECOMMENDED operations, GETXATTR,
SETXATTR, LISTXATTRS and REMOVEXATTR, to query, set, list and remove
xattrs respectively. A server MUST support all four operations if it
supports the xattr_support attribute. GETXATTR allows obtaining the
value of an xattr key, SETXATTR allows creating or replacing an xattr
key with a value, LISTXATTRS enumerates all the xattrs names, and
REMOVEXATTR allows deleting a single xattr.
7.4.1. GETXATTR - Get an extended attribute of a file
7.4.1.1. ARGUMENTS
<CODE BEGINS>
/// struct GETXATTR4args {
/// /* CURRENT_FH: file */
/// xattrkey4 gxa_name;
/// };
<CODE ENDS>
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7.4.1.2. RESULTS
<CODE BEGINS>
/// union GETXATTR4res switch (nfsstat4 gxr_status) {
/// case NFS4_OK:
/// xattrvalue4 gxr_value;
/// default:
/// void;
/// };
<CODE ENDS>
7.4.1.3. DESCRIPTION
The GETXATTR operation will obtain the value for the given extended
attribute key for the file system object specified by the current
filehandle.
The server will fetch the xattr value for the key that the client
requests if xattrs are supported by the server for the target file
system. If the server does not support xattrs on the target file
system, then it MUST NOT return a value and MUST return the
NFS4ERR_NOTSUPP error. The server also MUST return NFS4ERR_NOXATTR
if it supports xattrs on the target but cannot obtain the requested
data. If the xattr value contained in the server response is such as
to cause the channel's negotiated maximum response size to be
exceeded, then the server MUST return NFS4ERR_REP_TOO_BIG in
gxr_status.
7.4.1.4. IMPLEMENTATION
Clients that have cached an xattr may avoid the need to do a GETXATTR
by determining if the change attribute is the same as it was when the
xattr was fetched. If the client does not hold a delegation for the
file in question, it can do so with a GETATTR request to obtain the
change attribute and comparing its value to the change attribute
value fetched when the xattr value was obtained. This handling is
similar to how a client would revalidate other file attributes such
as ACLs.
When responding to such a GETATTR, the server will, if there is an
OPEN_DELEGATE_WRITE delegation held by another client for the file in
question, either obtain the actual current value of these attributes
from the client holding the delegation by using the CB_GETATTR
callback, or revoke the delegation. See Section 18.7.4 of [RFC5661]
for details.
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7.4.2. SETXATTR - Set an extended attribute of a file
7.4.2.1. ARGUMENTS
<CODE BEGINS>
/// enum setxattr_option4 {
/// SETXATTR4_NONE = 0,
/// SETXATTR4_CREATE = 1,
/// SETXATTR4_REPLACE = 2,
/// };
/// struct SETXATTR4args {
/// /* CURRENT_FH: file */
/// setxattr_option4 sxa_option;
/// xattrkey4 sxa_key;
/// xattrvalue4 sxa_value;
/// };
<CODE ENDS>
7.4.2.2. RESULTS
<CODE BEGINS>
/// struct SETXATTR4res switch (nfsstat4 sxr_status) {
/// case NFS4_OK:
/// change_info4 sxr_info;
/// default:
/// void;
/// };
<CODE ENDS>
7.4.2.3. DESCRIPTION
The SETXATTR operation changes one extended attribute of a file
system object. The change desired is specified by sxa_option.
SETXATTR4_CREATE is used to associate the given value with the given
extended attribute key for the file system object specified by the
current filehandle. The server MUST return NFS4ERR_EXIST if the
attribute key already exists. SETXATTR4_REPLACE is also used to set
an xattr, but the server MUST return NFS4ERR_NOXATTR if the attribute
key does not exist. By default (SETXATTR4_NONE), the extended
attribute will be created if need be, or its value will be replaced
if the attribute exists.
If the xattr key and value contained in the client request are such
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that the request would exceed the channel's negotiated maximum
request size, then the server MUST return NFS4ERR_REQ_TOO_BIG in
sxr_status. If the server file system imposes additional limits on
the size of key name or value, it MAY return NFS4ERR_XATTR2BIG.
A successful SETXATTR MUST change the file time_metadata and change
attributes if the xattr is created or the value assigned to xattr
changes. However, these attributes SHOULD NOT be changed if this
causes no actual change in the xattr value.
On success, the server returns the change_info4 information in
sxr_info. With the atomic field of the change_info4 data type, the
server will indicate if the before and after change attributes were
obtained atomically with respect to the SETXATTR operation. This
allows the client to determine if its cached xattrs are still valid
after the operation. See Section 7.6 for a discussion on xattr
caching.
7.4.2.4. IMPLEMENTATION
If the object whose xattr is being changed has a file delegation that
is held by a client other than the one doing the SETXATTR, the
delegation(s) must be recalled, and the operation cannot proceed to
actually change the xattr until each such delegation is returned or
revoked. In all cases in which delegations are recalled, the server
is likely to return one or more NFS4ERR_DELAY errors while the
delegation(s) remains outstanding, although it might not do that if
the delegations are returned quickly.
7.4.3. LISTXATTRS - List extended attributes of a file
7.4.3.1. ARGUMENTS
<CODE BEGINS>
/// struct LISTXATTRS4args {
/// /* CURRENT_FH: file */
/// nfs_cookie4 lxa_cookie;
/// verifier4 lxa_cookieverf;
/// count4 lxa_maxcount;
/// };
<CODE ENDS>
7.4.3.2. RESULTS
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<CODE BEGINS>
/// struct LISTXATTRS4resok {
/// nfs_cookie4 lxr_cookie;
/// verifier4 lxr_cookieverf;
/// xattrkey4 lxr_names<>;
/// bool lxr_eof;
/// };
/// union LISTXATTRS4res switch (nfsstat4 lxr_status) {
/// case NFS4_OK:
/// LISTXATTRS4resok lxr_value;
/// default:
/// void;
/// };
<CODE ENDS>
7.4.3.3. DESCRIPTION
The LISTXATTRS operation retrieves a variable number of extended
attribute keys from the file system object specified by the current
filehandle, along with information to allow the client to request
additional attribute keys in a subsequent LISTXATTRS.
The arguments contain a cookie value that represents where the
LISTXATTRS should start within the list of xattrs. A value of 0
(zero) for lxa_cookie is used to start reading at the beginning of
the list. For subsequent LISTXATTRS requests, the client specifies a
cookie value that is provided by the server on a previous LISTXATTRS
request.
The lxa_cookieverf value should be set to 0 (zero) when the
lxa_cookie value is 0 (zero) (first xattr read). On subsequent
requests, it should be lxr_cookieverf as returned by the server. The
lxa_cookieverf must match that returned by the LISTXATTRS in which
the cookie was acquired. If the server determines that the
lxa_cookieverf is no longer valid for the list, the error
NFS4ERR_NOT_SAME must be returned.
The lxa_maxcount value of the argument is the maximum number of bytes
for the result. This maximum size represents all of the data being
returned within the LISTXATTRS4resok structure and includes the XDR
overhead. The server may return less data. If the server is unable
to return a single xattr name within the maxcount limit, the error
NFS4ERR_TOOSMALL will be returned to the client.
On successful return, the server's response will provide a list of
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extended attribute keys. The "lxr_eof" flag has a value of TRUE if
there are no more keys for the object.
The cookie value is only meaningful to the server and is used as a
"bookmark" for the xattr key. As mentioned, this cookie is used by
the client for subsequent LISTXATTRS operations so that it may
continue listing keys. The cookie is similar in concept to a READDIR
cookie or the READ offset but should not be interpreted as such by
the client.
On success, the current filehandle retains its value.
7.4.3.4. IMPLEMENTATION
The handling of cookie/verifier is similar to that of the READDIR
operation. The verifier may be used by the server to help manage
cookie values that may become stale. It should be a rare occurrence
that a server is unable to continue properly listing xattrs with the
provided cookie/verifier pair. The server should make every effort
to avoid this condition since the application at the client may not
be able to properly handle this type of failure.
7.4.4. REMOVEXATTR - Remove an extended attribute of a file
7.4.4.1. ARGUMENTS
<CODE BEGINS>
/// struct REMOVEXATTR4args {
/// /* CURRENT_FH: file */
/// xattrkey4 rxa_name;
/// };
<CODE ENDS>
7.4.4.2. RESULTS
<CODE BEGINS>
/// struct REMOVEXATTR4res switch (nfsstat4 rxr_status) {
/// case NFS4_OK:
/// change_info4 rxr_info;
/// default:
/// void;
/// };
<CODE ENDS>
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7.4.4.3. DESCRIPTION
The REMOVEXATTR operation deletes one extended attribute of a file
system object specified by rxa_name. The server MUST return
NFS4ERR_NOXATTR if the attribute key does not exist.
A successful REMOVEXATTR SHOULD change the file time_metadata and
change attributes.
Similar to SETXATTR, the server communicates the value of the change
attribute immediately prior to, and immediately following, a
successful REMOVEXATTR operation in rxr_info. This allows the client
to determine if its cached xattrs are still valid after the
operation. See Section 7.6 for a discussion on xattr caching.
7.4.4.4. IMPLEMENTATION
If the object whose xattr is being removed has a file delegation that
is held by a client other than the one doing the REMOVEXATTR, the
delegation(s) must be recalled, and the operation cannot proceed to
delete the xattr until each such delegation is returned or revoked.
In all cases in which delegations are recalled, the server is likely
to return one or more NFS4ERR_DELAY errors while the delegation(s)
remains outstanding, although it might not do that if the delegations
are returned quickly.
7.4.5. Valid Errors
This section contains a table that gives the valid error returns for
each new protocol operation. The error code NFS4_OK (indicating no
error) is not listed but should be understood to be returnable by all
new operations. The error values for all other operations are
defined in Section 13.2 of [RFC7530].
Valid Error Returns for Each New Protocol Operation
+----------------------+--------------------------------------------+
| Operation | Errors |
+----------------------+--------------------------------------------+
| GETXATTR | NFS4ERR_ACCESS, NFS4ERR_BADXDR, |
| | NFS4ERR_DEADSESSION, NFS4ERR_DELAY, |
| | NFS4ERR_FHEXPIRED, NFS4ERR_INVAL, |
| | NFS4ERR_IO, NFS4ERR_MOVED, |
| | NFS4ERR_NAMETOOLONG, NFS4ERR_NOFILEHANDLE, |
| | NFS4ERR_NOTSUPP, NFS4ERR_OP_NOT_IN_SESSION,|
| | NFS4ERR_PERM, NFS4ERR_REP_TOO_BIG, |
| | NFS4ERR_REP_TOO_BIG_TO_CACHE, |
| | NFS4ERR_REQ_TOO_BIG, |
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| | NFS4ERR_RETRY_UNCACHED_REP, |
| | NFS4ERR_SERVERFAULT, NFS4ERR_STALE, |
| | NFS4ERR_TOO_MANY_OPS, NFS4ERR_WRONG_TYPE |
| SETXATTR | NFS4ERR_ACCESS, NFS4ERR_BADCHAR, |
| | NFS4ERR_BADXDR, NFS4ERR_DEADSESSION, |
| | NFS4ERR_DELAY, NFS4ERR_DQUOT, |
| | NFS4ERR_EXIST, NFS4ERR_FHEXPIRED, |
| | NFS4ERR_INVAL, NFS4ERR_IO, NFS4ERR_MOVED, |
| | NFS4ERR_NAMETOOLONG, NFS4ERR_NOFILEHANDLE, |
| | NFS4ERR_NOSPC, NFS4ERR_OP_NOT_IN_SESSION, |
| | NFS4ERR_PERM, NFS4ERR_REP_TOO_BIG, |
| | NFS4ERR_REP_TOO_BIG_TO_CACHE, |
| | NFS4ERR_REQ_TOO_BIG, |
| | NFS4ERR_RETRY_UNCACHED_REP, NFS4ERR_ROFS, |
| | NFS4ERR_SERVERFAULT, NFS4ERR_STALE, |
| | NFS4ERR_TOO_MANY_OPS, NFS4ERR_WRONG_TYPE |
| LISTXATTRS | NFS4ERR_ACCESS, NFS4ERR_DEADSESSION, |
| | NFS4ERR_DELAY, NFS4ERR_INVAL, NFS4ERR_IO, |
| | NFS4ERR_MOVED, NFS4ERR_NAMETOOLONG, |
| | NFS4ERR_NOFILEHANDLE, NFS4ERR_NOTSUPP, |
| | NFS4ERR_OP_NOT_IN_SESSION, |
| | NFS4ERR_PERM, NFS4ERR_REP_TOO_BIG, |
| | NFS4ERR_REP_TOO_BIG_TO_CACHE, |
| | NFS4ERR_REQ_TOO_BIG, |
| | NFS4ERR_RETRY_UNCACHED_REP, |
| | NFS4ERR_SERVERFAULT, NFS4ERR_STALE, |
| | NFS4ERR_TOO_MANY_OPS, NFS4ERR_WRONG_TYPE |
| REMOVEXATTR | NFS4ERR_ACCESS, NFS4ERR_BADCHAR, |
| | NFS4ERR_BADXDR, NFS4ERR_DEADSESSION, |
| | NFS4ERR_DELAY, NFS4ERR_DQUOT, |
| | NFS4ERR_EXIST, NFS4ERR_INVAL, NFS4ERR_IO, |
| | NFS4ERR_LOCKED, NFS4ERR_MOVED, |
| | NFS4ERR_NAMETOOLONG, NFS4ERR_NOFILEHANDLE, |
| | NFS4ERR_NOSPC, NFS4ERR_OLD_STATEID, |
| | NFS4ERR_OPENMODE, |
| | NFS4ERR_OP_NOT_IN_SESSION, NFS4ERR_PERM, |
| | NFS4ERR_RETRY_UNCACHED_REP, NFS4ERR_ROFS, |
| | NFS4ERR_SERVERFAULT, NFS4ERR_STALE, |
| | NFS4ERR_TOO_MANY_OPS, NFS4ERR_WRONG_TYPE |
+----------------------+--------------------------------------------+
7.5. Modifications to Existing Operations
In order to provide fine-grained access control to query or modify
extended attributes, additions are proposed to the set of access
rights that can be checked to determine if the client is permitted to
perform the xattr operation.
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Note that in general, as explained in Section 18.1.4 of [RFC5661], a
client cannot reliably perform an access check with only current file
attributes and must verify access with the server.
This section extends the semantics of the ACCESS operation documented
in Section 18.1 of [RFC5661]. Three new access permissions can be
requested:
ACCESS4_XAREAD Query a file or directory for its xattr value
given a key.
ACCESS4_XAWRITE Modify xattr keys and/or values of a file or
directory.
ACCESS4_XALIST Query a file or directory to list its xattr keys.
As with the existing access permissions, the results of ACCESS are
advisory in nature, with no implication that such access will be
allowed or denied in the future.
In addition, two new bitmask constants used for the access mask field
are added:
ACE4_READ_XATTRS Permission to interrogate the extended attributes
of a file with GETXATTR.
ACE4_WRITE_XATTRS Permission to change the extended attributes of a
file with SETXATTR or REMOVEXATTR.
ACE4_LIST_XATTRS Permission to list the extended attributes of a
file with LISTXATTRS.
The rules for the client and server follow:
o If the client is sending ACCESS in order to determine if the user
can read an xattr of the file with GETXATTR, the client SHOULD set
ACCESS4_XAREAD in the request's access field.
o If the client is sending ACCESS in order to determine if the user
can modify an xattr of the file with SETXATTR or REMOVEXATTR, the
client SHOULD set ACCESS4_XAWRITE in the request's access field.
o If the client is sending ACCESS in order to determine if the user
can list the xattr keys of the file with LISTXATTRS, the client
SHOULD set ACCESS4_XALIST in the request's access field.
o If the server supports the ACE4_READ_XATTRS permission bit, it
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MUST only check for it in the mode, acl, and dacl attributes when
it receives an ACCESS request with ACCESS4_XAREAD set in the
access field.
o If the server supports the ACE4_WRITE_XATTRS permission bit, it
MUST only check for it in the mode, acl, and dacl attributes when
it receives an ACCESS request with ACCESS4_XAWRITE set in the
access field.
o If the server supports the ACE4_LIST_XATTRS permission bit, it
MUST only check for it in the mode, acl, and dacl attributes when
it receives an ACCESS request with ACCESS4_XALIST set in the
access field.
Server implementations need not provide the granularity of control
that is implied by this list of masks. For example, POSIX-based
systems might not distinguish ACE4_XAREAD from ACE4_READ_ATTRIBUTES
(or ACE4_READ_DATA); both masks would be tied to a single "stat" (or
"read") permission. When such a server returns attributes to the
client, it would show both ACE4_READ_ATTRIBUTES (or ACE4_READ_DATA)
and ACE4_XAREAD if and only if the stat (or read) permission is
enabled.
If a server receives a SETXATTR request that it cannot accurately
implement, it should err in the direction of more restricted access.
For example, suppose a server supports xattrs, but cannot distinguish
modifying attributes from updating xattr. If a client submits an
ALLOW ACE where ACE4_WRITE_ATTRIBUTES is set but ACE4_WRITE_XATTR is
not (or vice versa), the server should either turn off
ACE4_WRITE_ATTRIBUTES or reject the request with NFS4ERR_ATTRNOTSUPP.
7.6. Numeric Values Assigned to Protocol Extensions
This section lists the numeric values assigned new attributes and
operations to implement the xattr feature. To avoid inconsistent
assignments, these have been checked against the most recent protocol
version [RFC5661], the current minor version [NFSv42], and all
extensions currently approved as working group documents.
Development of interoperable prototypes should be possible using
these values, although it is possible that these values may be
modified before eventual publication as a standard-track document.
<CODE BEGINS>
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/// /*
/// * ACCESS - Check Access Rights
/// */
/// const ACCESS4_XAREAD = 0x00000040;
/// const ACCESS4_XAWRITE = 0x00000080;
/// const ACCESS4_XALIST = 0x00000100;
/// /*
/// * ACE mask values
/// */
/// const ACE4_READ_XATTRS = 0x00200000;
/// const ACE4_WRITE_XATTRS = 0x00400000;
/// const ACE4_LIST_XATTRS = 0x00800000;
/// /*
/// * New NFSv4 attribute
/// */
/// typedef bool fattr4_xattr_support;
/// /*
/// * New RECOMMENDED Attribute
/// */
/// const FATTR4_XATTR_SUPPORT = 81;
/// /*
/// * New NFSv4 operations
/// */
/// /* Following lines are to be added to enum nfs_opnum4 */
/// /*
/// OP_GETXATTR = 72,
/// OP_SETXATTR = 73,
/// OP_LISTXATTRS = 74,
/// OP_REMOVEXATTR = 75,
/// */
<CODE ENDS>
7.7. Caching
The caching behavior for extended attributes is similar to other file
attributes such as ACLs and is affected by whether OPEN delegation
has been granted to a client or not.
Xattrs obtained from, or sent to, the server may be cached and
clients can use them to avoid subsequent GETXATTR requests, provided
that the client can ensure that the cached value has not been
subsequently modified by another client. Such assurance can depend
on the client holding a delegation for the file in question or the
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client interrogating the change attribute to make sure that any
cached value is still valid. Such caching may be read-only or write-
through.
When a delegation is in effect, some operations by a second client to
a delegated file will cause the server to recall the delegation
through a callback. For individual operations, we describe, under
IMPLEMENTATION, when such operations are required to effect a recall.
The result of local caching is that the individual xattrs maintained
on clients may not be up-to-date. Changes made in one order on the
server may be seen in a different order on one client and in a third
order on another client. In order to limit problems that may arise
due to separate operations to obtain individual xattrs and other file
attributes, a client should treat xattrs just like other file
attributes with respect to caching as detailed in section 10.6 of
[RFC7530]. A client may validate its cached version of an xattr for
a file by fetching the change attribute and assuming that if the
change attribute has the same value as it did when the attributes
were cached, then xattrs have not changed. If the client holds a
delegation that ensures that the change attribute cannot be modified
by another client, that it can dispense with actual interrogation of
the change attribute.
When a client is changing xattrs of a file, it needs to determine
whether there have been changes made to the file by other clients.
It does this by using the change attribute as reported before and
after the change operation (SETXATTR or REMOVEXATTR) in the
associated change_info4 value returned for the operation. The server
is able to communicate to the client whether the change_info4 data is
provided atomically with respect to the change operation. If the
change values are provided atomically, the client has a basis for
determining, given proper care, whether other clients are modifying
the file in question.
The simplest way to enable the client to make this determination is
for the client to serialize all xattr changes made to a specific
file. When this is done, and the server provides before and after
values of the change attribute atomically, the client can simply
compare the after value of the change attribute from one operation
with the before value on the subsequent change operation modifying
the file. When these are equal, the client is assured that no other
client is modifying the file in question.
If the comparison indicates that the file was updated by another
client, the xattr cache associated with the modified file is purged
from the client. If the comparison indicates no modification, the
xattr cache can be updated on the client to reflect the file
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operation and the associated timeout can be extended. The post-
operation change value needs to be saved as the basis for future
change_info4 comparisons.
Xattr caching requires that the client revalidate xattr cache data by
inspecting the change attribute of a file at the point when an xattr
was cached. This requires that the server update the change
attribute when xattrs are modified. For a client to use the
change_info4 information appropriately and correctly, the server must
report the pre- and post-operation change attribute values
atomically. When the server is unable to report the before and after
values atomically with respect to the xattr update operation, the
server must indicate that fact in the change_info4 return value.
When the information is not atomically reported, the client should
not assume that other clients have not changed the xattrs.
The protocol does not provide support for write-back caching of
xattrs. As such, all modifications to xattrs should be done by
requests to the server. The server should perform such updates
synchronously.
7.8. Xattrs and File Locking
Xattr operations, for the most part, function independent of
operations related to file locking state. For example, xattrs can be
interrogated and modified without a corresponding OPEN operation.
The server does not need to check for locks that conflict with xattr
access or modify operations. For example, another OPEN specified
with OPEN4_SHARE_DENY_READ or OPEN4_SHARE_DENY_BOTH does not prevent
access to or modification of xattrs. Note that the server MUST still
verify that the client is allowed to perform the xattr operation on
the basis of ACE access permissions.
However, the presence of delegations may dictate how xattr operations
interact with the state-related logic. Xattrs cannot be modified
when a delegation for the corresponding file is held by another
client. On the other hand, xattrs can be interrogated despite the
holding of a write delegation by another client since updates are
write-through to the server.
7.9. pNFS Considerations
All xattr operations are sent to the metadata server, which is
responsible for fetching data from and effecting necessary changes to
persistent storage.
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8. Security Considerations
Since xattrs are application data, security issues are exactly the
same as those relating to the storing of file data and named
attributes. These are all various sorts of application data and the
fact that the means of reference is slightly different in each case
should not be considered security-relevant. As such, the additions
to the NFS protocol for supporting extended attributes do not alter
the security considerations of the NFSv4.2 protocol [NFSv42].
9. IANA Considerations
The addition of xattr support to the NFSv4 protocol does not require
any actions by IANA. This document limits xattr names to the user
namespace, where application developers are allowed to define and use
attributes as needed. Unlike named attributes, there is no namespace
identifier associated with xattrs that may require registration.
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10. References
10.1. Normative References
[LEGAL] IETF Trust, "Legal Provisions Relating to IETF Documents",
November 2008, <http://trustee.ietf.org/docs/IETF-Trust-
License-Policy.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <http://www.rfc-editor.org/info/rfc4506>.
[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<http://www.rfc-editor.org/info/rfc5661>.
[RFC5662] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
External Data Representation Standard (XDR) Description",
RFC 5662, DOI 10.17487/RFC5662, January 2010,
<http://www.rfc-editor.org/info/rfc5662>.
[RFC7530] Haynes, T. and D. Noveck, "Network File System (NFS)
Version 4 Protocol", RFC 7530, March 2015.
10.2. Informative References
[NFSv42] Haynes, T., Ed., "NFS Version 4 Minor Version 2", January
2016, <http://www.ietf.org/id/draft-ietf-nfsv4-
minorversion2-41.txt>.
Work in progress.
[NFSv42-dot-x]
Haynes, T., Ed., "NFS Version 4 Minor Version 2 Protocol
External Data Representation Standard (XDR) Description",
January 2016, <http://www.ietf.org/id/draft-ietf-nfsv4-
minorversion2-dot-x-41.txt>.
Work in progress.
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[NFSv4-vers]
D. Noveck, "NFSv4 Version Management", January 2016,
<http://www.ietf.org/id/draft-ietf-nfsv4-versioning-
03.txt>.
Work in progress.
[freedesktop]
"Guidelines for extended attributes",
<http://www.freedesktop.org/wiki/CommonExtendedAttributes>.
[Linux] "Linux Programmer's Manual: xattr(7)",
<http://man7.org/linux/man-pages/man7/xattr.7.html>.
[Love] Love, R., "Linux System Programming: Talking Directly to
the Kernel and C Library", O'Reilly Media, Inc., 2007.
[FreeBSD] "FreeBSD Man Pages - extattr",
<http://www.freebsd.org/cgi/man.cgi?query=extattr&sektion=9>.
[fsattr] "Oracle Man Pages - fsattr",
<http://docs.oracle.com/cd/E19253-01/816-5175/6mbba7f02>.
[NTFS] "File Streams", <http://msdn.microsoft.com/en-
us/library/windows/desktop/aa364404(v=vs.85).aspx>.
[Swift] "Swift-on-File",
<https://github.com/stackforge/swiftonfile>.
[KDE] Handa, V., "KDE Planet",
<http://vhanda.in/blog/2014/08/extended-attributes-
updates/>.
Appendix A. Acknowledgements
This draft has attempted to capture the discussion on adding xattrs
to the NFSv4 protocol from many participants on the IETF NFSv4
mailing list. Those who provided valuable input and comments on
earlier revisions of this draft include: Tom Haynes, Christoph
Hellwig, Nico Williams, Dave Noveck, Benny Halevy and Andreas
Gruenbacher.
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Authors' Addresses
Manoj Naik
Nutanix
1740 Technology Drive, Suite 150,
San Jose, CA 95110
Email: manoj.naik@nutanix.com
Marc Eshel
IBM Almaden
650 Harry Rd
San Jose, CA 95120
Phone: +1 408-927-1894
Email: eshel@us.ibm.com
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