NFSv4 Working Group M. Naik
Internet Draft M. Eshel
Intended Status: Standards Track IBM Almaden
Expires: February 19, 2016 August 18, 2015
File System Extended Attributes in NFSv4
draft-ietf-nfsv4-xattrs-01
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
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Copyright and License Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. File System Support . . . . . . . . . . . . . . . . . . . . . 5
4. Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Differences from Named Attributes . . . . . . . . . . . . . . 6
6. XDR Description . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Code Components Licensing Notice . . . . . . . . . . . . . 7
7. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 9
7.1. New definition . . . . . . . . . . . . . . . . . . . . . . 9
7.1.1. xattr4 . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. New Attribute . . . . . . . . . . . . . . . . . . . . . . 10
7.2.1. xattr_support . . . . . . . . . . . . . . . . . . . . 10
7.3. New Operations . . . . . . . . . . . . . . . . . . . . . . 10
7.3.1. GETXATTR - Get an extended attribute of a file . . . . 11
7.3.2. SETXATTR - Set an extended attribute of a file . . . . 12
7.3.3. LISTXATTR - List extended attributes of a file . . . . 14
7.3.4. REMOVEXATTR - Remove an extended attribute of a file . 16
7.3.5. Valid Errors . . . . . . . . . . . . . . . . . . . . . 17
7.4. Modifications to Existing Operations . . . . . . . . . . . 18
7.5. Numeric Values Assigned to Protocol Extensions . . . . . . 20
7.6. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.7. Xattrs and File Locking . . . . . . . . . . . . . . . . . 22
7.8. pNFS Considerations . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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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 to 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
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
[Love]. 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 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.
This document provides for namespaces supporting user-managed
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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
lower level of service, and could fail to cooperate on a local
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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 definition
7.1.1. xattr4
The NFSv4 xattr4 structure is defined as follows:
<CODE BEGINS>
/// typedef component4 xattrname4;
/// typedef opaque xattrvalue4<>;
/// struct xattr4 {
/// xattrname4 xa_name;
/// xattrvalue4 xa_value;
/// };
<CODE ENDS>
Each xattr, defined by xattr4, is a key/value pair. An xattr4
consists of an xattrname4 which 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 xattrname4 with
regard to internationalization-related issues is the same as that for
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NFSv4 file names and named attribute names, as described in
[RFC7530]. Any regular file or directory may have set of extended
attributes, each consisting of a key and associated value. The NFS
client or server MUST NOT interpret the contents of xattr4.
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
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 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
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
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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, LISTXATTR and REMOVEXATTR, to query, set, list and remove
xattrs respectively. GETXATTR allows obtaining the value of an xattr
key, SETXATTR allows creating or replacing an xattr key with a value,
LISTXATTR enumerates all the xattrs names, and REMOVEXATTR allows
deleting a single xattr.
7.3.1. GETXATTR - Get an extended attribute of a file
7.3.1.1. ARGUMENTS
<CODE BEGINS>
/// struct GETXATTR4args {
/// /* CURRENT_FH: file */
/// xattrname4 ga_name;
/// };
<CODE ENDS>
7.3.1.2. RESULTS
<CODE BEGINS>
/// union GETXATTR4res switch (nfsstat4 gr_status) {
/// case NFS4_OK:
/// xattrvalue4 gr_value;
/// default:
/// void;
/// };
<CODE ENDS>
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7.3.1.3. 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 an error.
The server also MUST return an error if it supports xattrs on the
target but cannot obtain the requested data. In that case, no value
will be returned. 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 gr_status.
7.3.1.4. 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.
7.3.2. SETXATTR - Set an extended attribute of a file
7.3.2.1. ARGUMENTS
<CODE BEGINS>
/// enum setxattr_type4 {
/// SETXATTR4_CREATE = 0,
/// SETXATTR4_REPLACE = 1,
/// };
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/// struct SETXATTR4args {
/// /* CURRENT_FH: file */
/// setxattr_type4 sa_type;
/// xattr4 sa_xattr;
/// };
<CODE ENDS>
7.3.2.2. RESULTS
<CODE BEGINS>
/// struct SETXATTR4res switch (nfsstat4 sr_status) {
/// case NFS4_OK:
/// change_info4 sr_info;
/// default:
/// void;
/// };
<CODE ENDS>
7.3.2.3. DESCRIPTION
The SETXATTR operation changes one extended attribute of a file
system object. The change desired is specified by sa_type.
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 an error if the attribute
key already exists. SETXATTR4_REPLACE is also used to set an xattr,
but the server MUST return an error if the attribute key does not
exist.
If the xattr key and value contained in the client request are such
that the request would exceed the channel's negotiated maximum
request size, then the server MUST return NFS4ERR_REQ_TOO_BIG in
sr_status. If the server file system imposes additional limits on
the size of key name or value, it MAY return NFS4ERR_NAMETOOLONG.
A successful SETXATTR MUST change the file time_modify 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
sr_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
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after the operation. See Section 7.6 for a discussion on xattr
caching.
7.3.2.4. 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.
7.3.3. LISTXATTR - List extended attributes of a file
7.3.3.1. ARGUMENTS
<CODE BEGINS>
/// struct LISTXATTR4args {
/// /* CURRENT_FH: file */
/// nfs_cookie4 la_cookie;
/// verifier4 la_cookieverf;
/// count4 la_maxcount;
/// };
<CODE ENDS>
7.3.3.2. RESULTS
<CODE BEGINS>
/// struct LISTXATTR4resok (
/// nfs_cookie4 lr_cookie;
/// verifier4 lr_cookieverf;
/// bool lr_eof;
/// xattrname4 lr_names<>;
/// };
/// union LISTXATTR4res switch (nfsstat4 lr_status) {
/// case NFS4_OK:
/// LISTXATTR4resok lr_value;
/// default:
/// void;
/// };
<CODE ENDS>
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7.3.3.3. DESCRIPTION
The LISTXATTR 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 LISTXATTR.
The arguments contain a cookie value that represents where the
LISTXATTR should start within the list of xattrs. A value of 0
(zero) for la_cookie is used to start reading at the beginning of the
list. For subsequent LISTXATTR requests, the client specifies a
cookie value that is provided by the server on a previous LISTXATTR
request.
The la_cookieverf value should be set to 0 (zero) when the la_cookie
value is 0 (zero) (first xattr read). On subsequent requests, it
should be lr_cookieverf as returned by the server. The la_cookieverf
must match that returned by the LISTXATTR in which the cookie was
acquired. If the server determines that the la_cookieverf is no
longer valid for the directory, the error NFS4ERR_NOT_SAME must be
returned.
The la_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 LISTXATTR4resok 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
extended attribute keys. The "lr_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 LISTXATTR 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. Ideally, the cookie value should not change if the
object xattr values is modified since the client may be caching these
values.
On success, the current filehandle retains its value.
7.3.3.4. IMPLEMENTATION
The handling of ls_cookie and ls_cookieverf is similar to that of the
READDIR operation. The cookieverf may be used by the server to help
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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/cookieverf 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.
The use of the ls_cookieverf will also protect the client from using
LISTXATTR cookie values that may be stale. For example, if the file
system has been migrated, the server may or may not be able to use
the same cookie values to service LISTXATTR as the previous server
used. With the client providing the ls_cookieverf, the server is
able to provide the appropriate response to the client. This
prevents the case where the server may accept a cookie value but the
underlying object xattrs have changed and the response is invalid
from the client's context of its previous LISTXATTR.
7.3.4. REMOVEXATTR - Remove an extended attribute of a file
7.3.4.1. ARGUMENTS
<CODE BEGINS>
/// struct REMOVEXATTR4args {
/// /* CURRENT_FH: file */
/// xattrname4 ra_name;
/// };
<CODE ENDS>
7.3.4.2. RESULTS
<CODE BEGINS>
/// struct REMOVEXATTR4res switch (nfsstat4 rr_status) {
/// case NFS4_OK:
/// change_info4 rr_info;
/// default:
/// void;
/// };
<CODE ENDS>
7.3.4.3. DESCRIPTION
The REMOVEXATTR operation deletes one extended attribute of a file
system object specified by ra_name. The server MUST return an error
if the attribute key does not exist. If the xattr key contained in
the client request exceeds the channel's negotiated maximum request
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size, then the server MUST return NFS4ERR_REQ_TOO_BIG in rr_status.
A successful REMOVEXATTR SHOULD change the file time_modify and
change attributes. However, these attributes SHOULD NOT be changed
unless the xattr is not removed.
Similar to SETXATTR, the server communicates the value of the change
attribute immediately prior to, and immediately following, a
successful REMOVEXATTR operation in rr_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.
7.3.4.4. 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.
7.3.5. 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, |
| | NFS4ERR_RETRY_UNCACHED_REP, |
| | NFS4ERR_SERVERFAULT, NFS4ERR_STALE, |
| | NFS4ERR_TOO_MANY_OPS, NFS4ERR_WRONG_TYPE |
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| 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 |
| LISTXATTR | 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 |
+----------------------+--------------------------------------------+
7.4. 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.
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.
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This section extends the semantics of the ACCESS operation documented
in Section 18.1 of [RFC5661]. Two new access permissions can be
requested:
ACCESS4_XAREAD Query a file or directory for its xattr key and/or
value.
ACCESS4_XAWRITE Modify xattr keys and/or values of a file or
directory.
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 or LISTXATTR.
ACE4_WRITE_XATTRS Permission to change the extended attributes of a
file with SETXATTR or REMOVEXATTR.
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 or list the xattr keys
of the file with LISTXATTR, 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 server supports ACE4_READ_XATTRS permission bit, it 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 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.
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
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"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 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.
7.5. 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>
/// /*
/// * ACCESS - Check Access Rights
/// */
/// const ACCESS4_XAREAD = 0x00000040;
/// const ACCESS4_XAWRITE = 0x00000080;
/// /*
/// * ACE flag values
/// */
/// const ACE4_READ_XATTRS = 0x00200000;
/// const ACE4_WRITE_XATTRS = 0x00400000;
/// /*
/// * New NFSv4 attribute
/// */
/// typedef bool fattr4_xattr_support;
/// /*
/// * New RECOMMENDED Attribute
/// */
/// const FATTR4_XATTR_SUPPORT = 81;
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/// /*
/// * New NFSv4 operations
/// */
/// /* Following lines are to be added to enum nfs_opnum4 */
/// /*
/// OP_GETXATTR = 72,
/// OP_SETXATTR = 73,
/// OP_LISTXATTR = 74,
/// OP_REMOVEXATTR = 75,
/// */
<CODE ENDS>
7.6. 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
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.
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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
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.
7.7. Xattrs and File Locking
Xattr operations, for the most part, function independent of
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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.
7.8. 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.
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", April
2015, <http://www.ietf.org/id/draft-ietf-nfsv4-
minorversion2-38.txt>.
Work in progress.
[NFSv42-dot-x]
Haynes, T., Ed., "NFS Version 4 Minor Version 2 Protocol
External Data Representation Standard (XDR) Description",
April 2015, <http://www.ietf.org/id/draft-ietf-nfsv4-
minorversion2-dot-x-38.txt>.
Work in progress.
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[NFSv4-vers]
Haynes, T. and D. Noveck, "NFSv4 Version Management", July
2015, <http://www.ietf.org/id/draft-ietf-nfsv4-versioning-
01.txt>.
Work in progress.
[freedesktop]
"Guidelines for extended attributes",
<http://www.freedesktop.org/wiki/CommonExtendedAttributes>.
[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 and Nico Williams. Dave Noveck provided a comprehensive
review of the previous revision of this draft.
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Authors' Addresses
Manoj Naik
IBM Almaden
650 Harry Rd
San Jose, CA 95120
Phone: +1 408-927-1707
Email: mnaik@us.ibm.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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