FSATTR(5)FSATTR(5)NAMEfsattr - extended file attributes
DESCRIPTION
Attributes are logically supported as files within the file system.
The file system is therefore augmented with an orthogonal name space of
file attributes. Any file (including attribute files) can have an
arbitrarily deep attribute tree associated with it. Attribute values
are accessed by file descriptors obtained through a special attribute
interface. This logical view of "attributes as files" allows the
leveraging of existing file system interface functionality to support
the construction, deletion, and manipulation of attributes.
The special files "." and ".." retain their accustomed semantics within
the attribute hierarchy. The "." attribute file refers to the current
directory and the ".." attribute file refers to the parent directory.
The unnamed directory at the head of each attribute tree is considered
the "child" of the file it is associated with and the ".." file refers
to the associated file. For any non-directory file with attributes,
the ".." entry in the unnamed directory refers to a file that is not a
directory.
Conceptually, the attribute model is fully general. Extended attributes
can be any type of file (doors, links, directories, and so forth) and
can even have their own attributes (fully recursive). As a result, the
attributes associated with a file could be an arbitrarily deep direc‐
tory hierarchy where each attribute could have an equally complex
attribute tree associated with it. Not all implementations are able
to, or want to, support the full model. Implementation are therefore
permitted to reject operations that are not supported. For example,
the implementation for the UFS file system allows only regular files as
attributes (for example, no sub-directories) and rejects attempts to
place attributes on attributes.
The following list details the operations that are rejected in the cur‐
rent implementation:
link
Any attempt to create links between attribute
and non-attribute space is rejected to prevent
security-related or otherwise sensitive
attributes from being exposed, and therefore
manipulable, as regular files.
rename
Any attempt to rename between attribute and
non-attribute space is rejected to prevent an
already linked file from being renamed and
thereby circumventing the link restriction
above.
mkdir, symlink, mknod
Any attempt to create a "non-regular" file in
attribute space is rejected to reduce the
functionality, and therefore exposure and
risk, of the initial implementation.
The entire available name space has been allocated to "general use" to
bring the implementation in line with the NFSv4 draft standard [NFSv4].
That standard defines "named attributes" (equivalent to Solaris
Extended Attributes) with no naming restrictions. All Sun applications
making use of opaque extended attributes will use the prefix "SUNW".
Shell-level API
The command interface for extended attributes is the set of applica‐
tions provided by Solaris for the manipulation of attributes from the
command line. This interface consists of a set of existing utilities
that have been extended to be "attribute-aware", plus the runat utility
designed to "expose" the extended attribute space so that extended
attributes can be manipulated as regular files.
The -@ option enable utilities to manipulate extended attributes. As a
rule, this option enables the utility to enter into attribute space
when the utility is performing a recursive traversal of file system
space. This is a fully recursive concept. If the underlying file system
supports recursive attributes and directory structures, the -@ option
opens these spaces to the file tree-walking algorithms.
The following utilities accommodate extended attributes (see the indi‐
vidual manual pages for details):
cp
By default, cp ignores attributes and copies only file data.
This is intended to maintain the semantics implied by cp cur‐
rently, where attributes (such as owner and mode) are not
copied unless the -p option is specified. With the -@ (or -p)
option, cp attempts to copy all attributes along with the file
data.
cpio
The -@ option informs cpio to archive attributes, but by
default cpio ignores extended attributes. See Extended Archive
Formats below for a description of the new archive records.
du
File sizes computed include the space allocated for any
extended attributes present.
find
By default, find ignores attributes. The -xattr expression
provides support for searches involving attribute space. It
returns true if extended attributes are present on the current
file.
fsck
The fsck utility manages extended attribute data on the disk. A
file system with extended attributes can be mounted on versions
of Solaris that are not attribute-aware (versions prior to
Solaris 9), but the attributes will not be accessible and fsck
will strip them from the files and place them in lost+found.
Once the attributes have been stripped the file system is com‐
pletely stable on Solaris versions that are not attribute-
aware, but would now be considered corrupted on attribute-aware
versions of Solaris. The attribute-aware fsck utility should be
run to stabilize the file system before using it in an
attribute-aware environment.
fsdb
This fsdb utility is able to find the inode for the "hidden"
extended attribute directory.
ls
The ls -@ command displays an "@" following the mode informa‐
tion when extended attributes are present. More precisely, the
output line for a given file contains an "@" character follow‐
ing the mode characters if the pathconf(2) variable
XATTR_EXISTS is set to true. See the pathconf() section below.
The -@ option uses the same general output format as the -l
option.
mv
When a file is moved, all attributes are carried along with the
file rename. When a file is moved across a file system bound‐
ary, the copy command invoked is similar to the cp -p variant
described above and extended attributes are "moved". If the
extended file attributes cannot be replicated, the move opera‐
tion fails and the source file is not removed.
pax
The -@ option informs pax to archive attributes, but by default
pax ignores extended attributes. The pax(1) utility is a
generic replacement for both tar(1) and cpio(1) and is able to
produce either output format in its archive. See Extended Ar‐
chive Formats below for a description of the new archive
records.
tar
In the default case, tar does not attempt to place attributes
in the archive. If the -@ option is specified, however, tar
traverses into the attribute space of all files being placed in
the archive and attempts to add the attributes to the archive.
A new record type has been introduced for extended attribute
entries in tar archive files (the same is true for pax and cpio
archives) similar to the way ACLs records were defined. See
Extended Archive Formats below for a description of the new ar‐
chive records.
There is a class of utilities (chmod, chown, chgrp) that one might
expect to be modified in a manner similar to those listed above. For
example, one might expect that performing chmod on a file would not
only affect the file itself but would also affect at least the extended
attribute directory if not any existing extended attribute files. This
is not the case. The model chosen for extended attributes implies that
the attribute directory and the attributes themselves are all file
objects in their own right, and can therefore have independent file
status attributes associated with them (a given implementation cannot
support this, for example, for intrinsic attributes). The relationship
is left undefined and a fine-grained control mechanism (runat(1)) is
provided to allow manipulation of extended attribute status attributes
as necessary.
The runat utility has the following syntax:
runat filename [command]
The runat utility executes the supplied command in the context of the
"attribute space" associated with the indicated file. If no command
argument is supplied, a shell is invoked. See runat(1) for details.
Application-level API
The primary interface required to access extended attributes at the
programmatic level is the openat(2) function. Once a file descriptor
has been obtained for an attribute file by an openat() call, all normal
file system semantics apply. There is no attempt to place special
semantics on read(2), write(2), ftruncate(3C), or other functions when
applied to attribute file descriptors relative to "normal" file
descriptors.
The set of existing attributes can be browsed by calling openat() with
"." as the file name and the O_XATTR flag set, resulting in a file
descriptor for the attribute directory. The list of attributes is
obtained by calls to getdents(2) on the returned file descriptor. If
the target file did not previously have any attributes associated with
it, an empty top-level attribute directory is created for the file and
subsequent getdents() calls will return only "." and "..". While the
owner of the parent file owns the extended attribute directory, it is
not charged against its quota if the directory is empty. Attribute
files themselves, however, are charged against the user quota as any
other regular file.
Additional system calls have been provided as convenience functions.
These include the fchownat(2), fstatat(2), futimesat(2), renameat(2),
unlinkat(2). These new functions, along with openat(), provide a mecha‐
nism to access files relative to an arbitrary point in the file system,
rather than only the current working directory. This mechanism is par‐
ticularly useful in situations when a file descriptor is available with
no path. The openat() function, in particular, can be used in many con‐
texts where chdir() or fchdir() is currently required. See chdir(2).
Open a file relative to a file descriptor
int openat (int fd, const char *path, int oflag [, mode_t mode])
The openat(2) function behaves exactly as open(2) except when given a
relative path. Where open() resolves a relative path from the current
working directory, openat() resolves the path based on the vnode indi‐
cated by the supplied file descriptor. When oflag is O_XATTR, openat()
interprets the path argument as an extended attribute reference. The
following code fragment uses openat() to examine the attributes of some
already opened file:
dfd = openat(fd, ".", O_RDONLY|O_XATTR);
(void)getdents(dfd, buf, nbytes);
If openat() is passed the special value AT_FDCWD as its first (fd)
argument, its behavior is identical to open() and the relative path
arguments are interpreted relative to the current working directory. If
the O_XATTR flag is provided to openat() or to open(), the supplied
path is interpreted as a reference to an extended attribute on the cur‐
rent working directory.
Unlink a file relative to a directory file descriptor
int unlinkat (int dirfd, const char *pathflag, int flagflag)
The unlinkat(2) function deletes an entry from a directory. The path
argument indicates the name of the entry to remove. If path an absolute
path, the dirfd argument is ignored. If it is a relative path, it is
interpreted relative to the directory indicated by the dirfd argument.
If dirfd does not refer to a valid directory, the function returns
ENOTDIR. If the special value AT_FDCWD is specified for dirfd, a rela‐
tive path argument is resolved relative to the current working direc‐
tory. If the flag argument is 0, all other semantics of this function
are equivalent to unlink(2). If flag is set to AT_REMOVEDIR, all other
semantics of this function are equivalent to rmdir(2).
Rename a file relative to directories
int renameat (int fromfd, const char *old, int tofd, const char *new)
The renameat(2) function renames an entry in a directory, possibly mov‐
ing the entry into a different directory. The old argument indicates
the name of the entry to rename. If this argument is a relative path,
it is interpreted relative to the directory indicated by the fd argu‐
ment. If it is an absolute path, the fromfd argument is ignored. The
new argument indicates the new name for the entry. If this argument is
a relative path, it is interpreted relative to the directory indicated
by the tofd argument. If it is an absolute path, the tofd argument is
ignored.
In the relative path cases, if the directory file descriptor arguments
do not refer to a valid directory, the function returns ENOTDIR. All
other semantics of this function are equivalent to rename(2).
If a special value AT_FDCWD is specified for either the fromfd or tofd
arguments, their associated path arguments (old and new) are inter‐
preted relative to the current working directory if they are not speci‐
fied as absolute paths. Any attempt to use renameat() to move a file
that is not an extended attribute into an extended attribute directory
(so that it becomes an extended attribute) will fail. The same is true
for an attempt to move a file that is an extended attribute into a
directory that is not an extended attribute directory.
Obtain information about a file
int fstatat (int fd, const char *path, struct stat* buf, int flag)
The fstatat(2) function obtains information about a file. If the path
argument is relative, it is resolved relative to the fd argument file
descriptor, otherwise the fd argument is ignored. If the fd argument
is a special value AT_FDCWD the path is resolved relative to the cur‐
rent working directory. If the path argument is a null pointer, the
function returns information about the file referenced by the fd argu‐
ment. In all other relative path cases, if the fd argument does not
refer to a valid directory, the function returns ENOTDIR. If AT_SYM‐
LINK_NOFOLLOW is set in the flag argument, the function will not auto‐
matically traverse a symbolic link at the position of the path. If
_AT_TRIGGER is set in the flag argument and the vnode is a trigger
mount point, the mount is performed and the function returns the
attributes of the root of the mounted filesystem. The fstatat() func‐
tion is a multipurpose function that can be used in place of stat(),
lstat(), or fstat(). See stat(2)
The function call stat(path, buf) is identical to fstatat(AT_FDCWD,
path, buf, 0).
The function call lstat(path, buf) is identical to fstatat(AT_FDCWD,
path, buf, AT_SYMLINK_NOFOLLOW)
The function call fstat(fildes, buf) is identical to fstatat(fildes,
NULL, buf, 0).
Set owner and group ID
int fchownat (int fd, const char *path, uid_t owner, gid_t group, \
int flag)
The fchownat(2) function sets the owner ID and group ID for a file. If
the path argument is relative, it is resolved relative to the fd argu‐
ment file descriptor, otherwise the fd argument is ignored. If the fd
argument is a special value AT_FDCWD the path is resolved relative to
the current working directory. If the path argument is a null pointer,
the function sets the owner and group ID of the file referenced by the
fd argument. In all other relative path cases, if the fd argument does
not refer to a valid directory, the function returns ENOTDIR. If the
flag argument is set to AT_SYMLINK_NOFOLLOW, the function will not
automatically traverse a symbolic link at the position of the path. The
fchownat() function is a multi-purpose function that can be used in
place of chown(), lchown(), or fchown(). See chown(2).
The function call chown(path, owner, group) is equivalent to fchow‐
nat(AT_FDCWD, path, owner, group, 0).
The function call lchown(path, owner, group) is equivalent to fchow‐
nat(AT_FDCWD, path, owner, group, AT_SYMLINK_NOFOLLOW).
Set file access and modification times
int futimesat (int fd, const char *path, const struct timeval \
times[2])
The futimesat(2) function sets the access and modification times for a
file. If the path argument is relative, it is resolved relative to the
fd argument file descriptor; otherwise the fd argument is ignored. If
the fd argument is the special value AT_FDCWD, the path is resolved
relative to the current working directory. If the path argument is a
null pointer, the function sets the access and modification times of
the file referenced by the fd argument. In all other relative path
cases, if the fd argument does not refer to a valid directory, the
function returns ENOTDIR. The futimesat() function can be used in
place of utimes(2).
The function call utimes(path, times) is equivalent to futime‐
sat(AT_FDCWD, path, times).
New pathconf() functionality
long int pathconf(const char *path, int name)
Two variables have been added to pathconf(2) to provide enhanced sup‐
port for extended attribute manipulation. The XATTR_ENABLED variable
allows an application to determine if attribute support is currently
enabled for the file in question. The XATTR_EXISTS variable allows an
application to determine whether there are any extended attributes
associated with the supplied path.
Open/Create an attribute file
int attropen (const char *path, const char *attrpath, int oflag \
[, mode_t mode])
The attropen(3C) function returns a file descriptor for the named
attribute, attrpath, of the file indicated by path. The oflag and mode
arguments are identical to the open(2) arguments and are applied to the
open operation on the attribute file (for example, using the O_CREAT
flag creates a new attribute). Once opened, all normal file system
operations can be used on the attribute file descriptor. The
attropen() function is a convenience function and is equivalent to the
following sequence of operations:
fd = open (path, O_RDONLY);
attrfd = openat(fd, attrpath, oflag|O_XATTR, mode);
close(fd);
The set of existing attributes can be browsed by calling attropen()
with "." as the attribute name. The list of attributes is obtained by
calling getdents(2) (or fdopendir(3C) followed by readdir(3C), see
below) on the returned file descriptor.
Convert an open file descriptor for a directory into a directory descriptor
DIR * fdopendir (const int fd)
The fdopendir(3C) function promotes a file descriptor for a directory
to a directory pointer suitable for use with the readdir(3C) function.
The originating file descriptor should not be used again following the
call to fdopendir(). The directory pointer should be closed with a call
to closedir(3C). If the provided file descriptor does not reference a
directory, the function returns ENOTDIR. This function is useful in
circumstances where the only available handle on a directory is a file
descriptor. See attropen(3C) and openat(2).
Using the API
The following examples demonstrate how the API might be used to perform
basic operations on extended attributes:
Example 1 List extended attributes on a file.
attrdirfd = attropen("test", ".", O_RDONLY);
dirp = fdopendir(attrdirfd);
while (dp = readdir(dirp)) {
...
Example 2 Open an extended attribute.
attrfd = attropen("test", dp->d_name, O_RDONLY);
or
attrfd = openat(attrdirfd, dp->d_name, O_RDONLY);
Example 3 Read from an extended attribute.
while (read(attrfd, buf, 512) > 0) {
...
Example 4 Create an extended attribute.
newfd = attropen("test", "attr", O_CREAT|O_RDWR);
or
newfd = openat(attrdirfd, "attr", O_CREAT|O_RDWR);
Example 5 Write to an extended attribute.
count = write(newfd, buf, length);
Example 6 Delete an extended attribute.
error = unlinkat(attrdirfd, "attr");
Applications intending to access the interfaces defined here as well as
the POSIX and X/Open specification-conforming interfaces should define
the macro _ATFILE_SOURCE to be 1 and set whichever feature test macros
are appropriate to obtain the desired environment. See standards(5).
Extended Archive Formats
As noted above in the description of command utilities modified to pro‐
vide support for extended attributes, the archive formats for tar(1)
and cpio(1) have been extended to provide support for archiving
extended attributes. This section describes the specifics of the ar‐
chive format extensions.
Extended tar format
The tar archive is made up of a series of 512 byte blocks. Each
archived file is represented by a header block and zero or more data
blocks containing the file contents. The header block is structured as
shown in the following table.
Field Name Length (in Octets) Description
Name 100 File name string
Mode 8 12 file mode bits
Uid 8 User ID of file owner
Gid 8 Group ID of file owner
Size 12 Size of file
Mtime 12 File modification time
Chksum 8 File contents checksum
Typeflag 1 File type flag
Linkname 100 Link target name if file linked
Magic 6 "ustar"
Version 2 "00"
Uname 32 User name of file owner
Gname 32 Group name of file owner
Devmajor 8 Major device ID if special file
Devminor 8 Minor device ID if special file
Prefix 155 Path prefix string for file
The extended attribute project extends the above header format by
defining a new header type (for the Typeflag field). The type 'E' is
defined to be used for all extended attribute files. Attribute files
are stored in the tar archive as a sequence of two <header ,data>
pairs. The first file contains the data necessary to locate and name
the extended attribute in the file system. The second file contains the
actual attribute file data. Both files use an 'E' type header. The
prefix and name fields in extended attribute headers are ignored,
though they should be set to meaningful values for the benefit of
archivers that do not process these headers. Solaris archivers set the
prefix field to "/dev/null" to prevent archivers that do not understand
the type 'E' header from trying to restore extended attribute files in
inappropriate places.
Extended cpio format
The cpio archive format is octet-oriented rather than block-oriented.
Each file entry in the archive includes a header that describes the
file, followed by the file name, followed by the contents of the file.
These data are arranged as described in the following table.
Field Name Length (in Octets) Description
c_magic 6 70707
c_dev 6 First half of unique file ID
c_ino 6 Second half of unique file ID
c_mode 6 File mode bits
c_uid 6 User ID of file owner
c_gid 6 Group ID of file owner
c_nlink 6 Number of links referencing file
c_rdev 6 Information for special files
c_mtime 11 Modification time of file
c_namesize 6 Length of file pathname
c_filesize 11 Length of file content
c_name c_namesize File pathname
c_filedata c_filesize File content
The basic archive file structure is not changed for extended
attributes. The file type bits stored in the c_mode field for an
attribute file are set to 0xB000. As with the tar archive format,
extended attributes are stored in cpio archives as two consecutive file
entries. The first file describes the location/name for the extended
attribute. The second file contains the actual attribute file content.
The c_name field in extended attribute headers is ignored, though it
should be set to a meaningful value for the benefit of archivers that
do not process these headers. Solaris archivers start the pathname
with "/dev/null/"to prevent archivers that do not understand the type
'E' header from trying to restore extended attribute files in inappro‐
priate places.
Attribute identification data format
Both the tar and cpio archive formats can contain the special files
described above, always paired with the extended attribute data record,
for identifying the precise location of the extended attribute. These
special data files are necessary because there is no simple naming
mechanism for extended attribute files. Extended attributes are not
visible in the file system name space. The extended attribute name
space must be "tunneled into" using the openat() function. The
attribute identification data must support not only the flat naming
structure for extended attributes, but also the possibility of future
extensions allowing for attribute directory hierarchies and recursive
attributes. The data file is therefore composed of a sequence of
records. It begins with a fixed length header describing the content.
The following table describes the format of this data file.
Field Name Length (in Octets) Description
h_version 7 Name file version
h_size 10 Length of data file
h_component_len 10 Total length of all path segments
h_link_comp_len 10 Total length of all link segments
path h_component_len Complex path
link_path h_link_comp_len Complex link path
As demonstrated above, the header is followed by a record describing
the "path" to the attribute file. This path is composed of two or more
path segments separated by a null character. Each segment describes a
path rooted at the hidden extended attribute directory of the leaf file
of the previous segment, making it possible to name attributes on
attributes. The first segment is always the path to the parent file
that roots the entire sequence in the normal name space. The following
table describes the format of each segment.
Field Name Length (in Octets) Description
─────────────────────────────────────────────────────────────────────
h_namesz 7 Length of segment path
h_typeflag 1 Actual file type of attribute file
h_names h_namesz Parent path + segment path
If the attribute file is linked to another file, the path record is
followed by a second record describing the location of the referencing
file. The structure of this record is identical to the record
described above.
SEE ALSOcp(1), cpio(1), find(1), ls(1), mv(1), pax(1), runat(1), tar(1), du(1),
fsck(1M), chown(2), link(2), open(2), pathconf(2), rename(2), stat(2),
unlink(2), utimes(2), attropen(3C), standards(5)
Oct 10, 2007 FSATTR(5)
