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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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open_by_handle_at(2) System Calls Manual open_by_handle_at(2)
name_to_handle_at, open_by_handle_at - obtain handle for a
pathname and open file via a handle
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
int name_to_handle_at(int dirfd, const char *pathname,
struct file_handle *handle,
int *mount_id, int flags);
int open_by_handle_at(int mount_fd, struct file_handle *handle,
int flags);
The name_to_handle_at() and open_by_handle_at() system calls
split the functionality of openat(2) into two parts:
name_to_handle_at() returns an opaque handle that corresponds to
a specified file; open_by_handle_at() opens the file
corresponding to a handle returned by a previous call to
name_to_handle_at() and returns an open file descriptor.
name_to_handle_at()
The name_to_handle_at() system call returns a file handle and a
mount ID corresponding to the file specified by the dirfd and
pathname arguments. The file handle is returned via the argument
handle, which is a pointer to a structure of the following form:
struct file_handle {
unsigned int handle_bytes; /* Size of f_handle [in, out] */
int handle_type; /* Handle type [out] */
unsigned char f_handle[0]; /* File identifier (sized by
caller) [out] */
};
It is the caller's responsibility to allocate the structure with
a size large enough to hold the handle returned in f_handle.
Before the call, the handle_bytes field should be initialized to
contain the allocated size for f_handle. (The constant
MAX_HANDLE_SZ, defined in <fcntl.h>, specifies the maximum
expected size for a file handle. It is not a guaranteed upper
limit as future filesystems may require more space.) Upon
successful return, the handle_bytes field is updated to contain
the number of bytes actually written to f_handle.
The caller can discover the required size for the file_handle
structure by making a call in which handle->handle_bytes is zero;
in this case, the call fails with the error EOVERFLOW and
handle->handle_bytes is set to indicate the required size; the
caller can then use this information to allocate a structure of
the correct size (see EXAMPLES below). Some care is needed here
as EOVERFLOW can also indicate that no file handle is available
for this particular name in a filesystem which does normally
support file-handle lookup. This case can be detected when the
EOVERFLOW error is returned without handle_bytes being increased.
Other than the use of the handle_bytes field, the caller should
treat the file_handle structure as an opaque data type: the
handle_type and f_handle fields can be used in a subsequent call
to open_by_handle_at(). The caller can also use the opaque
file_handle to compare the identity of filesystem objects that
were queried at different times and possibly at different paths.
The fanotify(7) subsystem can report events with an information
record containing a file_handle to identify the filesystem
object.
The flags argument is a bit mask constructed by ORing together
zero or more of AT_HANDLE_FID, AT_EMPTY_PATH, and
AT_SYMLINK_FOLLOW, described below.
When flags contain the AT_HANDLE_FID (since Linux 6.5) flag, the
caller indicates that the returned file_handle is needed to
identify the filesystem object, and not for opening the file
later, so it should be expected that a subsequent call to
open_by_handle_at() with the returned file_handle may fail.
Together, the pathname and dirfd arguments identify the file for
which a handle is to be obtained. There are four distinct cases:
• If pathname is a nonempty string containing an absolute
pathname, then a handle is returned for the file referred to
by that pathname. In this case, dirfd is ignored.
• If pathname is a nonempty string containing a relative
pathname and dirfd has the special value AT_FDCWD, then
pathname is interpreted relative to the current working
directory of the caller, and a handle is returned for the file
to which it refers.
• If pathname is a nonempty string containing a relative
pathname and dirfd is a file descriptor referring to a
directory, then pathname is interpreted relative to the
directory referred to by dirfd, and a handle is returned for
the file to which it refers. (See openat(2) for an
explanation of why "directory file descriptors" are useful.)
• If pathname is an empty string and flags specifies the value
AT_EMPTY_PATH, then dirfd can be an open file descriptor
referring to any type of file, or AT_FDCWD, meaning the
current working directory, and a handle is returned for the
file to which it refers.
The mount_id argument returns an identifier for the filesystem
mount that corresponds to pathname. This corresponds to the
first field in one of the records in /proc/self/mountinfo.
Opening the pathname in the fifth field of that record yields a
file descriptor for the mount point; that file descriptor can be
used in a subsequent call to open_by_handle_at(). mount_id is
returned both for a successful call and for a call that results
in the error EOVERFLOW.
By default, name_to_handle_at() does not dereference pathname if
it is a symbolic link, and thus returns a handle for the link
itself. If AT_SYMLINK_FOLLOW is specified in flags, pathname is
dereferenced if it is a symbolic link (so that the call returns a
handle for the file referred to by the link).
name_to_handle_at() does not trigger a mount when the final
component of the pathname is an automount point. When a
filesystem supports both file handles and automount points, a
name_to_handle_at() call on an automount point will return with
error EOVERFLOW without having increased handle_bytes. This can
happen since Linux 4.13 with NFS when accessing a directory which
is on a separate filesystem on the server. In this case, the
automount can be triggered by adding a "/" to the end of the
pathname.
open_by_handle_at()
The open_by_handle_at() system call opens the file referred to by
handle, a file handle returned by a previous call to
name_to_handle_at().
The mount_fd argument is a file descriptor for any object (file,
directory, etc.) in the mounted filesystem with respect to which
handle should be interpreted. The special value AT_FDCWD can be
specified, meaning the current working directory of the caller.
The flags argument is as for open(2). If handle refers to a
symbolic link, the caller must specify the O_PATH flag, and the
symbolic link is not dereferenced; the O_NOFOLLOW flag, if
specified, is ignored.
The caller must have the CAP_DAC_READ_SEARCH capability to invoke
open_by_handle_at().
On success, name_to_handle_at() returns 0, and
open_by_handle_at() returns a file descriptor (a nonnegative
integer).
In the event of an error, both system calls return -1 and set
errno to indicate the error.
name_to_handle_at() and open_by_handle_at() can fail for the same
errors as openat(2). In addition, they can fail with the errors
noted below.
name_to_handle_at() can fail with the following errors:
EFAULT pathname, mount_id, or handle points outside your
accessible address space.
EINVAL flags includes an invalid bit value.
EINVAL handle->handle_bytes is greater than MAX_HANDLE_SZ.
ENOENT pathname is an empty string, but AT_EMPTY_PATH was not
specified in flags.
ENOTDIR
The file descriptor supplied in dirfd does not refer to a
directory, and it is not the case that both flags includes
AT_EMPTY_PATH and pathname is an empty string.
EOPNOTSUPP
The filesystem does not support decoding of a pathname to
a file handle.
EOVERFLOW
The handle->handle_bytes value passed into the call was
too small. When this error occurs, handle->handle_bytes
is updated to indicate the required size for the handle.
open_by_handle_at() can fail with the following errors:
EBADF mount_fd is not an open file descriptor.
EBADF pathname is relative but dirfd is neither AT_FDCWD nor a
valid file descriptor.
EFAULT handle points outside your accessible address space.
EINVAL handle->handle_bytes is greater than MAX_HANDLE_SZ or is
equal to zero.
ELOOP handle refers to a symbolic link, but O_PATH was not
specified in flags.
EPERM The caller does not have the CAP_DAC_READ_SEARCH
capability.
ESTALE The specified handle is not valid for opening a file.
This error will occur if, for example, the file has been
deleted. This error can also occur if the handle was
acquired using the AT_HANDLE_FID flag and the filesystem
does not support open_by_handle_at().
FreeBSD has a broadly similar pair of system calls in the form of
getfh() and fhopen().
Linux.
Linux 2.6.39, glibc 2.14.
A file handle can be generated in one process using
name_to_handle_at() and later used in a different process that
calls open_by_handle_at().
Some filesystem don't support the translation of pathnames to
file handles, for example, /proc, /sys, and various network
filesystems. Some filesystems support the translation of
pathnames to file handles, but do not support using those file
handles in open_by_handle_at().
A file handle may become invalid ("stale") if a file is deleted,
or for other filesystem-specific reasons. Invalid handles are
notified by an ESTALE error from open_by_handle_at().
These system calls are designed for use by user-space file
servers. For example, a user-space NFS server might generate a
file handle and pass it to an NFS client. Later, when the client
wants to open the file, it could pass the handle back to the
server. This sort of functionality allows a user-space file
server to operate in a stateless fashion with respect to the
files it serves.
If pathname refers to a symbolic link and flags does not specify
AT_SYMLINK_FOLLOW, then name_to_handle_at() returns a handle for
the link (rather than the file to which it refers). The process
receiving the handle can later perform operations on the symbolic
link by converting the handle to a file descriptor using
open_by_handle_at() with the O_PATH flag, and then passing the
file descriptor as the dirfd argument in system calls such as
readlinkat(2) and fchownat(2).
Obtaining a persistent filesystem ID
The mount IDs in /proc/self/mountinfo can be reused as
filesystems are unmounted and mounted. Therefore, the mount ID
returned by name_to_handle_at() (in *mount_id) should not be
treated as a persistent identifier for the corresponding mounted
filesystem. However, an application can use the information in
the mountinfo record that corresponds to the mount ID to derive a
persistent identifier.
For example, one can use the device name in the fifth field of
the mountinfo record to search for the corresponding device UUID
via the symbolic links in /dev/disks/by-uuid. (A more
comfortable way of obtaining the UUID is to use the libblkid(3)
library.) That process can then be reversed, using the UUID to
look up the device name, and then obtaining the corresponding
mount point, in order to produce the mount_fd argument used by
open_by_handle_at().
The two programs below demonstrate the use of name_to_handle_at()
and open_by_handle_at(). The first program
(t_name_to_handle_at.c) uses name_to_handle_at() to obtain the
file handle and mount ID for the file specified in its command-
line argument; the handle and mount ID are written to standard
output.
The second program (t_open_by_handle_at.c) reads a mount ID and
file handle from standard input. The program then employs
open_by_handle_at() to open the file using that handle. If an
optional command-line argument is supplied, then the mount_fd
argument for open_by_handle_at() is obtained by opening the
directory named in that argument. Otherwise, mount_fd is
obtained by scanning /proc/self/mountinfo to find a record whose
mount ID matches the mount ID read from standard input, and the
mount directory specified in that record is opened. (These
programs do not deal with the fact that mount IDs are not
persistent.)
The following shell session demonstrates the use of these two
programs:
$ echo 'Can you please think about it?' > cecilia.txt
$ ./t_name_to_handle_at cecilia.txt > fh
$ ./t_open_by_handle_at < fh
open_by_handle_at: Operation not permitted
$ sudo ./t_open_by_handle_at < fh # Need CAP_SYS_ADMIN
Read 31 bytes
$ rm cecilia.txt
Now we delete and (quickly) re-create the file so that it has the
same content and (by chance) the same inode. Nevertheless,
open_by_handle_at() recognizes that the original file referred to
by the file handle no longer exists.
$ stat --printf="%i\n" cecilia.txt # Display inode number
4072121
$ rm cecilia.txt
$ echo 'Can you please think about it?' > cecilia.txt
$ stat --printf="%i\n" cecilia.txt # Check inode number
4072121
$ sudo ./t_open_by_handle_at < fh
open_by_handle_at: Stale NFS file handle
Program source: t_name_to_handle_at.c
#define _GNU_SOURCE
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
int mount_id, fhsize, flags, dirfd;
char *pathname;
struct file_handle *fhp;
if (argc != 2) {
fprintf(stderr, "Usage: %s pathname\n", argv[0]);
exit(EXIT_FAILURE);
}
pathname = argv[1];
/* Allocate file_handle structure. */
fhsize = sizeof(*fhp);
fhp = malloc(fhsize);
if (fhp == NULL)
err(EXIT_FAILURE, "malloc");
/* Make an initial call to name_to_handle_at() to discover
the size required for file handle. */
dirfd = AT_FDCWD; /* For name_to_handle_at() calls */
flags = 0; /* For name_to_handle_at() calls */
fhp->handle_bytes = 0;
if (name_to_handle_at(dirfd, pathname, fhp,
&mount_id, flags) != -1
|| errno != EOVERFLOW)
{
fprintf(stderr, "Unexpected result from name_to_handle_at()\n");
exit(EXIT_FAILURE);
}
/* Reallocate file_handle structure with correct size. */
fhsize = sizeof(*fhp) + fhp->handle_bytes;
fhp = realloc(fhp, fhsize); /* Copies fhp->handle_bytes */
if (fhp == NULL)
err(EXIT_FAILURE, "realloc");
/* Get file handle from pathname supplied on command line. */
if (name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags) == -1)
err(EXIT_FAILURE, "name_to_handle_at");
/* Write mount ID, file handle size, and file handle to stdout,
for later reuse by t_open_by_handle_at.c. */
printf("%d\n", mount_id);
printf("%u %d ", fhp->handle_bytes, fhp->handle_type);
for (size_t j = 0; j < fhp->handle_bytes; j++)
printf(" %02x", fhp->f_handle[j]);
printf("\n");
exit(EXIT_SUCCESS);
}
Program source: t_open_by_handle_at.c
#define _GNU_SOURCE
#include <err.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
/* Scan /proc/self/mountinfo to find the line whose mount ID matches
'mount_id'. (An easier way to do this is to install and use the
'libmount' library provided by the 'util-linux' project.)
Open the corresponding mount path and return the resulting file
descriptor. */
static int
open_mount_path_by_id(int mount_id)
{
int mi_mount_id, found;
char mount_path[PATH_MAX];
char *linep;
FILE *fp;
size_t lsize;
ssize_t nread;
fp = fopen("/proc/self/mountinfo", "r");
if (fp == NULL)
err(EXIT_FAILURE, "fopen");
found = 0;
linep = NULL;
while (!found) {
nread = getline(&linep, &lsize, fp);
if (nread == -1)
break;
nread = sscanf(linep, "%d %*d %*s %*s %s",
&mi_mount_id, mount_path);
if (nread != 2) {
fprintf(stderr, "Bad sscanf()\n");
exit(EXIT_FAILURE);
}
if (mi_mount_id == mount_id)
found = 1;
}
free(linep);
fclose(fp);
if (!found) {
fprintf(stderr, "Could not find mount point\n");
exit(EXIT_FAILURE);
}
return open(mount_path, O_RDONLY);
}
int
main(int argc, char *argv[])
{
int mount_id, fd, mount_fd, handle_bytes;
char buf[1000];
#define LINE_SIZE 100
char line1[LINE_SIZE], line2[LINE_SIZE];
char *nextp;
ssize_t nread;
struct file_handle *fhp;
if ((argc > 1 && strcmp(argv[1], "--help") == 0) || argc > 2) {
fprintf(stderr, "Usage: %s [mount-path]\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Standard input contains mount ID and file handle information:
Line 1: <mount_id>
Line 2: <handle_bytes> <handle_type> <bytes of handle in hex>
*/
if (fgets(line1, sizeof(line1), stdin) == NULL ||
fgets(line2, sizeof(line2), stdin) == NULL)
{
fprintf(stderr, "Missing mount_id / file handle\n");
exit(EXIT_FAILURE);
}
mount_id = atoi(line1);
handle_bytes = strtoul(line2, &nextp, 0);
/* Given handle_bytes, we can now allocate file_handle structure. */
fhp = malloc(sizeof(*fhp) + handle_bytes);
if (fhp == NULL)
err(EXIT_FAILURE, "malloc");
fhp->handle_bytes = handle_bytes;
fhp->handle_type = strtoul(nextp, &nextp, 0);
for (size_t j = 0; j < fhp->handle_bytes; j++)
fhp->f_handle[j] = strtoul(nextp, &nextp, 16);
/* Obtain file descriptor for mount point, either by opening
the pathname specified on the command line, or by scanning
/proc/self/mounts to find a mount that matches the 'mount_id'
that we received from stdin. */
if (argc > 1)
mount_fd = open(argv[1], O_RDONLY);
else
mount_fd = open_mount_path_by_id(mount_id);
if (mount_fd == -1)
err(EXIT_FAILURE, "opening mount fd");
/* Open file using handle and mount point. */
fd = open_by_handle_at(mount_fd, fhp, O_RDONLY);
if (fd == -1)
err(EXIT_FAILURE, "open_by_handle_at");
/* Try reading a few bytes from the file. */
nread = read(fd, buf, sizeof(buf));
if (nread == -1)
err(EXIT_FAILURE, "read");
printf("Read %zd bytes\n", nread);
exit(EXIT_SUCCESS);
}
open(2), libblkid(3), blkid(8), findfs(8), mount(8)
The libblkid and libmount documentation in the latest util-linux
release at ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 open_by_handle_at(2)
Pages that refer to this page: fanotify_init(2), fanotify_mark(2), open(2), perf_event_open(2), statx(2), syscalls(2), capabilities(7), fanotify(7), symlink(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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