open_by_handle_at(2) System Calls Manual open_by_handle_at(2)
NAME
name_to_handle_at, open_by_handle_at - obtain handle for a pathname and
open file via a handle
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
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_han-
dle_at() opens the file corresponding to a handle returned by a previ-
ous 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 argu-
ments. 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 nor-
mally 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 are needed only by a subsequent call to open_by_han-
dle_at().
The flags argument is a bit mask constructed by ORing together zero or
more of AT_EMPTY_PATH and AT_SYMLINK_FOLLOW, described below.
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 han-
dle 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 direc-
tory, 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 re-
ferred 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 au-
tomount 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 han-
dle, 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, direc-
tory, 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().
RETURN VALUE
On success, name_to_handle_at() returns 0, and open_by_handle_at() re-
turns a file descriptor (a nonnegative integer).
In the event of an error, both system calls return -1 and set errno to
indicate the error.
ERRORS
name_to_handle_at() and open_by_handle_at() can fail for the same er-
rors 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 ad-
dress 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 direc-
tory, 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. This error will occur if,
for example, the file has been deleted.
VERSIONS
These system calls first appeared in Linux 2.6.39. Library support is
provided since glibc 2.14.
STANDARDS
These system calls are nonstandard Linux extensions.
FreeBSD has a broadly similar pair of system calls in the form of
getfh() and openfh().
NOTES
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 han-
dles, for example, /proc, /sys, and various network filesystems.
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 func-
tionality 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 con-
verting 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 ar-
gument 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 persis-
tent identifier for the corresponding mounted filesystem. However, an
application can use the information in the mountinfo record that corre-
sponds 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 ob-
taining 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().
EXAMPLES
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_han-
dle_at() to open the file using that handle. If an optional command-
line argument is supplied, then the mount_fd argument for open_by_han-
dle_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 per-
sistent.)
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_han-
dle_at() recognizes that the original file referred to by the file han-
dle 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 <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);
}
SEE ALSO
open(2), libblkid(3), blkid(8), findfs(8), mount(8)
The libblkid and libmount documentation in the latest util-linux re-
lease at ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩
Linux man-pages 6.03 2023-02-05 open_by_handle_at(2)
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