#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/stat.h>
#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);
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 are needed only by a subsequent call to open_by_handle_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:
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.
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().
In the event of an error, both system calls return -1 and set errno to indicate the error.
name_to_handle_at() can fail with the following errors:
open_by_handle_at() can fail with the following errors:
FreeBSD has a broadly similar pair of system calls in the form of getfh() and openfh().
Some filesystem don't support the translation of pathnames to file handles, 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 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).
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 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
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int
main(int argc, char *argv[])
{
struct file_handle *fhp;
int mount_id, fhsize, flags, dirfd;
char *pathname;
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)
errExit("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)
errExit("realloc");
/* Get file handle from pathname supplied on command line. */
if (name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags) == -1)
errExit("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 (int j = 0; j < fhp->handle_bytes; j++)
printf(" %02x", fhp->f_handle[j]);
printf("\n");
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
/* 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)
{
char *linep;
size_t lsize;
char mount_path[PATH_MAX];
int mi_mount_id, found;
ssize_t nread;
FILE *fp;
fp = fopen("/proc/self/mountinfo", "r");
if (fp == NULL)
errExit("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[])
{
struct file_handle *fhp;
int mount_id, fd, mount_fd, handle_bytes;
ssize_t nread;
char buf[1000];
#define LINE_SIZE 100
char line1[LINE_SIZE], line2[LINE_SIZE];
char *nextp;
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)
errExit("malloc");
fhp->handle_bytes = handle_bytes;
fhp->handle_type = strtoul(nextp, &nextp, 0);
for (int 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)
errExit("opening mount fd");
/* Open file using handle and mount point. */
fd = open_by_handle_at(mount_fd, fhp, O_RDONLY);
if (fd == -1)
errExit("open_by_handle_at");
/* Try reading a few bytes from the file. */
nread = read(fd, buf, sizeof(buf));
if (nread == -1)
errExit("read");
printf("Read %zd bytes\n", nread);
The libblkid and libmount documentation in the latest util-linux release at