open_by_handle_at (2)

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Copyright (c) 2014 by Michael Kerrisk <mtk.manpages@gmail.com>

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(The comments found at the beginning of the groff file "man2/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

SYNOPSIS

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <sys/types.h>
#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);

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_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 possible size for a file handle.) 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 EXAMPLE below).
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:

*

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().
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).

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().

RETURN VALUE

On success, name_to_handle_at() returns 0, and open_by_handle_at() returns a nonnegative file descriptor.
In the event of an error, both system calls return -1 and set errno to indicate the cause of the error.

ERRORS

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.

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 in glibc since version 2.14.

CONFORMING TO

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 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).

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().

EXAMPLE

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 <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

#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, j;
    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(struct file_handle) + 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("%d %d   ", fhp->handle_bytes, fhp->handle_type);
    for (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 <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

/* Scan /proc/self/mountinfo to find the line whose mount ID matches
   aqmount_idaq. (An easier way to do this is to install and use the
   aqlibmountaq library provided by the aqutil-linuxaq 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, j;
    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(struct file_handle) + handle_bytes);
    if (fhp == NULL)
        errExit("malloc");

    fhp->handle_bytes = handle_bytes;

    fhp->handle_type = strtoul(nextp, &nextp, 0);

    for (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 aqmount_idaq
       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);

    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 release at

COLOPHON

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