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OPEN_BY_HANDLE_AT(2)                                                    Linux Programmer's 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

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_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 han‐
       dle 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  cor‐
       rect  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 filesys‐
       tem 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:

       *  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 de‐
          scriptors" 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_FD‐
          CWD, 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().

RETURN VALUE
       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.

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.

       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 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().

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_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;
           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");

           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
          '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);

           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 ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩

Linux                                                                          2021-08-27                                                           OPEN_BY_HANDLE_AT(2)