name_to_handle_at

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_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 possible size for a file handle.)  Upon success-
       ful 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_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.

       *  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().

       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 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()
       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:
              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.

       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 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

   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
       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_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

       #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 */

       #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);

           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 '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");

           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/>

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-06-13              OPEN_BY_HANDLE_AT(2)
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