open_by_handle_at(2)



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 <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  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 EXAMPLE 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 path 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
       path.

   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 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 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.

       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
       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().

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
           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
          '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, 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");

           /* 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 re-
       lease at <https://www.kernel.org/pub/linux/utils/util-linux/>

COLOPHON
       This page is part of release 4.16 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15              OPEN_BY_HANDLE_AT(2)

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