open_by_handle_at

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_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 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:

       *  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().
       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 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().

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/COLOPHON
       This page is part of release 5.10 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                              2020-11-01               OPEN_BY_HANDLE_AT(2)