open_by_handle_at(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

OPEN_BY_HANDLE_AT(2)    Linux Programmer's Manual   OPEN_BY_HANDLE_AT(2)

NAME         top

       name_to_handle_at, open_by_handle_at - obtain handle for a
       pathname and open file via a handle

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       These system calls first appeared in Linux 2.6.39.  Library
       support is provided in glibc since version 2.14.

CONFORMING TO         top

       These system calls are nonstandard Linux extensions.

       FreeBSD has a broadly similar pair of system calls in the form of
       getfh() and openfh().

NOTES         top

       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         top

       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         top

       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         top

       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)

Pages that refer to this page: fanotify_init(2)open(2)perf_event_open(2)syscalls(2)capabilities(7)fanotify(7)symlink(7)