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man:name_to_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_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
     referred 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
     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 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()
     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:
     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 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
     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  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_han-
         dle_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_han-
     dle_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
     release at

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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