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NAME

       mount_setattr - change properties of a mount or mount tree

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <linux/fcntl.h> /* Definition of AT_* constants */
       #include <linux/mount.h> /* Definition of MOUNT_ATTR_* constants */
       #include <sys/syscall.h> /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_mount_setattr, int dirfd, const char *pathname,
                   unsigned int flags, struct mount_attr *attr, size_t size);

       Note: glibc provides no wrapper for mount_setattr(), necessitating the use of syscall(2).

DESCRIPTION

       The mount_setattr() system call changes the mount properties of a mount or an entire mount
       tree.  If pathname is a  relative  pathname,  then  it  is  interpreted  relative  to  the
       directory  referred  to  by  the  file  descriptor  dirfd.   If dirfd is the special value
       AT_FDCWD, then pathname is interpreted relative to the current working  directory  of  the
       calling process.  If pathname is the empty string and AT_EMPTY_PATH is specified in flags,
       then the mount properties of the mount identified by dirfd are  changed.   (See  openat(2)
       for an explanation of why the dirfd argument is useful.)

       The  mount_setattr() system call uses an extensible structure (struct mount_attr) to allow
       for future extensions.  Any non-flag extensions to mount_setattr() will be implemented  as
       new  fields  appended to the this structure, with a zero value in a new field resulting in
       the kernel behaving as though that extension field was not present.  Therefore, the caller
       must zero-fill this structure on initialization.  See the "Extensibility" subsection under
       NOTES for more details.

       The size argument should usually be specified as sizeof(struct mount_attr).   However,  if
       the  caller  is using a kernel that supports an extended struct mount_attr, but the caller
       does not intend to make use of these features, it is possible  to  pass  the  size  of  an
       earlier  version  of  the structure together with the extended structure.  This allows the
       kernel to not copy later parts of the  structure  that  aren't  used  anyway.   With  each
       extension  that changes the size of struct mount_attr, the kernel will expose a definition
       of the form MOUNT_ATTR_SIZE_VERnumber.  For example, the macro for the size of the initial
       version of struct mount_attr is MOUNT_ATTR_SIZE_VER0.

       The  flags  argument can be used to alter the pathname resolution behavior.  The supported
       values are:

       AT_EMPTY_PATH
              If pathname is the empty string, change the mount properties on dirfd itself.

       AT_RECURSIVE
              Change the mount properties of the entire mount tree.

       AT_SYMLINK_NOFOLLOW
              Don't follow trailing symbolic links.

       AT_NO_AUTOMOUNT
              Don't trigger automounts.

       The attr argument of mount_setattr() is a structure of the following form:

           struct mount_attr {
               __u64 attr_set;     /* Mount properties to set */
               __u64 attr_clr;     /* Mount properties to clear */
               __u64 propagation;  /* Mount propagation type */
               __u64 userns_fd;    /* User namespace file descriptor */
           };

       The attr_set and attr_clr members are used  to  specify  the  mount  properties  that  are
       supposed  to  be set or cleared for a mount or mount tree.  Flags set in attr_set enable a
       property on a mount or mount tree, and flags set in attr_clr  remove  a  property  from  a
       mount or mount tree.

       When  changing  mount  properties,  the kernel will first clear the flags specified in the
       attr_clr field, and then set the flags specified in  the  attr_set  field.   For  example,
       these settings:

           struct mount_attr attr = {
               .attr_clr = MOUNT_ATTR_NOEXEC | MOUNT_ATTR_NODEV,
               .attr_set = MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID,
           };

       are equivalent to the following steps:

           unsigned int current_mnt_flags = mnt->mnt_flags;

           /*
            * Clear all flags set in .attr_clr,
            * clearing MOUNT_ATTR_NOEXEC and MOUNT_ATTR_NODEV.
            */
           current_mnt_flags &= ~attr->attr_clr;

           /*
            * Now set all flags set in .attr_set,
            * applying MOUNT_ATTR_RDONLY and MOUNT_ATTR_NOSUID.
            */
           current_mnt_flags |= attr->attr_set;

           mnt->mnt_flags = current_mnt_flags;

       As  a  result  of  this  change,  the mount or mount tree (a) is read-only; (b) blocks the
       execution of set-user-ID and set-group-ID programs; (c) allows execution of programs;  and
       (d) allows access to devices.

       Multiple  changes  with  the  same  set  of  flags  requested in attr_clr and attr_set are
       guaranteed to be idempotent after the changes have been applied.

       The following mount attributes can be specified in the attr_set or attr_clr fields:

       MOUNT_ATTR_RDONLY
              If set in attr_set, makes the mount read-only.  If set  in  attr_clr,  removes  the
              read-only setting if set on the mount.

       MOUNT_ATTR_NOSUID
              If  set in attr_set, causes the mount not to honor the set-user-ID and set-group-ID
              mode bits and file capabilities when  executing  programs.   If  set  in  attr_clr,
              clears  the  set-user-ID,  set-group-ID,  and file capability restriction if set on
              this mount.

       MOUNT_ATTR_NODEV
              If set in attr_set, prevents access to devices on this mount.  If set in  attr_clr,
              removes the restriction that prevented accessing devices on this mount.

       MOUNT_ATTR_NOEXEC
              If set in attr_set, prevents executing programs on this mount.  If set in attr_clr,
              removes the restriction that prevented executing programs on this mount.

       MOUNT_ATTR_NOSYMFOLLOW
              If set in attr_set, prevents following symbolic links on this  mount.   If  set  in
              attr_clr,  removes  the restriction that prevented following symbolic links on this
              mount.

       MOUNT_ATTR_NODIRATIME
              If set in attr_set, prevents updating access time for directories  on  this  mount.
              If set in attr_clr, removes the restriction that prevented updating access time for
              directories.  Note that MOUNT_ATTR_NODIRATIME can be combined  with  other  access-
              time  settings  and  is  implied  by  the  noatime  setting.  All other access-time
              settings are mutually exclusive.

       MOUNT_ATTR__ATIME - changing access-time settings
              The access-time values listed below are an  enumeration  that  includes  the  value
              zero,  expressed  in  the  bits defined by the mask MOUNT_ATTR__ATIME.  Even though
              these bits are an enumeration (in  contrast  to  the  other  mount  flags  such  as
              MOUNT_ATTR_NOEXEC),  they  are  nonetheless  passed  in  attr_set  and attr_clr for
              consistency with fsmount(2), which introduced this behavior.

              Note that, since the access-time values are an enumeration rather than bit  values,
              a  caller  wanting  to  transition to a different access-time setting cannot simply
              specify  the   access-time   setting   in   attr_set,   but   must   also   include
              MOUNT_ATTR__ATIME   in   the   attr_clr   field.    The  kernel  will  verify  that
              MOUNT_ATTR__ATIME isn't partially set in attr_clr (i.e., either  all  bits  in  the
              MOUNT_ATTR__ATIME  bit  field  are  either set or clear), and that attr_set doesn't
              have any access-time bits set if MOUNT_ATTR__ATIME isn't set in attr_clr.

              MOUNT_ATTR_RELATIME
                     When a file is accessed via this mount, update the file's last  access  time
                     (atime)  only  if  the  current  value of atime is less than or equal to the
                     file's last modification time (mtime) or last status change time (ctime).

                     To  enable  this  access-time  setting   on   a   mount   or   mount   tree,
                     MOUNT_ATTR_RELATIME  must  be  set in attr_set and MOUNT_ATTR__ATIME must be
                     set in the attr_clr field.

              MOUNT_ATTR_NOATIME
                     Do not update access times for (all types of) files on this mount.

                     To  enable  this  access-time  setting   on   a   mount   or   mount   tree,
                     MOUNT_ATTR_NOATIME must be set in attr_set and MOUNT_ATTR__ATIME must be set
                     in the attr_clr field.

              MOUNT_ATTR_STRICTATIME
                     Always update the last access time (atime) when files are accessed  on  this
                     mount.

                     To   enable   this   access-time   setting   on   a  mount  or  mount  tree,
                     MOUNT_ATTR_STRICTATIME must be set in attr_set and MOUNT_ATTR__ATIME must be
                     set in the attr_clr field.

       MOUNT_ATTR_IDMAP
              If  set  in attr_set, creates an ID-mapped mount.  The ID mapping is taken from the
              user namespace specified in userns_fd and attached to the mount.

              Since it is not supported to change the ID mapping of a mount after it has been  ID
              mapped, it is invalid to specify MOUNT_ATTR_IDMAP in attr_clr.

              For further details, see the subsection "ID-mapped mounts" under NOTES.

       The  propagation field is used to specify the propagation type of the mount or mount tree.
       This field either has the value zero, meaning leave the propagation type unchanged, or  it
       has one of the following values:

       MS_PRIVATE
              Turn all mounts into private mounts.

       MS_SHARED
              Turn all mounts into shared mounts.

       MS_SLAVE
              Turn all mounts into dependent mounts.

       MS_UNBINDABLE
              Turn all mounts into unbindable mounts.

       For further details on the above propagation types, see mount_namespaces(7).

RETURN VALUE

       On  success,  mount_setattr()  returns zero.  On error, -1 is returned and errno is set to
       indicate the cause of the error.

ERRORS

       EBADF  pathname is relative but dirfd is neither AT_FDCWD nor a valid file descriptor.

       EBADF  userns_fd is not a valid file descriptor.

       EBUSY  The caller tried to change the mount to  MOUNT_ATTR_RDONLY,  but  the  mount  still
              holds files open for writing.

       EBUSY  The  caller  tried  to  create  an  ID-mapped  mount  raising  MOUNT_ATTR_IDMAP and
              specifying userns_fd but the mount still holds files open for writing.

       EINVAL The pathname specified via the dirfd  and  pathname  arguments  to  mount_setattr()
              isn't a mount point.

       EINVAL An unsupported value was set in flags.

       EINVAL An unsupported value was specified in the attr_set field of mount_attr.

       EINVAL An unsupported value was specified in the attr_clr field of mount_attr.

       EINVAL An unsupported value was specified in the propagation field of mount_attr.

       EINVAL More  than  one of MS_SHARED, MS_SLAVE, MS_PRIVATE, or MS_UNBINDABLE was set in the
              propagation field of mount_attr.

       EINVAL An  access-time   setting   was   specified   in   the   attr_set   field   without
              MOUNT_ATTR__ATIME being set in the attr_clr field.

       EINVAL MOUNT_ATTR_IDMAP was specified in attr_clr.

       EINVAL A file descriptor value was specified in userns_fd which exceeds INT_MAX.

       EINVAL A  valid  file descriptor value was specified in userns_fd, but the file descriptor
              did not refer to a user namespace.

       EINVAL The underlying filesystem does not support ID-mapped mounts.

       EINVAL The mount that is to be ID mapped is not a detached mount; that is, the  mount  has
              not previously been visible in a mount namespace.

       EINVAL A   partial   access-time   setting   was   specified   in   attr_clr   instead  of
              MOUNT_ATTR__ATIME being set.

       EINVAL The mount is located outside the caller's mount namespace.

       EINVAL The underlying filesystem has been mounted in a mount namespace that is owned by  a
              noninitial user namespace

       ENOENT A pathname was empty or had a nonexistent component.

       ENOMEM When  changing  mount  propagation  to  MS_SHARED,  a new peer group ID needs to be
              allocated for all mounts without a peer  group  ID  set.   This  allocation  failed
              because there was not enough memory to allocate the relevant internal structures.

       ENOSPC When  changing  mount  propagation  to  MS_SHARED,  a new peer group ID needs to be
              allocated for all mounts without a peer  group  ID  set.   This  allocation  failed
              because the kernel has run out of IDs.

       EPERM  One  of  the  mounts  had  at  least  one  of MOUNT_ATTR_NOATIME, MOUNT_ATTR_NODEV,
              MOUNT_ATTR_NODIRATIME, MOUNT_ATTR_NOEXEC, MOUNT_ATTR_NOSUID,  or  MOUNT_ATTR_RDONLY
              set and the flag is locked.  Mount attributes become locked on a mount if:

              •  A  new  mount  or  mount  tree  is created causing mount propagation across user
                 namespaces (i.e., propagation to a mount namespace owned  by  a  different  user
                 namespace).   The  kernel  will  lock  the aforementioned flags to prevent these
                 sensitive properties from being altered.

              •  A new mount and user namespace pair is created.  This happens for  example  when
                 specifying  CLONE_NEWUSER  |  CLONE_NEWNS in unshare(2), clone(2), or clone3(2).
                 The aforementioned flags become locked in the new  mount  namespace  to  prevent
                 sensitive  mount  properties  from being altered.  Since the newly created mount
                 namespace will be owned by the newly created user namespace, a  calling  process
                 that  is  privileged  in  the  new  user  namespace would—in the absence of such
                 locking—be able to alter sensitive mount properties (e.g., to  remount  a  mount
                 that was marked read-only as read-write in the new mount namespace).

       EPERM  A  valid  file descriptor value was specified in userns_fd, but the file descriptor
              refers to the initial user namespace.

       EPERM  An attempt was made to add an ID mapping to a mount that is already ID mapped.

       EPERM  The caller does not have CAP_SYS_ADMIN in the initial user namespace.

VERSIONS

       mount_setattr() first appeared in Linux 5.12.

STANDARDS

       mount_setattr() is Linux-specific.

NOTES

   ID-mapped mounts
       Creating an ID-mapped mount makes it possible to change the ownership of all files located
       under a mount.  Thus, ID-mapped mounts make it possible to change ownership in a temporary
       and localized way.  It is a localized change because the  ownership  changes  are  visible
       only  via a specific mount.  All other users and locations where the filesystem is exposed
       are unaffected.  It is a temporary change because the ownership changes are  tied  to  the
       lifetime of the mount.

       Whenever  callers  interact with the filesystem through an ID-mapped mount, the ID mapping
       of the mount will be applied to user and group IDs  associated  with  filesystem  objects.
       This  encompasses  the  user  and  group IDs associated with inodes and also the following
       xattr(7) keys:

       •  security.capability, whenever filesystem capabilities are stored  or  returned  in  the
          VFS_CAP_REVISION_3  format, which stores a root user ID alongside the capabilities (see
          capabilities(7)).

       •  system.posix_acl_access and system.posix_acl_default, whenever user IDs  or  group  IDs
          are stored in ACL_USER or ACL_GROUP entries.

       The following conditions must be met in order to create an ID-mapped mount:

       •  The  caller must have the CAP_SYS_ADMIN capability in the user namespace the filesystem
          was mounted in.

       •  The underlying filesystem must support  ID-mapped  mounts.   Currently,  the  following
          filesystems support ID-mapped mounts:

          •  xfs(5) (since Linux 5.12)
          •  ext4(5) (since Linux 5.12)
          •  FAT (since Linux 5.12)
          •  btrfs(5) (since Linux 5.15)
          •  ntfs3 (since Linux 5.15)
          •  f2fs (since Linux 5.18)
          •  erofs (since Linux 5.19)
          •  overlayfs (ID-mapped lower and upper layers supported since Linux 5.19)

       •  The  mount  must  not already be ID-mapped.  This also implies that the ID mapping of a
          mount cannot be altered.

       •  The mount must be a detached mount; that is, it must have been created by calling

       •  The mount must not have any writers.  open_tree(2) with the OPEN_TREE_CLONE flag and it
          must  not  already have been visible in a mount namespace.  (To put things another way:
          the mount must not have been attached to the filesystem hierarchy with  a  system  call
          such as move_mount(2).)

       ID  mappings  can  be  created for user IDs, group IDs, and project IDs.  An ID mapping is
       essentially a mapping of a range of user or group IDs into another or the  same  range  of
       user  or  group  IDs.   ID mappings are written to map files as three numbers separated by
       white space.  The first two numbers specify the starting user or group ID in each  of  the
       two  user  namespaces.   The  third  number  specifies  the  range of the ID mapping.  For
       example, a mapping for user IDs such as "1000 1001 1" would indicate that user ID 1000  in
       the  caller's  user  namespace  is  mapped to user ID 1001 in its ancestor user namespace.
       Since the map range is 1, only user ID 1000 is mapped.

       It is possible to specify up to 340 ID mappings for each ID mapping type.  If any user IDs
       or group IDs are not mapped, all files owned by that unmapped user or group ID will appear
       as being owned by the overflow user ID or overflow group ID respectively.

       Further details on setting up ID mappings can be found in user_namespaces(7).

       In the common case, the user namespace passed in userns_fd (together with MOUNT_ATTR_IDMAP
       in  attr_set)  to create an ID-mapped mount will be the user namespace of a container.  In
       other scenarios it will be a dedicated user  namespace  associated  with  a  user's  login
       session  as is the case for portable home directories in systemd-homed.service(8)).  It is
       also perfectly fine to create a dedicated user namespace for the  sake  of  ID  mapping  a
       mount.

       ID-mapped mounts can be useful in the following and a variety of other scenarios:

       •  Sharing files or filesystems between multiple users or multiple machines, especially in
          complex scenarios.  For example, ID-mapped mounts are used to implement  portable  home
          directories  in  systemd-homed.service(8),  where  they  allow users to move their home
          directory to an external storage device and use it on multiple computers where they are
          assigned  different  user  IDs  and  group  IDs.  This effectively makes it possible to
          assign random user IDs and group IDs at login time.

       •  Sharing files or filesystems from the host with unprivileged containers.  This allows a
          user to avoid having to change ownership permanently through chown(2).

       •  ID  mapping  a  container's  root  filesystem.   Users  don't  need to change ownership
          permanently through chown(2).  Especially for large root  filesystems,  using  chown(2)
          can be prohibitively expensive.

       •  Sharing files or filesystems between containers with non-overlapping ID mappings.

       •  Implementing  discretionary  access (DAC) permission checking for filesystems lacking a
          concept of ownership.

       •  Efficiently changing ownership on a per-mount basis.  In contrast to chown(2), changing
          ownership  of  large  sets  of  files  is instantaneous with ID-mapped mounts.  This is
          especially useful when ownership of an entire root filesystem of a virtual  machine  or
          container  is  to  be  changed  as  mentioned  above.   With ID-mapped mounts, a single
          mount_setattr() system call will be sufficient to change the ownership of all files.

       •  Taking the current ownership into account.  ID mappings specify precisely what  a  user
          or  group ID is supposed to be mapped to.  This contrasts with the chown(2) system call
          which cannot by itself take the current ownership of the files it changes into account.
          It simply changes the ownership to the specified user ID and group ID.

       •  Locally  and  temporarily  restricted  ownership  changes.   ID-mapped  mounts  make it
          possible to change ownership locally, restricting the  ownership  changes  to  specific
          mounts,  and  temporarily  as  the  ownership  changes  only apply as long as the mount
          exists.  By contrast, changing ownership via  the  chown(2)  system  call  changes  the
          ownership globally and permanently.

   Extensibility
       In  order  to  allow  for  future  extensibility,  mount_setattr() requires the user-space
       application to specify the size of the  mount_attr  structure  that  it  is  passing.   By
       providing  this  information, it is possible for mount_setattr() to provide both forwards-
       and backwards-compatibility, with size acting as an implicit version number.  (Because new
       extension  fields will always be appended, the structure size will always increase.)  This
       extensibility design is very similar  to  other  system  calls  such  as  perf_setattr(2),
       perf_event_open(2), clone3(2) and openat2(2).

       Let usize be the size of the structure as specified by the user-space application, and let
       ksize be the size of the structure which the kernel supports, then there are  three  cases
       to consider:

       •  If ksize equals usize, then there is no version mismatch and attr can be used verbatim.

       •  If  ksize  is  larger  than usize, then there are some extension fields that the kernel
          supports which the user-space application is unaware of.  Because a zero value  in  any
          added  extension field signifies a no-op, the kernel treats all of the extension fields
          not provided by the user-space  application  as  having  zero  values.   This  provides
          backwards-compatibility.

       •  If  ksize  is  smaller than usize, then there are some extension fields which the user-
          space application is aware of but which the  kernel  does  not  support.   Because  any
          extension field must have its zero values signify a no-op, the kernel can safely ignore
          the unsupported extension fields if they are all zero.  If  any  unsupported  extension
          fields  are  non-zero,  then  -1  is returned and errno is set to E2BIG.  This provides
          forwards-compatibility.

       Because the definition of struct mount_attr may change in  the  future  (with  new  fields
       being  added  when  system  headers are updated), user-space applications should zero-fill
       struct mount_attr to ensure that recompiling the program with new headers will not  result
       in spurious errors at runtime.  The simplest way is to use a designated initializer:

           struct mount_attr attr = {
               .attr_set = MOUNT_ATTR_RDONLY,
               .attr_clr = MOUNT_ATTR_NODEV
           };

       Alternatively, the structure can be zero-filled using memset(3) or similar functions:

           struct mount_attr attr;
           memset(&attr, 0, sizeof(attr));
           attr.attr_set = MOUNT_ATTR_RDONLY;
           attr.attr_clr = MOUNT_ATTR_NODEV;

       A  user-space  application  that  wishes  to determine which extensions the running kernel
       supports can do so by conducting a binary search on size with a structure which has  every
       byte nonzero (to find the largest value which doesn't produce an error of E2BIG).

EXAMPLES

       /*
        * This program allows the caller to create a new detached mount
        * and set various properties on it.
        */
       #define _GNU_SOURCE
       #include <err.h>
       #include <fcntl.h>
       #include <getopt.h>
       #include <linux/mount.h>
       #include <linux/types.h>
       #include <stdbool.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/syscall.h>
       #include <unistd.h>

       static inline int
       mount_setattr(int dirfd, const char *pathname, unsigned int flags,
                     struct mount_attr *attr, size_t size)
       {
           return syscall(SYS_mount_setattr, dirfd, pathname, flags,
                          attr, size);
       }

       static inline int
       open_tree(int dirfd, const char *filename, unsigned int flags)
       {
           return syscall(SYS_open_tree, dirfd, filename, flags);
       }

       static inline int
       move_mount(int from_dirfd, const char *from_pathname,
                  int to_dirfd, const char *to_pathname, unsigned int flags)
       {
           return syscall(SYS_move_mount, from_dirfd, from_pathname,
                          to_dirfd, to_pathname, flags);
       }

       static const struct option longopts[] = {
           {"map-mount",       required_argument,  NULL,  'a'},
           {"recursive",       no_argument,        NULL,  'b'},
           {"read-only",       no_argument,        NULL,  'c'},
           {"block-setid",     no_argument,        NULL,  'd'},
           {"block-devices",   no_argument,        NULL,  'e'},
           {"block-exec",      no_argument,        NULL,  'f'},
           {"no-access-time",  no_argument,        NULL,  'g'},
           { NULL,             0,                  NULL,   0 },
       };

       int
       main(int argc, char *argv[])
       {
           int                fd_userns = -1;
           int                fd_tree;
           int                index = 0;
           int                ret;
           bool               recursive = false;
           const char         *source;
           const char         *target;
           struct mount_attr  *attr = &(struct mount_attr){};

           while ((ret = getopt_long_only(argc, argv, "",
                                          longopts, &index)) != -1) {
               switch (ret) {
               case 'a':
                   fd_userns = open(optarg, O_RDONLY | O_CLOEXEC);
                   if (fd_userns == -1)
                       err(EXIT_FAILURE, "open(%s)", optarg);
                   break;
               case 'b':
                   recursive = true;
                   break;
               case 'c':
                   attr->attr_set |= MOUNT_ATTR_RDONLY;
                   break;
               case 'd':
                   attr->attr_set |= MOUNT_ATTR_NOSUID;
                   break;
               case 'e':
                   attr->attr_set |= MOUNT_ATTR_NODEV;
                   break;
               case 'f':
                   attr->attr_set |= MOUNT_ATTR_NOEXEC;
                   break;
               case 'g':
                   attr->attr_set |= MOUNT_ATTR_NOATIME;
                   attr->attr_clr |= MOUNT_ATTR__ATIME;
                   break;
               default:
                   errx(EXIT_FAILURE, "Invalid argument specified");
               }
           }

           if ((argc - optind) < 2)
               errx(EXIT_FAILURE, "Missing source or target mount point");

           source = argv[optind];
           target = argv[optind + 1];

           /* In the following, -1 as the 'dirfd' argument ensures that
              open_tree() fails if 'source' is not an absolute pathname. */

           fd_tree = open_tree(-1, source,
                               OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC |
                               AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0));
           if (fd_tree == -1)
               err(EXIT_FAILURE, "open(%s)", source);

           if (fd_userns >= 0) {
               attr->attr_set  |= MOUNT_ATTR_IDMAP;
               attr->userns_fd = fd_userns;
           }

           ret = mount_setattr(fd_tree, "",
                               AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0),
                               attr, sizeof(struct mount_attr));
           if (ret == -1)
               err(EXIT_FAILURE, "mount_setattr");

           close(fd_userns);

           /* In the following, -1 as the 'to_dirfd' argument ensures that
              open_tree() fails if 'target' is not an absolute pathname. */

           ret = move_mount(fd_tree, "", -1, target,
                            MOVE_MOUNT_F_EMPTY_PATH);
           if (ret == -1)
               err(EXIT_FAILURE, "move_mount() to %s", target);

           close(fd_tree);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       newgidmap(1),  newuidmap(1),  clone(2),  mount(2),  unshare(2),  proc(5), capabilities(7),
       mount_namespaces(7), user_namespaces(7), xattr(7)