Miscellaneous Device control operations for the autofs4 kernel module
  ====================================================================
  
  The problem
  ===========
  
  There is a problem with active restarts in autofs (that is to say
  restarting autofs when there are busy mounts).
  
  During normal operation autofs uses a file descriptor opened on the
  directory that is being managed in order to be able to issue control
  operations. Using a file descriptor gives ioctl operations access to
  autofs specific information stored in the super block. The operations
  are things such as setting an autofs mount catatonic, setting the
  expire timeout and requesting expire checks. As is explained below,
  certain types of autofs triggered mounts can end up covering an autofs
  mount itself which prevents us being able to use open(2) to obtain a
  file descriptor for these operations if we don't already have one open.
  
  Currently autofs uses "umount -l" (lazy umount) to clear active mounts
  at restart. While using lazy umount works for most cases, anything that
  needs to walk back up the mount tree to construct a path, such as
  getcwd(2) and the proc file system /proc/<pid>/cwd, no longer works
  because the point from which the path is constructed has been detached
  from the mount tree.
  
  The actual problem with autofs is that it can't reconnect to existing
  mounts. Immediately one thinks of just adding the ability to remount
  autofs file systems would solve it, but alas, that can't work. This is
  because autofs direct mounts and the implementation of "on demand mount
  and expire" of nested mount trees have the file system mounted directly
  on top of the mount trigger directory dentry.
  
  For example, there are two types of automount maps, direct (in the kernel
  module source you will see a third type called an offset, which is just
  a direct mount in disguise) and indirect.
  
  Here is a master map with direct and indirect map entries:
  
  /-      /etc/auto.direct
  /test   /etc/auto.indirect
  
  and the corresponding map files:
  
  /etc/auto.direct:
  
  /automount/dparse/g6  budgie:/autofs/export1
  /automount/dparse/g1  shark:/autofs/export1
  and so on.
  
  /etc/auto.indirect:
  
  g1    shark:/autofs/export1
  g6    budgie:/autofs/export1
  and so on.
  
  For the above indirect map an autofs file system is mounted on /test and
  mounts are triggered for each sub-directory key by the inode lookup
  operation. So we see a mount of shark:/autofs/export1 on /test/g1, for
  example.
  
  The way that direct mounts are handled is by making an autofs mount on
  each full path, such as /automount/dparse/g1, and using it as a mount
  trigger. So when we walk on the path we mount shark:/autofs/export1 "on
  top of this mount point". Since these are always directories we can
  use the follow_link inode operation to trigger the mount.
  
  But, each entry in direct and indirect maps can have offsets (making
  them multi-mount map entries).
  
  For example, an indirect mount map entry could also be:
  
  g1  \
     /        shark:/autofs/export5/testing/test \
     /s1      shark:/autofs/export/testing/test/s1 \
     /s2      shark:/autofs/export5/testing/test/s2 \
     /s1/ss1  shark:/autofs/export1 \
     /s2/ss2  shark:/autofs/export2
  
  and a similarly a direct mount map entry could also be:
  
  /automount/dparse/g1 \
      /       shark:/autofs/export5/testing/test \
      /s1     shark:/autofs/export/testing/test/s1 \
      /s2     shark:/autofs/export5/testing/test/s2 \
      /s1/ss1 shark:/autofs/export2 \
      /s2/ss2 shark:/autofs/export2
  
  One of the issues with version 4 of autofs was that, when mounting an
  entry with a large number of offsets, possibly with nesting, we needed
  to mount and umount all of the offsets as a single unit. Not really a
  problem, except for people with a large number of offsets in map entries.
  This mechanism is used for the well known "hosts" map and we have seen
  cases (in 2.4) where the available number of mounts are exhausted or
  where the number of privileged ports available is exhausted.
  
  In version 5 we mount only as we go down the tree of offsets and
  similarly for expiring them which resolves the above problem. There is
  somewhat more detail to the implementation but it isn't needed for the
  sake of the problem explanation. The one important detail is that these
  offsets are implemented using the same mechanism as the direct mounts
  above and so the mount points can be covered by a mount.
  
  The current autofs implementation uses an ioctl file descriptor opened
  on the mount point for control operations. The references held by the
  descriptor are accounted for in checks made to determine if a mount is
  in use and is also used to access autofs file system information held
  in the mount super block. So the use of a file handle needs to be
  retained.
  
  
  The Solution
  ============
  
  To be able to restart autofs leaving existing direct, indirect and
  offset mounts in place we need to be able to obtain a file handle
  for these potentially covered autofs mount points. Rather than just
  implement an isolated operation it was decided to re-implement the
  existing ioctl interface and add new operations to provide this
  functionality.
  
  In addition, to be able to reconstruct a mount tree that has busy mounts,
  the uid and gid of the last user that triggered the mount needs to be
  available because these can be used as macro substitution variables in
  autofs maps. They are recorded at mount request time and an operation
  has been added to retrieve them.
  
  Since we're re-implementing the control interface, a couple of other
  problems with the existing interface have been addressed. First, when
  a mount or expire operation completes a status is returned to the
  kernel by either a "send ready" or a "send fail" operation. The
  "send fail" operation of the ioctl interface could only ever send
  ENOENT so the re-implementation allows user space to send an actual
  status. Another expensive operation in user space, for those using
  very large maps, is discovering if a mount is present. Usually this
  involves scanning /proc/mounts and since it needs to be done quite
  often it can introduce significant overhead when there are many entries
  in the mount table. An operation to lookup the mount status of a mount
  point dentry (covered or not) has also been added.
  
  Current kernel development policy recommends avoiding the use of the
  ioctl mechanism in favor of systems such as Netlink. An implementation
  using this system was attempted to evaluate its suitability and it was
  found to be inadequate, in this case. The Generic Netlink system was
  used for this as raw Netlink would lead to a significant increase in
  complexity. There's no question that the Generic Netlink system is an
  elegant solution for common case ioctl functions but it's not a complete
  replacement probably because its primary purpose in life is to be a
  message bus implementation rather than specifically an ioctl replacement.
  While it would be possible to work around this there is one concern
  that lead to the decision to not use it. This is that the autofs
  expire in the daemon has become far to complex because umount
  candidates are enumerated, almost for no other reason than to "count"
  the number of times to call the expire ioctl. This involves scanning
  the mount table which has proved to be a big overhead for users with
  large maps. The best way to improve this is try and get back to the
  way the expire was done long ago. That is, when an expire request is
  issued for a mount (file handle) we should continually call back to
  the daemon until we can't umount any more mounts, then return the
  appropriate status to the daemon. At the moment we just expire one
  mount at a time. A Generic Netlink implementation would exclude this
  possibility for future development due to the requirements of the
  message bus architecture.
  
  
  autofs4 Miscellaneous Device mount control interface
  ====================================================
  
  The control interface is opening a device node, typically /dev/autofs.
  
  All the ioctls use a common structure to pass the needed parameter
  information and return operation results:
  
  struct autofs_dev_ioctl {
  	__u32 ver_major;
  	__u32 ver_minor;
  	__u32 size;             /* total size of data passed in
  				 * including this struct */
  	__s32 ioctlfd;          /* automount command fd */
  
  	__u32 arg1;             /* Command parameters */
  	__u32 arg2;
  
  	char path[0];
  };
  
  The ioctlfd field is a mount point file descriptor of an autofs mount
  point. It is returned by the open call and is used by all calls except
  the check for whether a given path is a mount point, where it may
  optionally be used to check a specific mount corresponding to a given
  mount point file descriptor, and when requesting the uid and gid of the
  last successful mount on a directory within the autofs file system.
  
  The fields arg1 and arg2 are used to communicate parameters and results of
  calls made as described below.
  
  The path field is used to pass a path where it is needed and the size field
  is used account for the increased structure length when translating the
  structure sent from user space.
  
  This structure can be initialized before setting specific fields by using
  the void function call init_autofs_dev_ioctl(struct autofs_dev_ioctl *).
  
  All of the ioctls perform a copy of this structure from user space to
  kernel space and return -EINVAL if the size parameter is smaller than
  the structure size itself, -ENOMEM if the kernel memory allocation fails
  or -EFAULT if the copy itself fails. Other checks include a version check
  of the compiled in user space version against the module version and a
  mismatch results in a -EINVAL return. If the size field is greater than
  the structure size then a path is assumed to be present and is checked to
  ensure it begins with a "/" and is NULL terminated, otherwise -EINVAL is
  returned. Following these checks, for all ioctl commands except
  AUTOFS_DEV_IOCTL_VERSION_CMD, AUTOFS_DEV_IOCTL_OPENMOUNT_CMD and
  AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD the ioctlfd is validated and if it is
  not a valid descriptor or doesn't correspond to an autofs mount point
  an error of -EBADF, -ENOTTY or -EINVAL (not an autofs descriptor) is
  returned.
  
  
  The ioctls
  ==========
  
  An example of an implementation which uses this interface can be seen
  in autofs version 5.0.4 and later in file lib/dev-ioctl-lib.c of the
  distribution tar available for download from kernel.org in directory
  /pub/linux/daemons/autofs/v5.
  
  The device node ioctl operations implemented by this interface are:
  
  
  AUTOFS_DEV_IOCTL_VERSION
  ------------------------
  
  Get the major and minor version of the autofs4 device ioctl kernel module
  implementation. It requires an initialized struct autofs_dev_ioctl as an
  input parameter and sets the version information in the passed in structure.
  It returns 0 on success or the error -EINVAL if a version mismatch is
  detected.
  
  
  AUTOFS_DEV_IOCTL_PROTOVER_CMD and AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD
  ------------------------------------------------------------------
  
  Get the major and minor version of the autofs4 protocol version understood
  by loaded module. This call requires an initialized struct autofs_dev_ioctl
  with the ioctlfd field set to a valid autofs mount point descriptor
  and sets the requested version number in structure field arg1. These
  commands return 0 on success or one of the negative error codes if
  validation fails.
  
  
  AUTOFS_DEV_IOCTL_OPENMOUNT and AUTOFS_DEV_IOCTL_CLOSEMOUNT
  ----------------------------------------------------------
  
  Obtain and release a file descriptor for an autofs managed mount point
  path. The open call requires an initialized struct autofs_dev_ioctl with
  the the path field set and the size field adjusted appropriately as well
  as the arg1 field set to the device number of the autofs mount. The
  device number can be obtained from the mount options shown in
  /proc/mounts. The close call requires an initialized struct
  autofs_dev_ioct with the ioctlfd field set to the descriptor obtained
  from the open call. The release of the file descriptor can also be done
  with close(2) so any open descriptors will also be closed at process exit.
  The close call is included in the implemented operations largely for
  completeness and to provide for a consistent user space implementation.
  
  
  AUTOFS_DEV_IOCTL_READY_CMD and AUTOFS_DEV_IOCTL_FAIL_CMD
  --------------------------------------------------------
  
  Return mount and expire result status from user space to the kernel.
  Both of these calls require an initialized struct autofs_dev_ioctl
  with the ioctlfd field set to the descriptor obtained from the open
  call and the arg1 field set to the wait queue token number, received
  by user space in the foregoing mount or expire request. The arg2 field
  is set to the status to be returned. For the ready call this is always
  0 and for the fail call it is set to the errno of the operation.
  
  
  AUTOFS_DEV_IOCTL_SETPIPEFD_CMD
  ------------------------------
  
  Set the pipe file descriptor used for kernel communication to the daemon.
  Normally this is set at mount time using an option but when reconnecting
  to a existing mount we need to use this to tell the autofs mount about
  the new kernel pipe descriptor. In order to protect mounts against
  incorrectly setting the pipe descriptor we also require that the autofs
  mount be catatonic (see next call).
  
  The call requires an initialized struct autofs_dev_ioctl with the
  ioctlfd field set to the descriptor obtained from the open call and
  the arg1 field set to descriptor of the pipe. On success the call
  also sets the process group id used to identify the controlling process
  (eg. the owning automount(8) daemon) to the process group of the caller.
  
  
  AUTOFS_DEV_IOCTL_CATATONIC_CMD
  ------------------------------
  
  Make the autofs mount point catatonic. The autofs mount will no longer
  issue mount requests, the kernel communication pipe descriptor is released
  and any remaining waits in the queue released.
  
  The call requires an initialized struct autofs_dev_ioctl with the
  ioctlfd field set to the descriptor obtained from the open call.
  
  
  AUTOFS_DEV_IOCTL_TIMEOUT_CMD
  ----------------------------
  
  Set the expire timeout for mounts within an autofs mount point.
  
  The call requires an initialized struct autofs_dev_ioctl with the
  ioctlfd field set to the descriptor obtained from the open call.
  
  
  AUTOFS_DEV_IOCTL_REQUESTER_CMD
  ------------------------------
  
  Return the uid and gid of the last process to successfully trigger a the
  mount on the given path dentry.
  
  The call requires an initialized struct autofs_dev_ioctl with the path
  field set to the mount point in question and the size field adjusted
  appropriately as well as the arg1 field set to the device number of the
  containing autofs mount. Upon return the struct field arg1 contains the
  uid and arg2 the gid.
  
  When reconstructing an autofs mount tree with active mounts we need to
  re-connect to mounts that may have used the original process uid and
  gid (or string variations of them) for mount lookups within the map entry.
  This call provides the ability to obtain this uid and gid so they may be
  used by user space for the mount map lookups.
  
  
  AUTOFS_DEV_IOCTL_EXPIRE_CMD
  ---------------------------
  
  Issue an expire request to the kernel for an autofs mount. Typically
  this ioctl is called until no further expire candidates are found.
  
  The call requires an initialized struct autofs_dev_ioctl with the
  ioctlfd field set to the descriptor obtained from the open call. In
  addition an immediate expire, independent of the mount timeout, can be
  requested by setting the arg1 field to 1. If no expire candidates can
  be found the ioctl returns -1 with errno set to EAGAIN.
  
  This call causes the kernel module to check the mount corresponding
  to the given ioctlfd for mounts that can be expired, issues an expire
  request back to the daemon and waits for completion.
  
  AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD
  ------------------------------
  
  Checks if an autofs mount point is in use.
  
  The call requires an initialized struct autofs_dev_ioctl with the
  ioctlfd field set to the descriptor obtained from the open call and
  it returns the result in the arg1 field, 1 for busy and 0 otherwise.
  
  
  AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD
  ---------------------------------
  
  Check if the given path is a mountpoint.
  
  The call requires an initialized struct autofs_dev_ioctl. There are two
  possible variations. Both use the path field set to the path of the mount
  point to check and the size field adjusted appropriately. One uses the
  ioctlfd field to identify a specific mount point to check while the other
  variation uses the path and optionally arg1 set to an autofs mount type.
  The call returns 1 if this is a mount point and sets arg1 to the device
  number of the mount and field arg2 to the relevant super block magic
  number (described below) or 0 if it isn't a mountpoint. In both cases
  the the device number (as returned by new_encode_dev()) is returned
  in field arg1.
  
  If supplied with a file descriptor we're looking for a specific mount,
  not necessarily at the top of the mounted stack. In this case the path
  the descriptor corresponds to is considered a mountpoint if it is itself
  a mountpoint or contains a mount, such as a multi-mount without a root
  mount. In this case we return 1 if the descriptor corresponds to a mount
  point and and also returns the super magic of the covering mount if there
  is one or 0 if it isn't a mountpoint.
  
  If a path is supplied (and the ioctlfd field is set to -1) then the path
  is looked up and is checked to see if it is the root of a mount. If a
  type is also given we are looking for a particular autofs mount and if
  a match isn't found a fail is returned. If the the located path is the
  root of a mount 1 is returned along with the super magic of the mount
  or 0 otherwise.