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-
-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.
-