411 lines
18 KiB
ReStructuredText
411 lines
18 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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====================================================================
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Miscellaneous Device control operations for the autofs kernel module
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====================================================================
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The problem
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===========
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There is a problem with active restarts in autofs (that is to say
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restarting autofs when there are busy mounts).
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During normal operation autofs uses a file descriptor opened on the
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directory that is being managed in order to be able to issue control
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operations. Using a file descriptor gives ioctl operations access to
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autofs specific information stored in the super block. The operations
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are things such as setting an autofs mount catatonic, setting the
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expire timeout and requesting expire checks. As is explained below,
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certain types of autofs triggered mounts can end up covering an autofs
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mount itself which prevents us being able to use open(2) to obtain a
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file descriptor for these operations if we don't already have one open.
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Currently autofs uses "umount -l" (lazy umount) to clear active mounts
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at restart. While using lazy umount works for most cases, anything that
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needs to walk back up the mount tree to construct a path, such as
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getcwd(2) and the proc file system /proc/<pid>/cwd, no longer works
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because the point from which the path is constructed has been detached
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from the mount tree.
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The actual problem with autofs is that it can't reconnect to existing
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mounts. Immediately one thinks of just adding the ability to remount
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autofs file systems would solve it, but alas, that can't work. This is
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because autofs direct mounts and the implementation of "on demand mount
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and expire" of nested mount trees have the file system mounted directly
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on top of the mount trigger directory dentry.
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For example, there are two types of automount maps, direct (in the kernel
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module source you will see a third type called an offset, which is just
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a direct mount in disguise) and indirect.
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Here is a master map with direct and indirect map entries::
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/- /etc/auto.direct
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/test /etc/auto.indirect
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and the corresponding map files::
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/etc/auto.direct:
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/automount/dparse/g6 budgie:/autofs/export1
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/automount/dparse/g1 shark:/autofs/export1
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and so on.
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/etc/auto.indirect::
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g1 shark:/autofs/export1
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g6 budgie:/autofs/export1
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and so on.
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For the above indirect map an autofs file system is mounted on /test and
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mounts are triggered for each sub-directory key by the inode lookup
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operation. So we see a mount of shark:/autofs/export1 on /test/g1, for
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example.
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The way that direct mounts are handled is by making an autofs mount on
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each full path, such as /automount/dparse/g1, and using it as a mount
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trigger. So when we walk on the path we mount shark:/autofs/export1 "on
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top of this mount point". Since these are always directories we can
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use the follow_link inode operation to trigger the mount.
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But, each entry in direct and indirect maps can have offsets (making
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them multi-mount map entries).
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For example, an indirect mount map entry could also be::
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g1 \
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/ shark:/autofs/export5/testing/test \
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/s1 shark:/autofs/export/testing/test/s1 \
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/s2 shark:/autofs/export5/testing/test/s2 \
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/s1/ss1 shark:/autofs/export1 \
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/s2/ss2 shark:/autofs/export2
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and a similarly a direct mount map entry could also be::
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/automount/dparse/g1 \
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/ shark:/autofs/export5/testing/test \
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/s1 shark:/autofs/export/testing/test/s1 \
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/s2 shark:/autofs/export5/testing/test/s2 \
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/s1/ss1 shark:/autofs/export2 \
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/s2/ss2 shark:/autofs/export2
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One of the issues with version 4 of autofs was that, when mounting an
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entry with a large number of offsets, possibly with nesting, we needed
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to mount and umount all of the offsets as a single unit. Not really a
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problem, except for people with a large number of offsets in map entries.
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This mechanism is used for the well known "hosts" map and we have seen
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cases (in 2.4) where the available number of mounts are exhausted or
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where the number of privileged ports available is exhausted.
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In version 5 we mount only as we go down the tree of offsets and
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similarly for expiring them which resolves the above problem. There is
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somewhat more detail to the implementation but it isn't needed for the
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sake of the problem explanation. The one important detail is that these
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offsets are implemented using the same mechanism as the direct mounts
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above and so the mount points can be covered by a mount.
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The current autofs implementation uses an ioctl file descriptor opened
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on the mount point for control operations. The references held by the
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descriptor are accounted for in checks made to determine if a mount is
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in use and is also used to access autofs file system information held
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in the mount super block. So the use of a file handle needs to be
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retained.
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The Solution
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============
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To be able to restart autofs leaving existing direct, indirect and
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offset mounts in place we need to be able to obtain a file handle
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for these potentially covered autofs mount points. Rather than just
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implement an isolated operation it was decided to re-implement the
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existing ioctl interface and add new operations to provide this
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functionality.
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In addition, to be able to reconstruct a mount tree that has busy mounts,
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the uid and gid of the last user that triggered the mount needs to be
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available because these can be used as macro substitution variables in
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autofs maps. They are recorded at mount request time and an operation
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has been added to retrieve them.
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Since we're re-implementing the control interface, a couple of other
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problems with the existing interface have been addressed. First, when
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a mount or expire operation completes a status is returned to the
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kernel by either a "send ready" or a "send fail" operation. The
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"send fail" operation of the ioctl interface could only ever send
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ENOENT so the re-implementation allows user space to send an actual
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status. Another expensive operation in user space, for those using
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very large maps, is discovering if a mount is present. Usually this
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involves scanning /proc/mounts and since it needs to be done quite
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often it can introduce significant overhead when there are many entries
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in the mount table. An operation to lookup the mount status of a mount
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point dentry (covered or not) has also been added.
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Current kernel development policy recommends avoiding the use of the
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ioctl mechanism in favor of systems such as Netlink. An implementation
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using this system was attempted to evaluate its suitability and it was
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found to be inadequate, in this case. The Generic Netlink system was
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used for this as raw Netlink would lead to a significant increase in
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complexity. There's no question that the Generic Netlink system is an
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elegant solution for common case ioctl functions but it's not a complete
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replacement probably because its primary purpose in life is to be a
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message bus implementation rather than specifically an ioctl replacement.
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While it would be possible to work around this there is one concern
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that lead to the decision to not use it. This is that the autofs
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expire in the daemon has become far to complex because umount
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candidates are enumerated, almost for no other reason than to "count"
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the number of times to call the expire ioctl. This involves scanning
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the mount table which has proved to be a big overhead for users with
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large maps. The best way to improve this is try and get back to the
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way the expire was done long ago. That is, when an expire request is
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issued for a mount (file handle) we should continually call back to
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the daemon until we can't umount any more mounts, then return the
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appropriate status to the daemon. At the moment we just expire one
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mount at a time. A Generic Netlink implementation would exclude this
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possibility for future development due to the requirements of the
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message bus architecture.
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autofs Miscellaneous Device mount control interface
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====================================================
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The control interface is opening a device node, typically /dev/autofs.
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All the ioctls use a common structure to pass the needed parameter
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information and return operation results::
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struct autofs_dev_ioctl {
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__u32 ver_major;
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__u32 ver_minor;
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__u32 size; /* total size of data passed in
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* including this struct */
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__s32 ioctlfd; /* automount command fd */
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/* Command parameters */
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union {
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struct args_protover protover;
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struct args_protosubver protosubver;
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struct args_openmount openmount;
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struct args_ready ready;
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struct args_fail fail;
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struct args_setpipefd setpipefd;
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struct args_timeout timeout;
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struct args_requester requester;
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struct args_expire expire;
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struct args_askumount askumount;
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struct args_ismountpoint ismountpoint;
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};
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char path[0];
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};
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The ioctlfd field is a mount point file descriptor of an autofs mount
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point. It is returned by the open call and is used by all calls except
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the check for whether a given path is a mount point, where it may
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optionally be used to check a specific mount corresponding to a given
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mount point file descriptor, and when requesting the uid and gid of the
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last successful mount on a directory within the autofs file system.
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The union is used to communicate parameters and results of calls made
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as described below.
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The path field is used to pass a path where it is needed and the size field
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is used account for the increased structure length when translating the
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structure sent from user space.
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This structure can be initialized before setting specific fields by using
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the void function call init_autofs_dev_ioctl(``struct autofs_dev_ioctl *``).
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All of the ioctls perform a copy of this structure from user space to
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kernel space and return -EINVAL if the size parameter is smaller than
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the structure size itself, -ENOMEM if the kernel memory allocation fails
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or -EFAULT if the copy itself fails. Other checks include a version check
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of the compiled in user space version against the module version and a
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mismatch results in a -EINVAL return. If the size field is greater than
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the structure size then a path is assumed to be present and is checked to
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ensure it begins with a "/" and is NULL terminated, otherwise -EINVAL is
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returned. Following these checks, for all ioctl commands except
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AUTOFS_DEV_IOCTL_VERSION_CMD, AUTOFS_DEV_IOCTL_OPENMOUNT_CMD and
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AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD the ioctlfd is validated and if it is
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not a valid descriptor or doesn't correspond to an autofs mount point
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an error of -EBADF, -ENOTTY or -EINVAL (not an autofs descriptor) is
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returned.
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The ioctls
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==========
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An example of an implementation which uses this interface can be seen
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in autofs version 5.0.4 and later in file lib/dev-ioctl-lib.c of the
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distribution tar available for download from kernel.org in directory
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/pub/linux/daemons/autofs/v5.
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The device node ioctl operations implemented by this interface are:
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AUTOFS_DEV_IOCTL_VERSION
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------------------------
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Get the major and minor version of the autofs device ioctl kernel module
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implementation. It requires an initialized struct autofs_dev_ioctl as an
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input parameter and sets the version information in the passed in structure.
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It returns 0 on success or the error -EINVAL if a version mismatch is
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detected.
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AUTOFS_DEV_IOCTL_PROTOVER_CMD and AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD
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------------------------------------------------------------------
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Get the major and minor version of the autofs protocol version understood
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by loaded module. This call requires an initialized struct autofs_dev_ioctl
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with the ioctlfd field set to a valid autofs mount point descriptor
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and sets the requested version number in version field of struct args_protover
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or sub_version field of struct args_protosubver. These commands return
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0 on success or one of the negative error codes if validation fails.
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AUTOFS_DEV_IOCTL_OPENMOUNT and AUTOFS_DEV_IOCTL_CLOSEMOUNT
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----------------------------------------------------------
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Obtain and release a file descriptor for an autofs managed mount point
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path. The open call requires an initialized struct autofs_dev_ioctl with
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the path field set and the size field adjusted appropriately as well
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as the devid field of struct args_openmount set to the device number of
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the autofs mount. The device number can be obtained from the mount options
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shown in /proc/mounts. The close call requires an initialized struct
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autofs_dev_ioct with the ioctlfd field set to the descriptor obtained
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from the open call. The release of the file descriptor can also be done
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with close(2) so any open descriptors will also be closed at process exit.
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The close call is included in the implemented operations largely for
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completeness and to provide for a consistent user space implementation.
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AUTOFS_DEV_IOCTL_READY_CMD and AUTOFS_DEV_IOCTL_FAIL_CMD
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--------------------------------------------------------
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Return mount and expire result status from user space to the kernel.
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Both of these calls require an initialized struct autofs_dev_ioctl
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with the ioctlfd field set to the descriptor obtained from the open
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call and the token field of struct args_ready or struct args_fail set
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to the wait queue token number, received by user space in the foregoing
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mount or expire request. The status field of struct args_fail is set to
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the errno of the operation. It is set to 0 on success.
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AUTOFS_DEV_IOCTL_SETPIPEFD_CMD
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------------------------------
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Set the pipe file descriptor used for kernel communication to the daemon.
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Normally this is set at mount time using an option but when reconnecting
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to a existing mount we need to use this to tell the autofs mount about
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the new kernel pipe descriptor. In order to protect mounts against
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incorrectly setting the pipe descriptor we also require that the autofs
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mount be catatonic (see next call).
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The call requires an initialized struct autofs_dev_ioctl with the
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ioctlfd field set to the descriptor obtained from the open call and
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the pipefd field of struct args_setpipefd set to descriptor of the pipe.
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On success the call also sets the process group id used to identify the
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controlling process (eg. the owning automount(8) daemon) to the process
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group of the caller.
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AUTOFS_DEV_IOCTL_CATATONIC_CMD
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------------------------------
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Make the autofs mount point catatonic. The autofs mount will no longer
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issue mount requests, the kernel communication pipe descriptor is released
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and any remaining waits in the queue released.
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The call requires an initialized struct autofs_dev_ioctl with the
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ioctlfd field set to the descriptor obtained from the open call.
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AUTOFS_DEV_IOCTL_TIMEOUT_CMD
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----------------------------
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Set the expire timeout for mounts within an autofs mount point.
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The call requires an initialized struct autofs_dev_ioctl with the
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ioctlfd field set to the descriptor obtained from the open call.
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AUTOFS_DEV_IOCTL_REQUESTER_CMD
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------------------------------
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Return the uid and gid of the last process to successfully trigger a the
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mount on the given path dentry.
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The call requires an initialized struct autofs_dev_ioctl with the path
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field set to the mount point in question and the size field adjusted
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appropriately. Upon return the uid field of struct args_requester contains
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the uid and gid field the gid.
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When reconstructing an autofs mount tree with active mounts we need to
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re-connect to mounts that may have used the original process uid and
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gid (or string variations of them) for mount lookups within the map entry.
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This call provides the ability to obtain this uid and gid so they may be
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used by user space for the mount map lookups.
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AUTOFS_DEV_IOCTL_EXPIRE_CMD
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---------------------------
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Issue an expire request to the kernel for an autofs mount. Typically
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this ioctl is called until no further expire candidates are found.
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The call requires an initialized struct autofs_dev_ioctl with the
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ioctlfd field set to the descriptor obtained from the open call. In
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addition an immediate expire that's independent of the mount timeout,
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and a forced expire that's independent of whether the mount is busy,
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can be requested by setting the how field of struct args_expire to
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AUTOFS_EXP_IMMEDIATE or AUTOFS_EXP_FORCED, respectively . If no
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expire candidates can be found the ioctl returns -1 with errno set to
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EAGAIN.
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This call causes the kernel module to check the mount corresponding
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to the given ioctlfd for mounts that can be expired, issues an expire
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request back to the daemon and waits for completion.
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AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD
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------------------------------
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Checks if an autofs mount point is in use.
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The call requires an initialized struct autofs_dev_ioctl with the
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ioctlfd field set to the descriptor obtained from the open call and
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it returns the result in the may_umount field of struct args_askumount,
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1 for busy and 0 otherwise.
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AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD
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---------------------------------
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Check if the given path is a mountpoint.
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The call requires an initialized struct autofs_dev_ioctl. There are two
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possible variations. Both use the path field set to the path of the mount
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point to check and the size field adjusted appropriately. One uses the
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ioctlfd field to identify a specific mount point to check while the other
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variation uses the path and optionally in.type field of struct args_ismountpoint
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set to an autofs mount type. The call returns 1 if this is a mount point
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and sets out.devid field to the device number of the mount and out.magic
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field to the relevant super block magic number (described below) or 0 if
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it isn't a mountpoint. In both cases the device number (as returned
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by new_encode_dev()) is returned in out.devid field.
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If supplied with a file descriptor we're looking for a specific mount,
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not necessarily at the top of the mounted stack. In this case the path
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the descriptor corresponds to is considered a mountpoint if it is itself
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a mountpoint or contains a mount, such as a multi-mount without a root
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mount. In this case we return 1 if the descriptor corresponds to a mount
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point and also returns the super magic of the covering mount if there
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is one or 0 if it isn't a mountpoint.
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If a path is supplied (and the ioctlfd field is set to -1) then the path
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is looked up and is checked to see if it is the root of a mount. If a
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type is also given we are looking for a particular autofs mount and if
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a match isn't found a fail is returned. If the located path is the
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root of a mount 1 is returned along with the super magic of the mount
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or 0 otherwise.
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