autofs4: device node ioctl documentation

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 it's 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 withing 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 optionaly 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.