394 lines
17 KiB
Plaintext
394 lines
17 KiB
Plaintext
|
|
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.
|
|
|