kernel.txt

Linux下fuse用户文件系统的的源代码

Definitions
~~~~~~~~~~~

Userspace filesystem:

  A filesystem in which data and metadata are provided by an ordinary
  userspace process.  The filesystem can be accessed normally through
  the kernel interface.

Filesystem daemon:

  The process(es) providing the data and metadata of the filesystem.

Non-privileged mount (or user mount):

  A userspace filesystem mounted by a non-privileged (non-root) user.
  The filesystem daemon is running with the privileges of the mounting
  user.  NOTE: this is not the same as mounts allowed with the "user"
  option in /etc/fstab, which is not discussed here.

Filesystem connection:

  A connection between the filesystem daemon and the kernel.  The
  connection exists until either the daemon dies, or the filesystem is
  umounted.  Note that detaching (or lazy umounting) the filesystem
  does _not_ break the connection, in this case it will exist until
  the last reference to the filesystem is released.

Mount owner:

  The user who does the mounting.

User:

  The user who is performing filesystem operations.

What is FUSE?
~~~~~~~~~~~~~

FUSE is a userspace filesystem framework.  It consists of a kernel
module (fuse.ko), a userspace library (libfuse.*) and a mount utility
(fusermount).

One of the most important features of FUSE is allowing secure,
non-privileged mounts.  This opens up new possibilities for the use of
filesystems.  A good example is sshfs: a secure network filesystem
using the sftp protocol.

The userspace library and utilities are available from the FUSE
homepage:

  http://fuse.sourceforge.net/

Filesystem type
~~~~~~~~~~~~~~~

The filesystem type given to mount(2) can be one of the following:

'fuse'

  This is the usual way to mount a FUSE filesystem.  The first
  argument of the mount system call may contain an arbitrary string,
  which is not interpreted by the kernel.

'fuseblk'

  The filesystem is block device based.  The first argument of the
  mount system call is interpreted as the name of the device.

Mount options
~~~~~~~~~~~~~

'fd=N'

  The file descriptor to use for communication between the userspace
  filesystem and the kernel.  The file descriptor must have been
  obtained by opening the FUSE device ('/dev/fuse').

'rootmode=M'

  The file mode of the filesystem's root in octal representation.

'user_id=N'

  The numeric user id of the mount owner.

'group_id=N'

  The numeric group id of the mount owner.

'default_permissions'

  By default FUSE doesn't check file access permissions, the
  filesystem is free to implement it's access policy or leave it to
  the underlying file access mechanism (e.g. in case of network
  filesystems).  This option enables permission checking, restricting
  access based on file mode.  It is usually useful together with the
  'allow_other' mount option.

'allow_other'

  This option overrides the security measure restricting file access
  to the user mounting the filesystem.  This option is by default only
  allowed to root, but this restriction can be removed with a
  (userspace) configuration option.

'max_read=N'

  With this option the maximum size of read operations can be set.
  The default is infinite.  Note that the size of read requests is
  limited anyway to 32 pages (which is 128kbyte on i386).

'blksize=N'

  Set the block size for the filesystem.  The default is 512.  This
  option is only valid for 'fuseblk' type mounts.

Control filesystem
~~~~~~~~~~~~~~~~~~

There's a control filesystem for FUSE, which can be mounted by:

  mount -t fusectl none /sys/fs/fuse/connections

Mounting it under the '/sys/fs/fuse/connections' directory makes it
backwards compatible with earlier versions.

Under the fuse control filesystem each connection has a directory
named by a unique number.

For each connection the following files exist within this directory:

 'waiting'

  The number of requests which are waiting to be transferred to
  userspace or being processed by the filesystem daemon.  If there is
  no filesystem activity and 'waiting' is non-zero, then the
  filesystem is hung or deadlocked.

 'abort'

  Writing anything into this file will abort the filesystem
  connection.  This means that all waiting requests will be aborted an
  error returned for all aborted and new requests.

Only the owner of the mount may read or write these files.

Interrupting filesystem operations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If a process issuing a FUSE filesystem request is interrupted, the
following will happen:

  1) If the request is not yet sent to userspace AND the signal is
     fatal (SIGKILL or unhandled fatal signal), then the request is
     dequeued and returns immediately.

  2) If the request is not yet sent to userspace AND the signal is not
     fatal, then an 'interrupted' flag is set for the request.  When
     the request has been successfully transferred to userspace and
     this flag is set, an INTERRUPT request is queued.

  3) If the request is already sent to userspace, then an INTERRUPT
     request is queued.

INTERRUPT requests take precedence over other requests, so the
userspace filesystem will receive queued INTERRUPTs before any others.

The userspace filesystem may ignore the INTERRUPT requests entirely,
or may honor them by sending a reply to the _original_ request, with
the error set to EINTR.

It is also possible that there's a race between processing the
original request and it's INTERRUPT request.  There are two possibilities:

  1) The INTERRUPT request is processed before the original request is
     processed

  2) The INTERRUPT request is processed after the original request has
     been answered

If the filesystem cannot find the original request, it should wait for
some timeout and/or a number of new requests to arrive, after which it
should reply to the INTERRUPT request with an EAGAIN error.  In case
1) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
reply will be ignored.

Aborting a filesystem connection
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is possible to get into certain situations where the filesystem is
not responding.  Reasons for this may be:

  a) Broken userspace filesystem implementation

  b) Network connection down

  c) Accidental deadlock

  d) Malicious deadlock

(For more on c) and d) see later sections)

In either of these cases it may be useful to abort the connection to
the filesystem.  There are several ways to do this:

  - Kill the filesystem daemon.  Works in case of a) and b)

  - Kill the filesystem daemon and all users of the filesystem.  Works
    in all cases except some malicious deadlocks

  - Use forced umount (umount -f).  Works in all cases but only if