[Linux-kernel-mentees] [RFC PATCH] Documentation: filesystems: convert fuse to RST
Daniel W. S. Almeida
dwlsalmeida at gmail.com
Fri Nov 8 16:46:19 UTC 2019
From: "Daniel W. S. Almeida" <dwlsalmeida at gmail.com>
Converts fuse.txt to reStructuredText format, improving the presentation
without changing much of the underlying content.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida at gmail.com>
---
.../filesystems/{fuse.txt => fuse.rst} | 216 ++++++++----------
Documentation/filesystems/index.rst | 1 +
2 files changed, 102 insertions(+), 115 deletions(-)
rename Documentation/filesystems/{fuse.txt => fuse.rst} (72%)
diff --git a/Documentation/filesystems/fuse.txt b/Documentation/filesystems/fuse.rst
similarity index 72%
rename from Documentation/filesystems/fuse.txt
rename to Documentation/filesystems/fuse.rst
index 13af4a49e7db..5741b1071244 100644
--- a/Documentation/filesystems/fuse.txt
+++ b/Documentation/filesystems/fuse.rst
@@ -1,95 +1,86 @@
-Definitions
-~~~~~~~~~~~
+==============
+FUSE
+==============
-Userspace filesystem:
+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:
-
+**Filesystem daemon:**
The process(es) providing the data and metadata of the filesystem.
-Non-privileged mount (or user mount):
-
+**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:
-
+**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
+ does *not* break the connection, in this case it will exist until
the last reference to the filesystem is released.
-Mount owner:
-
+**Mount owner:**
The user who does the mounting.
-User:
-
+**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).
+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
+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/
+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.
+ **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.
+ **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'
+=============
+**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'
+ obtained by opening the FUSE device *('/dev/fuse')*.
+**rootmode=M**
The file mode of the filesystem's root in octal representation.
-'user_id=N'
-
+**user_id=N**
The numeric user id of the mount owner.
-'group_id=N'
-
+**group_id=N**
The numeric group id of the mount owner.
-'default_permissions'
-
+**default_permissions**
By default FUSE doesn't check file access permissions, the
filesystem is free to implement its access policy or leave it to
the underlying file access mechanism (e.g. in case of network
@@ -97,32 +88,29 @@ Mount options
access based on file mode. It is usually useful together with the
'allow_other' mount option.
-'allow_other'
-
+**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'
-
+**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'
-
+**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:
+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
+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
@@ -130,63 +118,61 @@ named by a unique number.
For each connection the following files exist within this directory:
- 'waiting'
+ **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.
- 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.
+ **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
+ #. 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
+ #. 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.
+ this flag is set, an *INTERRUPT* request is queued.
- 3) If the request is already sent to userspace, then an INTERRUPT
+ #. If the request is already sent to userspace, then an *INTERRUPT*
request is queued.
-INTERRUPT requests take precedence over other requests, so the
+*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.
+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 its INTERRUPT request. There are two possibilities:
- 1) The INTERRUPT request is processed before the original request is
+ #. The *INTERRUPT* request is processed before the original request is
processed
- 2) The INTERRUPT request is processed after the original request has
+ #. 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
+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:
@@ -209,14 +195,14 @@ the filesystem. There are several ways to do this:
- 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
+ - Use forced umount ``(umount -f)``. Works in all cases but only if
filesystem is still attached (it hasn't been lazy unmounted)
- Abort filesystem through the FUSE control filesystem. Most
powerful method, always works.
How do non-privileged mounts work?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+==================================
Since the mount() system call is a privileged operation, a helper
program (fusermount) is needed, which is installed setuid root.
@@ -235,20 +221,18 @@ system. Obvious requirements arising from this are:
other users' or the super user's processes
How are requirements fulfilled?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+===============================
A) The mount owner could gain elevated privileges by either:
- 1) creating a filesystem containing a device file, then opening
- this device
+ 1. creating a filesystem containing a device file, then opening this device
- 2) creating a filesystem containing a suid or sgid application,
- then executing this application
+ 2. creating a filesystem containing a suid or sgid application, then executing this application
- The solution is not to allow opening device files and ignore
+ **The solution is not to allow opening device files and ignore
setuid and setgid bits when executing programs. To ensure this
fusermount always adds "nosuid" and "nodev" to the mount options
- for non-privileged mounts.
+ for non-privileged mounts.**
B) If another user is accessing files or directories in the
filesystem, the filesystem daemon serving requests can record the
@@ -256,7 +240,7 @@ How are requirements fulfilled?
information is otherwise inaccessible to the mount owner, so this
counts as an information leak.
- The solution to this problem will be presented in point 2) of C).
+ **The solution to this problem will be presented in point 2) of C).**
C) There are several ways in which the mount owner can induce
undesired behavior in other users' processes, such as:
@@ -275,47 +259,46 @@ How are requirements fulfilled?
of other users' processes.
i) It can slow down or indefinitely delay the execution of a
- filesystem operation creating a DoS against the user or the
- whole system. For example a suid application locking a
- system file, and then accessing a file on the mount owner's
- filesystem could be stopped, and thus causing the system
- file to be locked forever.
+ filesystem operation creating a DoS against the user or the
+ whole system. For example a suid application locking a
+ system file, and then accessing a file on the mount owner's
+ filesystem could be stopped, and thus causing the system
+ file to be locked forever.
ii) It can present files or directories of unlimited length, or
- directory structures of unlimited depth, possibly causing a
- system process to eat up diskspace, memory or other
- resources, again causing DoS.
+ directory structures of unlimited depth, possibly causing a
+ system process to eat up diskspace, memory or other
+ resources, again causing *DoS*.
- The solution to this as well as B) is not to allow processes
+ **The solution to this as well as B) is not to allow processes
to access the filesystem, which could otherwise not be
- monitored or manipulated by the mount owner. Since if the
+ monitored or manipulated by the mount owner.** Since if the
mount owner can ptrace a process, it can do all of the above
without using a FUSE mount, the same criteria as used in
ptrace can be used to check if a process is allowed to access
the filesystem or not.
- Note that the ptrace check is not strictly necessary to
+ Note that the *ptrace* check is not strictly necessary to
prevent B/2/i, it is enough to check if mount owner has enough
privilege to send signal to the process accessing the
- filesystem, since SIGSTOP can be used to get a similar effect.
+ filesystem, since *SIGSTOP* can be used to get a similar effect.
I think these limitations are unacceptable?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+===========================================
If a sysadmin trusts the users enough, or can ensure through other
measures, that system processes will never enter non-privileged
-mounts, it can relax the last limitation with a "user_allow_other"
+mounts, it can relax the last limitation with a ``user_allow_other``
config option. If this config option is set, the mounting user can
-add the "allow_other" mount option which disables the check for other
+add the ``allow_other`` mount option which disables the check for other
users' processes.
Kernel - userspace interface
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+============================
The following diagram shows how a filesystem operation (in this
-example unlink) is performed in FUSE.
+example unlink) is performed in ``FUSE``. ::
-NOTE: everything in this description is greatly simplified
| "rm /mnt/fuse/file" | FUSE filesystem daemon
| |
@@ -357,12 +340,15 @@ NOTE: everything in this description is greatly simplified
| <fuse_unlink() |
| <sys_unlink() |
-There are a couple of ways in which to deadlock a FUSE filesystem.
+.. note:: Everything in the description above is greatly simplified
+
+There are a couple of ways in which to deadlock a ``FUSE`` filesystem.
Since we are talking about unprivileged userspace programs,
something must be done about these.
-Scenario 1 - Simple deadlock
------------------------------
+**Scenario 1 - Simple deadlock**
+
+::
| "rm /mnt/fuse/file" | FUSE filesystem daemon
| |
@@ -379,12 +365,12 @@ Scenario 1 - Simple deadlock
The solution for this is to allow the filesystem to be aborted.
-Scenario 2 - Tricky deadlock
-----------------------------
+**Scenario 2 - Tricky deadlock**
+
This one needs a carefully crafted filesystem. It's a variation on
the above, only the call back to the filesystem is not explicit,
-but is caused by a pagefault.
+but is caused by a pagefault. ::
| Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2
| |
@@ -410,7 +396,7 @@ but is caused by a pagefault.
| | [lock page]
| | * DEADLOCK *
-Solution is basically the same as above.
+The solution is basically the same as above.
An additional problem is that while the write buffer is being copied
to the request, the request must not be interrupted/aborted. This is
@@ -419,5 +405,5 @@ request has returned.
This is solved with doing the copy atomically, and allowing abort
while the page(s) belonging to the write buffer are faulted with
-get_user_pages(). The 'req->locked' flag indicates when the copy is
+get_user_pages(). The ``'req->locked'`` flag indicates when the copy is
taking place, and abort is delayed until this flag is unset.
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 2c3a9f761205..03a7c4ed7f15 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -46,4 +46,5 @@ Documentation for filesystem implementations.
.. toctree::
:maxdepth: 2
+ fuse
virtiofs
--
2.24.0
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