devices: fs: Add Filesystem trait

Add the `Filesystem` trait, which is the main interface between the
transport and the actual file system implementation.

BUG=b:136128319
TEST=`tast run vm.VirtioFs`

Change-Id: Ic8bc9e231652020501e10ad0be810a9f66e90b8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1757241
Auto-Submit: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-by: Stephen Barber <smbarber@chromium.org>
Tested-by: Chirantan Ekbote <chirantan@chromium.org>
Commit-Queue: Chirantan Ekbote <chirantan@chromium.org>

1 files changed, 1096 insertions, 0 deletions

diff --git a/devices/src/virtio/fs/filesystem.rs b/devices/src/virtio/fs/filesystem.rs
new file mode 100644
index 0000000..53a3f1e
--- /dev/null
+++ b/devices/src/virtio/fs/filesystem.rs
@@ -0,0 +1,1096 @@
+// Copyright 2019 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use std::convert::TryInto;
+use std::ffi::CStr;
+use std::fs::File;
+use std::io;
+use std::mem;
+use std::time::Duration;
+
+use libc;
+
+use crate::virtio::fs::fuse;
+
+pub use fuse::FsOptions;
+pub use fuse::OpenOptions;
+pub use fuse::SetattrValid;
+pub use fuse::ROOT_ID;
+
+/// Information about a path in the filesystem.
+pub struct Entry {
+    /// An `Inode` that uniquely identifies this path. During `lookup`, setting this to `0` means a
+    /// negative entry. Returning `ENOENT` also means a negative entry but setting this to `0`
+    /// allows the kernel to cache the negative result for `entry_timeout`. The value should be
+    /// produced by converting a `FileSystem::Inode` into a `u64`.
+    pub inode: u64,
+
+    /// The generation number for this `Entry`. Typically used for network file systems. An `inode`
+    /// / `generation` pair must be unique over the lifetime of the file system (rather than just
+    /// the lifetime of the mount). In other words, if a `FileSystem` implementation re-uses an
+    /// `Inode` after it has been deleted then it must assign a new, previously unused generation
+    /// number to the `Inode` at the same time.
+    pub generation: u64,
+
+    /// Inode attributes. Even if `attr_timeout` is zero, `attr` must be correct. For example, for
+    /// `open()`, FUSE uses `attr.st_size` from `lookup()` to determine how many bytes to request.
+    /// If this value is not correct, incorrect data will be returned.
+    pub attr: libc::stat64,
+
+    /// How long the values in `attr` should be considered valid. If the attributes of the `Entry`
+    /// are only modified by the FUSE client, then this should be set to a very large value.
+    pub attr_timeout: Duration,
+
+    /// How long the name associated with this `Entry` should be considered valid. If directory
+    /// entries are only changed or deleted by the FUSE client, then this should be set to a very
+    /// large value.
+    pub entry_timeout: Duration,
+}
+
+impl From<Entry> for fuse::EntryOut {
+    fn from(entry: Entry) -> fuse::EntryOut {
+        fuse::EntryOut {
+            nodeid: entry.inode,
+            generation: entry.generation,
+            entry_valid: entry.entry_timeout.as_secs(),
+            attr_valid: entry.attr_timeout.as_secs(),
+            entry_valid_nsec: entry.entry_timeout.subsec_nanos(),
+            attr_valid_nsec: entry.attr_timeout.subsec_nanos(),
+            attr: entry.attr.into(),
+        }
+    }
+}
+
+/// Represents information about an entry in a directory.
+pub struct DirEntry<'a> {
+    /// The inode number for this entry. This does NOT have to be the same as the `Inode` for this
+    /// directory entry. However, it must be the same as the `attr.st_ino` field of the `Entry` that
+    /// would be returned by a `lookup` request in the parent directory for `name`.
+    pub ino: libc::ino64_t,
+
+    /// Any non-zero value that the kernel can use to identify the current point in the directory
+    /// entry stream. It does not need to be the actual physical position. A value of `0` is
+    /// reserved to mean "from the beginning" and should never be used. The `offset` value of the
+    /// first entry in a stream should point to the beginning of the second entry and so on.
+    pub offset: u64,
+
+    /// The type of this directory entry. Valid values are any of the `libc::DT_*` constants.
+    pub type_: u32,
+
+    /// The name of this directory entry. There are no requirements for the contents of this field
+    /// and any sequence of bytes is considered valid.
+    pub name: &'a [u8],
+}
+
+/// A reply to a `getxattr` method call.
+pub enum GetxattrReply {
+    /// The value of the requested extended attribute. This can be arbitrary textual or binary data
+    /// and does not need to be nul-terminated.
+    Value(Vec<u8>),
+
+    /// The size of the buffer needed to hold the value of the requested extended attribute. Should
+    /// be returned when the `size` parameter is 0. Callers should note that it is still possible
+    /// for the size of the value to change in between `getxattr` calls and should not assume that a
+    /// subsequent call to `getxattr` with the returned count will always succeed.
+    Count(u32),
+}
+
+/// A reply to a `listxattr` method call.
+pub enum ListxattrReply {
+    /// A buffer containing a nul-separated list of the names of all the extended attributes
+    /// associated with this `Inode`. This list of names may be unordered and includes a namespace
+    /// prefix. There may be several disjoint namespaces associated with a single `Inode`.
+    Names(Vec<u8>),
+
+    /// This size of the buffer needed to hold the full list of extended attribute names associated
+    /// with this `Inode`. Should be returned when the `size` parameter is 0. Callers should note
+    /// that it is still possible for the set of extended attributes to change between `listxattr`
+    /// calls and so should not assume that a subsequent call to `listxattr` with the returned count
+    /// will always succeed.
+    Count(u32),
+}
+
+/// A trait for directly copying data from the fuse transport into a `File` without first storing it
+/// in an intermediate buffer.
+pub trait ZeroCopyReader {
+    /// Copies at most `count` bytes from `self` directly into `f` at offset `off` without storing
+    /// it in any intermediate buffers. If the return value is `Ok(n)` then it must be guaranteed
+    /// that `0 <= n <= count`. If `n` is `0`, then it can indicate one of 3 possibilities:
+    ///
+    /// 1. There is no more data left in `self`.
+    /// 2. There is no more space in `f`.
+    /// 3. `count` was `0`.
+    ///
+    /// # Errors
+    ///
+    /// If any error is returned then the implementation must guarantee that no bytes were copied
+    /// from `self`. If the underlying write to `f` returns `0` then the implementation must return
+    /// an error of the kind `io::ErrorKind::WriteZero`.
+    fn read_to(&mut self, f: &mut File, count: usize, off: u64) -> io::Result<usize>;
+
+    /// Copies exactly `count` bytes of data from `self` into `f` at offset `off`. `off + count`
+    /// must be less than `u64::MAX`.
+    ///
+    /// # Errors
+    ///
+    /// If an error is returned then the number of bytes copied from `self` is unspecified but it
+    /// will never be more than `count`.
+    fn read_exact_to(&mut self, f: &mut File, mut count: usize, mut off: u64) -> io::Result<()> {
+        let c = count
+            .try_into()
+            .map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;
+        if off.checked_add(c).is_none() {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "`off` + `count` must be less than u64::MAX",
+            ));
+        }
+
+        while count > 0 {
+            match self.read_to(f, count, off) {
+                Ok(0) => {
+                    return Err(io::Error::new(
+                        io::ErrorKind::WriteZero,
+                        "failed to fill whole buffer",
+                    ))
+                }
+                Ok(n) => {
+                    count -= n;
+                    off += n as u64;
+                }
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Copies all remaining bytes from `self` into `f` at offset `off`. Equivalent to repeatedly
+    /// calling `read_to` until it returns either `Ok(0)` or a non-`ErrorKind::Interrupted` error.
+    ///
+    /// # Errors
+    ///
+    /// If an error is returned then the number of bytes copied from `self` is unspecified.
+    fn copy_to_end(&mut self, f: &mut File, mut off: u64) -> io::Result<usize> {
+        let mut out = 0;
+        loop {
+            match self.read_to(f, ::std::usize::MAX, off) {
+                Ok(0) => return Ok(out),
+                Ok(n) => {
+                    off = off.saturating_add(n as u64);
+                    out += n;
+                }
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+    }
+}
+
+impl<'a, R: ZeroCopyReader> ZeroCopyReader for &'a mut R {
+    fn read_to(&mut self, f: &mut File, count: usize, off: u64) -> io::Result<usize> {
+        (**self).read_to(f, count, off)
+    }
+    fn read_exact_to(&mut self, f: &mut File, count: usize, off: u64) -> io::Result<()> {
+        (**self).read_exact_to(f, count, off)
+    }
+    fn copy_to_end(&mut self, f: &mut File, off: u64) -> io::Result<usize> {
+        (**self).copy_to_end(f, off)
+    }
+}
+
+/// A trait for directly copying data from a `File` into the fuse transport without first storing
+/// it in an intermediate buffer.
+pub trait ZeroCopyWriter {
+    /// Copies at most `count` bytes from `f` at offset `off` directly into `self` without storing
+    /// it in any intermediate buffers. If the return value is `Ok(n)` then it must be guaranteed
+    /// that `0 <= n <= count`. If `n` is `0`, then it can indicate one of 3 possibilities:
+    ///
+    /// 1. There is no more data left in `f`.
+    /// 2. There is no more space in `self`.
+    /// 3. `count` was `0`.
+    ///
+    /// # Errors
+    ///
+    /// If any error is returned then the implementation must guarantee that no bytes were copied
+    /// from `f`. If the underlying read from `f` returns `0` then the implementation must return an
+    /// error of the kind `io::ErrorKind::UnexpectedEof`.
+    fn write_from(&mut self, f: &mut File, count: usize, off: u64) -> io::Result<usize>;
+
+    /// Copies exactly `count` bytes of data from `f` at offset `off` into `self`. `off + count`
+    /// must be less than `u64::MAX`.
+    ///
+    /// # Errors
+    ///
+    /// If an error is returned then the number of bytes copied from `self` is unspecified but it
+    /// well never be more than `count`.
+    fn write_all_from(&mut self, f: &mut File, mut count: usize, mut off: u64) -> io::Result<()> {
+        let c = count
+            .try_into()
+            .map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;
+        if off.checked_add(c).is_none() {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "`off` + `count` must be less than u64::MAX",
+            ));
+        }
+
+        while count > 0 {
+            match self.write_from(f, count, off) {
+                Ok(0) => {
+                    return Err(io::Error::new(
+                        io::ErrorKind::UnexpectedEof,
+                        "failed to write whole buffer",
+                    ))
+                }
+                Ok(n) => {
+                    // No need for checked math here because we verified that `off + count` will not
+                    // overflow and `n` must be <= `count`.
+                    count -= n;
+                    off += n as u64;
+                }
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Copies all remaining bytes from `f` at offset `off` into `self`. Equivalent to repeatedly
+    /// calling `write_from` until it returns either `Ok(0)` or a non-`ErrorKind::Interrupted`
+    /// error.
+    ///
+    /// # Errors
+    ///
+    /// If an error is returned then the number of bytes copied from `f` is unspecified.
+    fn copy_to_end(&mut self, f: &mut File, mut off: u64) -> io::Result<usize> {
+        let mut out = 0;
+        loop {
+            match self.write_from(f, ::std::usize::MAX, off) {
+                Ok(0) => return Ok(out),
+                Ok(n) => {
+                    off = off.saturating_add(n as u64);
+                    out += n;
+                }
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+    }
+}
+
+impl<'a, W: ZeroCopyWriter> ZeroCopyWriter for &'a mut W {
+    fn write_from(&mut self, f: &mut File, count: usize, off: u64) -> io::Result<usize> {
+        (**self).write_from(f, count, off)
+    }
+    fn write_all_from(&mut self, f: &mut File, count: usize, off: u64) -> io::Result<()> {
+        (**self).write_all_from(f, count, off)
+    }
+    fn copy_to_end(&mut self, f: &mut File, off: u64) -> io::Result<usize> {
+        (**self).copy_to_end(f, off)
+    }
+}
+
+/// Additional context associated with requests.
+#[derive(Clone, Copy, Debug)]
+pub struct Context {
+    /// The user ID of the calling process.
+    pub uid: libc::uid_t,
+
+    /// The group ID of the calling process.
+    pub gid: libc::gid_t,
+
+    /// The thread group ID of the calling process.
+    pub pid: libc::pid_t,
+}
+
+impl From<fuse::InHeader> for Context {
+    fn from(source: fuse::InHeader) -> Self {
+        Context {
+            uid: source.uid,
+            gid: source.gid,
+            pid: source.pid as i32,
+        }
+    }
+}
+
+/// The main trait that connects a file system with a transport.
+#[allow(unused_variables)]
+pub trait FileSystem {
+    /// Represents a location in the filesystem tree and can be used to perform operations that act
+    /// on the metadata of a file/directory (e.g., `getattr` and `setattr`). Can also be used as the
+    /// starting point for looking up paths in the filesystem tree. An `Inode` may support operating
+    /// directly on the content of the path that to which it points. `FileSystem` implementations
+    /// that support this should set the `FsOptions::ZERO_MESSAGE_OPEN` option in the return value
+    /// of the `init` function. On linux based systems, an `Inode` is equivalent to opening a file
+    /// or directory with the `libc::O_PATH` flag.
+    ///
+    /// # Lookup Count
+    ///
+    /// The `FileSystem` implementation is required to keep a "lookup count" for every `Inode`.
+    /// Every time an `Entry` is returned by a `FileSystem` trait method, this lookup count should
+    /// increase by 1. The lookup count for an `Inode` decreases when the kernel sends a `forget`
+    /// request. `Inode`s with a non-zero lookup count may receive requests from the kernel even
+    /// after calls to `unlink`, `rmdir` or (when overwriting an existing file) `rename`.
+    /// `FileSystem` implementations must handle such requests properly and it is recommended to
+    /// defer removal of the `Inode` until the lookup count reaches zero. Calls to `unlink`, `rmdir`
+    /// or `rename` will be followed closely by `forget` unless the file or directory is open, in
+    /// which case the kernel issues `forget` only after the `release` or `releasedir` calls.
+    ///
+    /// Note that if a file system will be exported over NFS the `Inode`'s lifetime must extend even
+    /// beyond `forget`. See the `generation` field in `Entry`.
+    type Inode: From<u64> + Into<u64>;
+
+    /// Represents a file or directory that is open for reading/writing.
+    type Handle: From<u64> + Into<u64>;
+
+    /// Initialize the file system.
+    ///
+    /// This method is called when a connection to the FUSE kernel module is first established. The
+    /// `capable` parameter indicates the features that are supported by the kernel module. The
+    /// implementation should return the options that it supports. Any options set in the returned
+    /// `FsOptions` that are not also set in `capable` are silently dropped.
+    fn init(&self, capable: FsOptions) -> io::Result<FsOptions> {
+        Ok(FsOptions::empty())
+    }
+
+    /// Clean up the file system.
+    ///
+    /// Called when the filesystem exits. All open `Handle`s should be closed and the lookup count
+    /// for all open `Inode`s implicitly goes to zero. At this point the connection to the FUSE
+    /// kernel module may already be gone so implementations should not rely on being able to
+    /// communicate with the kernel.
+    fn destroy(&self) {}
+
+    /// Look up a directory entry by name and get its attributes.
+    ///
+    /// If this call is successful then the lookup count of the `Inode` associated with the returned
+    /// `Entry` must be increased by 1.
+    fn lookup(&self, ctx: Context, parent: Self::Inode, name: &CStr) -> io::Result<Entry> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Forget about an inode.
+    ///
+    /// Called when the kernel removes an inode from its internal caches. `count` indicates the
+    /// amount by which the lookup count for the inode should be decreased. If reducing the lookup
+    /// count by `count` causes it to go to zero, then the implementation may delete the `Inode`.
+    fn forget(&self, ctx: Context, inode: Self::Inode, count: u64) {}
+
+    /// Forget about multiple inodes.
+    ///
+    /// `requests` is a vector of `(inode, count)` pairs. See the documentation for `forget` for
+    /// more information.
+    fn batch_forget(&self, ctx: Context, requests: Vec<(Self::Inode, u64)>) {
+        for (inode, count) in requests {
+            self.forget(ctx, inode, count)
+        }
+    }
+
+    /// Get attributes for a file / directory.
+    ///
+    /// If `handle` is not `None`, then it contains the handle previously returned by the
+    /// implementation after a call to `open` or `opendir`. However, implementations should still
+    /// take care to verify the handle if they do not trust the client (e.g., virtio-fs).
+    ///
+    /// If writeback caching is enabled (`FsOptions::WRITEBACK_CACHE`), then the kernel module
+    /// likely has a better idea of the length of the file than the file system (for
+    /// example, if there was a write that extended the size of the file but has not yet been
+    /// flushed). In this case, the `st_size` field of the returned struct is ignored.
+    ///
+    /// The returned `Duration` indicates how long the returned attributes should be considered
+    /// valid by the client. If the attributes are only changed via the FUSE kernel module (i.e.,
+    /// the kernel module has exclusive access), then this should be a very large value.
+    fn getattr(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Option<Self::Handle>,
+    ) -> io::Result<(libc::stat64, Duration)> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Set attributes for a file / directory.
+    ///
+    /// If `handle` is not `None`, then it contains the handle previously returned by the
+    /// implementation after a call to `open` or `opendir`. However, implementations should still
+    /// take care to verify the handle if they do not trust the client (e.g., virtio-fs).
+    ///
+    /// The `valid` parameter indicates the fields of `attr` that may be considered valid and should
+    /// be set by the file system. The content of all other fields in `attr` is undefined.
+    ///
+    /// If the `FsOptions::HANDLE_KILLPRIV` was set during `init`, then the implementation is
+    /// expected to reset the setuid and setgid bits if the file size or owner is being changed.
+    ///
+    /// This method returns the new attributes after making the modifications requested by the
+    /// client. The returned `Duration` indicates how long the returned attributes should be
+    /// considered valid by the client. If the attributes are only changed via the FUSE kernel
+    /// module (i.e., the kernel module has exclusive access), then this should be a very large
+    /// value.
+    fn setattr(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        attr: libc::stat64,
+        handle: Option<Self::Handle>,
+        valid: SetattrValid,
+    ) -> io::Result<(libc::stat64, Duration)> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Read a symbolic link.
+    fn readlink(&self, ctx: Context, inode: Self::Inode) -> io::Result<Vec<u8>> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Create a symbolic link.
+    ///
+    /// The file system must create a symbolic link named `name` in the directory represented by
+    /// `parent`, which contains the string `linkname`. Returns an `Entry` for the newly created
+    /// symlink.
+    ///
+    /// If this call is successful then the lookup count of the `Inode` associated with the returned
+    /// `Entry` must be increased by 1.
+    fn symlink(
+        &self,
+        ctx: Context,
+        linkname: &CStr,
+        parent: Self::Inode,
+        name: &CStr,
+    ) -> io::Result<Entry> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Create a file node.
+    ///
+    /// Create a regular file, character device, block device, fifo, or socket node named `name` in
+    /// the directory represented by `inode`. Valid values for `mode` and `rdev` are the same as
+    /// those accepted by the `mknod(2)` system call. Returns an `Entry` for the newly created node.
+    ///
+    /// When the `FsOptions::DONT_MASK` feature is set, the file system is responsible for setting
+    /// the permissions of the created node to `mode & !umask`.
+    ///
+    /// If this call is successful then the lookup count of the `Inode` associated with the returned
+    /// `Entry` must be increased by 1.
+    fn mknod(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        name: &CStr,
+        mode: u32,
+        rdev: u32,
+        umask: u32,
+    ) -> io::Result<Entry> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Create a directory.
+    ///
+    /// When the `FsOptions::DONT_MASK` feature is set, the file system is responsible for setting
+    /// the permissions of the created directory to `mode & !umask`. Returns an `Entry` for the
+    /// newly created directory.
+    ///
+    /// If this call is successful then the lookup count of the `Inode` associated with the returned
+    /// `Entry` must be increased by 1.
+    fn mkdir(
+        &self,
+        ctx: Context,
+        parent: Self::Inode,
+        name: &CStr,
+        mode: u32,
+        umask: u32,
+    ) -> io::Result<Entry> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Remove a file.
+    ///
+    /// If the file's inode lookup count is non-zero, then the file system is expected to delay
+    /// removal of the inode until the lookup count goes to zero. See the documentation of the
+    /// `forget` function for more information.
+    fn unlink(&self, ctx: Context, parent: Self::Inode, name: &CStr) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Remove a directory.
+    ///
+    /// If the directory's inode lookup count is non-zero, then the file system is expected to delay
+    /// removal of the inode until the lookup count goes to zero. See the documentation of the
+    /// `forget` function for more information.
+    fn rmdir(&self, ctx: Context, parent: Self::Inode, name: &CStr) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Rename a file / directory.
+    ///
+    /// If the destination exists, it should be atomically replaced. If the destination's inode
+    /// lookup count is non-zero, then the file system is expected to delay removal of the inode
+    /// until the lookup count goes to zero. See the documentation of the `forget` function for more
+    /// information.
+    ///
+    /// `flags` may be `libc::RENAME_EXCHANGE` or `libc::RENAME_NOREPLACE`. If
+    /// `libc::RENAME_NOREPLACE` is specified, the implementation must not overwrite `newname` if it
+    /// exists and must return an error instead. If `libc::RENAME_EXCHANGE` is specified, the
+    /// implementation must atomically exchange the two files, i.e., both must exist and neither may
+    /// be deleted.
+    fn rename(
+        &self,
+        ctx: Context,
+        olddir: Self::Inode,
+        oldname: &CStr,
+        newdir: Self::Inode,
+        newname: &CStr,
+        flags: u32,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Create a hard link.
+    ///
+    /// Create a hard link from `inode` to `newname` in the directory represented by `newparent`.
+    ///
+    /// If this call is successful then the lookup count of the `Inode` associated with the returned
+    /// `Entry` must be increased by 1.
+    fn link(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        newparent: Self::Inode,
+        newname: &CStr,
+    ) -> io::Result<Entry> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Open a file.
+    ///
+    /// Open the file associated with `inode` for reading / writing. All values accepted by the
+    /// `open(2)` system call are valid values for `flags` and must be handled by the file system.
+    /// However, there are some additional rules:
+    ///
+    /// * Creation flags (`libc::O_CREAT`, `libc::O_EXCL`, `libc::O_NOCTTY`) will be filtered out
+    ///   and handled by the kernel.
+    ///
+    /// * The file system should check the access modes (`libc::O_RDONLY`, `libc::O_WRONLY`,
+    ///   `libc::O_RDWR`) to determine if the operation is permitted. If the file system was mounted
+    ///   with the `-o default_permissions` mount option, then this check will also be carried out
+    ///   by the kernel before sending the open request.
+    ///
+    /// * When writeback caching is enabled (`FsOptions::WRITEBACK_CACHE`) the kernel may send read
+    ///   requests even for files opened with `libc::O_WRONLY`. The file system should be prepared
+    ///   to handle this.
+    ///
+    /// * When writeback caching is enabled, the kernel will handle the `libc::O_APPEND` flag.
+    ///   However, this will not work reliably unless the kernel has exclusive access to the file.
+    ///   In this case the file system may either ignore the `libc::O_APPEND` flag or return an
+    ///   error to indicate that reliable `libc::O_APPEND` handling is not available.
+    ///
+    /// * When writeback caching is disabled, the file system is expected to properly handle
+    ///   `libc::O_APPEND` and ensure that each write is appended to the end of the file.
+    ///
+    /// The file system may choose to return a `Handle` to refer to the newly opened file. The
+    /// kernel will then use this `Handle` for all operations on the content of the file (`read`,
+    /// `write`, `flush`, `release`, `fsync`). If the file system does not return a
+    /// `Handle` then the kernel will use the `Inode` for the file to operate on its contents. In
+    /// this case the file system may wish to enable the `FsOptions::ZERO_MESSAGE_OPEN` feature if
+    /// it is supported by the kernel (see below).
+    ///
+    /// The returned `OpenOptions` allow the file system to change the way the opened file is
+    /// handled by the kernel. See the documentation of `OpenOptions` for more information.
+    ///
+    /// If the `FsOptions::ZERO_MESSAGE_OPEN` feature is enabled by both the file system
+    /// implementation and the kernel, then the file system may return an error of `ENOSYS`. This
+    /// will be interpreted by the kernel as success and future calls to `open` and `release` will
+    /// be handled by the kernel without being passed on to the file system.
+    fn open(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        flags: u32,
+    ) -> io::Result<(Option<Self::Handle>, OpenOptions)> {
+        // Matches the behavior of libfuse.
+        Ok((None, OpenOptions::empty()))
+    }
+
+    /// Create and open a file.
+    ///
+    /// If the file does not already exist, the file system should create it with the specified
+    /// `mode`. When the `FsOptions::DONT_MASK` feature is set, the file system is responsible for
+    /// setting the permissions of the created file to `mode & !umask`.
+    ///
+    /// If the file system returns an `ENOSYS` error, then the kernel will treat this method as
+    /// unimplemented and all future calls to `create` will be handled by calling the `mknod` and
+    /// `open` methods instead.
+    ///
+    /// See the documentation for the `open` method for more information about opening the file. In
+    /// addition to the optional `Handle` and the `OpenOptions`, the file system must also return an
+    /// `Entry` for the file. This increases the lookup count for the `Inode` associated with the
+    /// file by 1.
+    fn create(
+        &self,
+        ctx: Context,
+        parent: Self::Inode,
+        name: &CStr,
+        mode: u32,
+        flags: u32,
+        umask: u32,
+    ) -> io::Result<(Entry, Option<Self::Handle>, OpenOptions)> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Read data from a file.
+    ///
+    /// Returns `size` bytes of data starting from offset `off` from the file associated with
+    /// `inode` or `handle`.
+    ///
+    /// `flags` contains the flags used to open the file. Similarly, `handle` is the `Handle`
+    /// returned by the file system from the `open` method, if any. If the file system
+    /// implementation did not return a `Handle` from `open` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// This method should return exactly the number of bytes requested by the kernel, except in the
+    /// case of error or EOF. Otherwise, the kernel will substitute the rest of the data with
+    /// zeroes. An exception to this rule is if the file was opened with the "direct I/O" option
+    /// (`libc::O_DIRECT`), in which case the kernel will forward the return code from this method
+    /// to the userspace application that made the system call.
+    fn read<W: io::Write + ZeroCopyWriter>(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        w: W,
+        size: u32,
+        offset: u64,
+        lock_owner: Option<u64>,
+        flags: u32,
+    ) -> io::Result<usize> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Write data to a file.
+    ///
+    /// Writes `size` bytes of data starting from offset `off` to the file associated with `inode`
+    /// or `handle`.
+    ///
+    /// `flags` contains the flags used to open the file. Similarly, `handle` is the `Handle`
+    /// returned by the file system from the `open` method, if any. If the file system
+    /// implementation did not return a `Handle` from `open` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// If the `FsOptions::HANDLE_KILLPRIV` feature is not enabled then then the file system is
+    /// expected to clear the setuid and setgid bits.
+    ///
+    /// If `delayed_write` is true then it indicates that this is a write for buffered data.
+    ///
+    /// This method should return exactly the number of bytes requested by the kernel, except in the
+    /// case of error. An exception to this rule is if the file was opened with the "direct I/O"
+    /// option (`libc::O_DIRECT`), in which case the kernel will forward the return code from this
+    /// method to the userspace application that made the system call.
+    fn write<R: io::Read + ZeroCopyReader>(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        r: R,
+        size: u32,
+        offset: u64,
+        lock_owner: Option<u64>,
+        delayed_write: bool,
+        flags: u32,
+    ) -> io::Result<usize> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Flush the contents of a file.
+    ///
+    /// This method is called on every `close()` of a file descriptor. Since it is possible to
+    /// duplicate file descriptors there may be many `flush` calls for one call to `open`.
+    ///
+    /// File systems should not make any assumptions about when `flush` will be
+    /// called or even if it will be called at all.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `open` method, if any. If the
+    /// file system did not return a `Handle` from `open` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// Unlike `fsync`, the file system is not required to flush pending writes. One reason to flush
+    /// data is if the file system wants to return write errors during close. However, this is not
+    /// portable because POSIX does not require `close` to wait for delayed I/O to complete.
+    ///
+    /// If the `FsOptions::POSIX_LOCKS` feature is enabled, then the file system must remove all
+    /// locks belonging to `lock_owner`.
+    ///
+    /// If this method returns an `ENOSYS` error then the kernel will treat it as success and all
+    /// subsequent calls to `flush` will be handled by the kernel without being forwarded to the
+    /// file system.
+    fn flush(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        lock_owner: u64,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Synchronize file contents.
+    ///
+    /// File systems must ensure that the file contents have been flushed to disk before returning
+    /// from this method. If `datasync` is true then only the file data (but not the metadata) needs
+    /// to be flushed.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `open` method, if any. If the
+    /// file system did not return a `Handle` from `open` then the contents of
+    /// `handle` are undefined.
+    ///
+    /// If this method returns an `ENOSYS` error then the kernel will treat it as success and all
+    /// subsequent calls to `fsync` will be handled by the kernel without being forwarded to the
+    /// file system.
+    fn fsync(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        datasync: bool,
+        handle: Self::Handle,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Allocate requested space for file data.
+    ///
+    /// If this function returns success, then the file sytem must guarantee that it is possible to
+    /// write up to `length` bytes of data starting at `offset` without failing due to a lack of
+    /// free space on the disk.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `open` method, if any. If the
+    /// file system did not return a `Handle` from `open` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// If this method returns an `ENOSYS` error then the kernel will treat that as a permanent
+    /// failure: all future calls to `fallocate` will fail with `EOPNOTSUPP` without being forwarded
+    /// to the file system.
+    fn fallocate(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        mode: u32,
+        offset: u64,
+        length: u64,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Release an open file.
+    ///
+    /// This method is called when there are no more references to an open file: all file
+    /// descriptors are closed and all memory mappings are unmapped.
+    ///
+    /// For every `open` call there will be exactly one `release` call (unless the file system is
+    /// force-unmounted).
+    ///
+    /// The file system may reply with an error, but error values are not returned to the `close()`
+    /// or `munmap()` which triggered the release.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `open` method, if any. If the
+    /// file system did not return a `Handle` from `open` then the contents of
+    /// `handle` are undefined.
+    ///
+    /// If `flush` is `true` then the contents of the file should also be flushed to disk.
+    fn release(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        flags: u32,
+        handle: Self::Handle,
+        flush: bool,
+        flock_release: bool,
+        lock_owner: Option<u64>,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Get information about the file system.
+    fn statfs(&self, ctx: Context, inode: Self::Inode) -> io::Result<libc::statvfs64> {
+        // Safe because we are zero-initializing a struct with only POD fields.
+        let mut st: libc::statvfs64 = unsafe { mem::zeroed() };
+
+        // This matches the behavior of libfuse as it returns these values if the
+        // filesystem doesn't implement this method.
+        st.f_namemax = 255;
+        st.f_bsize = 512;
+
+        Ok(st)
+    }
+
+    /// Set an extended attribute.
+    ///
+    /// If this method fails with an `ENOSYS` error, then the kernel will treat that as a permanent
+    /// failure. The kernel will return `EOPNOTSUPP` for all future calls to `setxattr` without
+    /// forwarding them to the file system.
+    ///
+    /// Valid values for flags are the same as those accepted by the `setxattr(2)` system call and
+    /// have the same behavior.
+    fn setxattr(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        name: &CStr,
+        value: &[u8],
+        flags: u32,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Get an extended attribute.
+    ///
+    /// If `size` is 0, then the file system should respond with `GetxattrReply::Count` and the
+    /// number of bytes needed to hold the value. If `size` is large enough to hold the value, then
+    /// the file system should reply with `GetxattrReply::Value` and the value of the extended
+    /// attribute. If `size` is not 0 but is also not large enough to hold the value, then the file
+    /// system should reply with an `ERANGE` error.
+    ///
+    /// If this method fails with an `ENOSYS` error, then the kernel will treat that as a permanent
+    /// failure. The kernel will return `EOPNOTSUPP` for all future calls to `getxattr` without
+    /// forwarding them to the file system.
+    fn getxattr(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        name: &CStr,
+        size: u32,
+    ) -> io::Result<GetxattrReply> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// List extended attribute names.
+    ///
+    /// If `size` is 0, then the file system should respond with `ListxattrReply::Count` and the
+    /// number of bytes needed to hold a `\0` byte separated list of the names of all the extended
+    /// attributes. If `size` is large enough to hold the `\0` byte separated list of the attribute
+    /// names, then the file system should reply with `ListxattrReply::Names` and the list. If
+    /// `size` is not 0 but is also not large enough to hold the list, then the file system should
+    /// reply with an `ERANGE` error.
+    ///
+    /// If this method fails with an `ENOSYS` error, then the kernel will treat that as a permanent
+    /// failure. The kernel will return `EOPNOTSUPP` for all future calls to `listxattr` without
+    /// forwarding them to the file system.
+    fn listxattr(&self, ctx: Context, inode: Self::Inode, size: u32) -> io::Result<ListxattrReply> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Remove an extended attribute.
+    ///
+    /// If this method fails with an `ENOSYS` error, then the kernel will treat that as a permanent
+    /// failure. The kernel will return `EOPNOTSUPP` for all future calls to `removexattr` without
+    /// forwarding them to the file system.
+    fn removexattr(&self, ctx: Context, inode: Self::Inode, name: &CStr) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Open a directory for reading.
+    ///
+    /// The file system may choose to return a `Handle` to refer to the newly opened directory. The
+    /// kernel will then use this `Handle` for all operations on the content of the directory
+    /// (`readdir`, `readdirplus`, `fsyncdir`, `releasedir`). If the file system does not return a
+    /// `Handle` then the kernel will use the `Inode` for the directory to operate on its contents.
+    /// In this case the file system may wish to enable the `FsOptions::ZERO_MESSAGE_OPENDIR`
+    /// feature if it is supported by the kernel (see below).
+    ///
+    /// The returned `OpenOptions` allow the file system to change the way the opened directory is
+    /// handled by the kernel. See the documentation of `OpenOptions` for more information.
+    ///
+    /// If the `FsOptions::ZERO_MESSAGE_OPENDIR` feature is enabled by both the file system
+    /// implementation and the kernel, then the file system may return an error of `ENOSYS`. This
+    /// will be interpreted by the kernel as success and future calls to `opendir` and `releasedir`
+    /// will be handled by the kernel without being passed on to the file system.
+    fn opendir(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        flags: u32,
+    ) -> io::Result<(Option<Self::Handle>, OpenOptions)> {
+        // Matches the behavior of libfuse.
+        Ok((None, OpenOptions::empty()))
+    }
+
+    /// Read a directory.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `opendir` method, if any. If
+    /// the file system did not return a `Handle` from `opendir` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// `size` indicates the maximum number of bytes that should be returned by this method.
+    ///
+    /// If `offset` is non-zero then it corresponds to one of the `offset` values from a `DirEntry`
+    /// that was previously returned by a call to `readdir` for the same handle. In this case the
+    /// file system should skip over the entries before the position defined by the `offset` value.
+    /// If entries were added or removed while the `Handle` is open then the file system may still
+    /// include removed entries or skip newly created entries. However, adding or removing entries
+    /// should never cause the file system to skip over unrelated entries or include an entry more
+    /// than once. This means that `offset` cannot be a simple index and must include sufficient
+    /// information to uniquely determine the next entry in the list even when the set of entries is
+    /// being changed.
+    ///
+    /// The file system may return entries for the current directory (".") and parent directory
+    /// ("..") but is not required to do so. If the file system does not return these entries, then
+    /// they are implicitly added by the kernel.
+    ///
+    /// The lookup count for `Inode`s associated with the returned directory entries is **NOT**
+    /// affected by this method.
+    ///
+    // TODO(chirantan): Change method signature to return `Iterator<DirEntry>` rather than using an
+    // `FnMut` for adding entries.
+    fn readdir<F>(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        size: u32,
+        offset: u64,
+        add_entry: F,
+    ) -> io::Result<()>
+    where
+        F: FnMut(DirEntry) -> io::Result<usize>,
+    {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Read a directory with entry attributes.
+    ///
+    /// Like `readdir` but also includes the attributes for each directory entry.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `opendir` method, if any. If
+    /// the file system did not return a `Handle` from `opendir` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// `size` indicates the maximum number of bytes that should be returned by this method.
+    ///
+    /// Unlike `readdir`, the lookup count for `Inode`s associated with the returned directory
+    /// entries **IS** affected by this method (since it returns an `Entry` for each `DirEntry`).
+    /// The count for each `Inode` should be increased by 1.
+    ///
+    /// File systems that implement this method should enable the `FsOptions::DO_READDIRPLUS`
+    /// feature when supported by the kernel. The kernel will not call this method unless that
+    /// feature is enabled.
+    ///
+    /// Additionally, file systems that implement both `readdir` and `readdirplus` should enable the
+    /// `FsOptions::READDIRPLUS_AUTO` feature to allow the kernel to issue both `readdir` and
+    /// `readdirplus` requests, depending on how much information is expected to be required.
+    ///
+    /// TODO(chirantan): Change method signature to return `Iterator<(DirEntry, Entry)>` rather than
+    /// using an `FnMut` for adding entries.
+    fn readdirplus<F>(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        size: u32,
+        offset: u64,
+        add_entry: F,
+    ) -> io::Result<()>
+    where
+        F: FnMut(DirEntry, Entry) -> io::Result<usize>,
+    {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Synchronize the contents of a directory.
+    ///
+    /// File systems must ensure that the directory contents have been flushed to disk before
+    /// returning from this method. If `datasync` is true then only the directory data (but not the
+    /// metadata) needs to be flushed.
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `opendir` method, if any. If
+    /// the file system did not return a `Handle` from `opendir` then the contents of
+    /// `handle` are undefined.
+    ///
+    /// If this method returns an `ENOSYS` error then the kernel will treat it as success and all
+    /// subsequent calls to `fsyncdir` will be handled by the kernel without being forwarded to the
+    /// file system.
+    fn fsyncdir(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        datasync: bool,
+        handle: Self::Handle,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Release an open directory.
+    ///
+    /// For every `opendir` call there will be exactly one `releasedir` call (unless the file system
+    /// is force-unmounted).
+    ///
+    /// `handle` is the `Handle` returned by the file system from the `opendir` method, if any. If
+    /// the file system did not return a `Handle` from `opendir` then the contents of `handle` are
+    /// undefined.
+    ///
+    /// `flags` contains used the flags used to open the directory in `opendir`.
+    fn releasedir(
+        &self,
+        ctx: Context,
+        inode: Self::Inode,
+        flags: u32,
+        handle: Self::Handle,
+    ) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// Check file access permissions.
+    ///
+    /// This method is called when a userspace process in the client makes an `access()` or
+    /// `chdir()` system call. If the file system was mounted with the `-o default_permissions`
+    /// mount option, then the kernel will perform these checks itself and this method will not be
+    /// called.
+    ///
+    /// If this method returns an `ENOSYS` error, then the kernel will treat it as a permanent
+    /// success: all future calls to `access` will return success without being forwarded to the
+    /// file system.
+    fn access(&self, ctx: Context, inode: Self::Inode, mask: u32) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn getlk(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn setlk(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn setlkw(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn ioctl(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn bmap(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn poll(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn notify_reply(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+
+    /// TODO: support this
+    fn lseek(&self) -> io::Result<()> {
+        Err(io::Error::from_raw_os_error(libc::ENOSYS))
+    }
+}