xref: /external/crosvm/devices/src/virtio/fs/passthrough.rs

// 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::borrow::Cow;
use std::cmp;
use std::collections::btree_map;
use std::collections::BTreeMap;
use std::ffi::{CStr, CString};
use std::fs::File;
use std::io;
use std::mem::{self, size_of, MaybeUninit};
use std::os::raw::{c_int, c_long};
use std::str::FromStr;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::Arc;
use std::time::Duration;

use base::{
    error, ioctl_ior_nr, ioctl_iow_nr, ioctl_iowr_nr, ioctl_with_mut_ptr, ioctl_with_ptr,
    AsRawDescriptor, FromRawDescriptor, RawDescriptor,
};
use data_model::DataInit;
use fuse::filesystem::{
    Context, DirectoryIterator, Entry, FileSystem, FsOptions, GetxattrReply, IoctlFlags,
    IoctlReply, ListxattrReply, OpenOptions, RemoveMappingOne, SetattrValid, ZeroCopyReader,
    ZeroCopyWriter, ROOT_ID,
};
use fuse::sys::WRITE_KILL_PRIV;
use fuse::Mapper;
use sync::Mutex;

use crate::virtio::fs::caps::{Capability, Caps, Set as CapSet, Value as CapValue};
use crate::virtio::fs::multikey::MultikeyBTreeMap;
use crate::virtio::fs::read_dir::ReadDir;

const EMPTY_CSTR: &[u8] = b"\0";
const ROOT_CSTR: &[u8] = b"/\0";
const PROC_CSTR: &[u8] = b"/proc\0";

const USER_VIRTIOFS_XATTR: &[u8] = b"user.virtiofs.";
const SECURITY_XATTR: &[u8] = b"security.";
const SELINUX_XATTR: &[u8] = b"security.selinux";

const FSCRYPT_KEY_DESCRIPTOR_SIZE: usize = 8;
const FSCRYPT_KEY_IDENTIFIER_SIZE: usize = 16;

#[repr(C)]
#[derive(Clone, Copy)]
struct fscrypt_policy_v1 {
    _version: u8,
    _contents_encryption_mode: u8,
    _filenames_encryption_mode: u8,
    _flags: u8,
    _master_key_descriptor: [u8; FSCRYPT_KEY_DESCRIPTOR_SIZE],
}
unsafe impl DataInit for fscrypt_policy_v1 {}

#[repr(C)]
#[derive(Clone, Copy)]
struct fscrypt_policy_v2 {
    _version: u8,
    _contents_encryption_mode: u8,
    _filenames_encryption_mode: u8,
    _flags: u8,
    __reserved: [u8; 4],
    master_key_identifier: [u8; FSCRYPT_KEY_IDENTIFIER_SIZE],
}
unsafe impl DataInit for fscrypt_policy_v2 {}

#[repr(C)]
#[derive(Copy, Clone)]
union fscrypt_policy {
    _version: u8,
    _v1: fscrypt_policy_v1,
    _v2: fscrypt_policy_v2,
}
unsafe impl DataInit for fscrypt_policy {}

#[repr(C)]
#[derive(Copy, Clone)]
struct fscrypt_get_policy_ex_arg {
    policy_size: u64,       /* input/output */
    policy: fscrypt_policy, /* output */
}
unsafe impl DataInit for fscrypt_get_policy_ex_arg {}

ioctl_iowr_nr!(FS_IOC_GET_ENCRYPTION_POLICY_EX, 'f' as u32, 22, [u8; 9]);

#[repr(C)]
#[derive(Clone, Copy)]
struct fsxattr {
    _fsx_xflags: u32,     /* xflags field value (get/set) */
    _fsx_extsize: u32,    /* extsize field value (get/set)*/
    _fsx_nextents: u32,   /* nextents field value (get)	*/
    _fsx_projid: u32,     /* project identifier (get/set) */
    _fsx_cowextsize: u32, /* CoW extsize field value (get/set)*/
    _fsx_pad: [u8; 8],
}
unsafe impl DataInit for fsxattr {}

ioctl_ior_nr!(FS_IOC_FSGETXATTR, 'X' as u32, 31, fsxattr);
ioctl_iow_nr!(FS_IOC_FSSETXATTR, 'X' as u32, 32, fsxattr);

ioctl_ior_nr!(FS_IOC_GETFLAGS, 'f' as u32, 1, c_long);
ioctl_iow_nr!(FS_IOC_SETFLAGS, 'f' as u32, 2, c_long);

ioctl_ior_nr!(FS_IOC32_GETFLAGS, 'f' as u32, 1, u32);
ioctl_iow_nr!(FS_IOC32_SETFLAGS, 'f' as u32, 2, u32);

ioctl_ior_nr!(FS_IOC64_GETFLAGS, 'f' as u32, 1, u64);
ioctl_iow_nr!(FS_IOC64_SETFLAGS, 'f' as u32, 2, u64);

type Inode = u64;
type Handle = u64;

#[derive(Clone, Copy, PartialOrd, Ord, PartialEq, Eq)]
struct InodeAltKey {
    ino: libc::ino64_t,
    dev: libc::dev_t,
}

#[derive(PartialEq, Eq)]
enum FileType {
    Regular,
    Directory,
    Other,
}

impl From<libc::mode_t> for FileType {
    fn from(mode: libc::mode_t) -> Self {
        match mode & libc::S_IFMT {
            libc::S_IFREG => FileType::Regular,
            libc::S_IFDIR => FileType::Directory,
            _ => FileType::Other,
        }
    }
}

struct InodeData {
    inode: Inode,
    // (File, open_flags)
    file: Mutex<(File, libc::c_int)>,
    refcount: AtomicU64,
    filetype: FileType,
}

impl AsRawDescriptor for InodeData {
    fn as_raw_descriptor(&self) -> RawDescriptor {
        self.file.lock().0.as_raw_descriptor()
    }
}

struct HandleData {
    inode: Inode,
    file: Mutex<File>,
}

impl AsRawDescriptor for HandleData {
    fn as_raw_descriptor(&self) -> RawDescriptor {
        self.file.lock().as_raw_descriptor()
    }
}

macro_rules! scoped_cred {
    ($name:ident, $ty:ty, $syscall_nr:expr) => {
        #[derive(Debug)]
        struct $name {
            old: $ty,
        }

        impl $name {
            // Changes the effective uid/gid of the current thread to `val`. Changes the thread's
            // credentials back to `old` when the returned struct is dropped.
            fn new(val: $ty, old: $ty) -> io::Result<Option<$name>> {
                if val == old {
                    // Nothing to do since we already have the correct value.
                    return Ok(None);
                }

                // We want credential changes to be per-thread because otherwise
                // we might interfere with operations being carried out on other
                // threads with different uids/gids.  However, posix requires that
                // all threads in a process share the same credentials.  To do this
                // libc uses signals to ensure that when one thread changes its
                // credentials the other threads do the same thing.
                //
                // So instead we invoke the syscall directly in order to get around
                // this limitation.  Another option is to use the setfsuid and
                // setfsgid systems calls.   However since those calls have no way to
                // return an error, it's preferable to do this instead.

                // This call is safe because it doesn't modify any memory and we
                // check the return value.
                let res = unsafe { libc::syscall($syscall_nr, -1, val, -1) };
                if res == 0 {
                    Ok(Some($name { old }))
                } else {
                    Err(io::Error::last_os_error())
                }
            }
        }

        impl Drop for $name {
            fn drop(&mut self) {
                let res = unsafe { libc::syscall($syscall_nr, -1, self.old, -1) };
                if res < 0 {
                    error!(
                        "failed to change credentials back to {}: {}",
                        self.old,
                        io::Error::last_os_error(),
                    );
                }
            }
        }
    };
}
#[cfg(not(target_arch = "arm"))]
scoped_cred!(ScopedUid, libc::uid_t, libc::SYS_setresuid);
#[cfg(target_arch = "arm")]
scoped_cred!(ScopedUid, libc::uid_t, libc::SYS_setresuid32);

#[cfg(not(target_arch = "arm"))]
scoped_cred!(ScopedGid, libc::gid_t, libc::SYS_setresgid);
#[cfg(target_arch = "arm")]
scoped_cred!(ScopedGid, libc::gid_t, libc::SYS_setresgid32);

#[cfg(not(target_arch = "arm"))]
const SYS_GETEUID: libc::c_long = libc::SYS_geteuid;
#[cfg(target_arch = "arm")]
const SYS_GETEUID: libc::c_long = libc::SYS_geteuid32;

#[cfg(not(target_arch = "arm"))]
const SYS_GETEGID: libc::c_long = libc::SYS_getegid;
#[cfg(target_arch = "arm")]
const SYS_GETEGID: libc::c_long = libc::SYS_getegid32;

thread_local! {
    // Both these calls are safe because they take no parameters, and only return an integer value.
    // The kernel also guarantees that they can never fail.
    static THREAD_EUID: libc::uid_t = unsafe { libc::syscall(SYS_GETEUID) as libc::uid_t };
    static THREAD_EGID: libc::gid_t = unsafe { libc::syscall(SYS_GETEGID) as libc::gid_t };
}

fn set_creds(
    uid: libc::uid_t,
    gid: libc::gid_t,
) -> io::Result<(Option<ScopedUid>, Option<ScopedGid>)> {
    let olduid = THREAD_EUID.with(|uid| *uid);
    let oldgid = THREAD_EGID.with(|gid| *gid);

    // We have to change the gid before we change the uid because if we change the uid first then we
    // lose the capability to change the gid.  However changing back can happen in any order.
    ScopedGid::new(gid, oldgid).and_then(|gid| Ok((ScopedUid::new(uid, olduid)?, gid)))
}

struct ScopedUmask<'a> {
    old: libc::mode_t,
    mask: libc::mode_t,
    _factory: &'a mut Umask,
}

impl<'a> Drop for ScopedUmask<'a> {
    fn drop(&mut self) {
        // Safe because this doesn't modify any memory and always succeeds.
        let previous = unsafe { libc::umask(self.old) };
        debug_assert_eq!(
            previous, self.mask,
            "umask changed while holding ScopedUmask"
        );
    }
}

struct Umask;

impl Umask {
    fn set(&mut self, mask: libc::mode_t) -> ScopedUmask {
        ScopedUmask {
            // Safe because this doesn't modify any memory and always succeeds.
            old: unsafe { libc::umask(mask) },
            mask,
            _factory: self,
        }
    }
}

struct ScopedFsetid(Caps);
impl Drop for ScopedFsetid {
    fn drop(&mut self) {
        if let Err(e) = raise_cap_fsetid(&mut self.0) {
            error!(
                "Failed to restore CAP_FSETID: {}.  Some operations may be broken.",
                e
            )
        }
    }
}

fn raise_cap_fsetid(c: &mut Caps) -> io::Result<()> {
    c.update(&[Capability::Fsetid], CapSet::Effective, CapValue::Set)?;
    c.apply()
}

// Drops CAP_FSETID from the effective set for the current thread and returns an RAII guard that
// adds the capability back when it is dropped.
fn drop_cap_fsetid() -> io::Result<ScopedFsetid> {
    let mut caps = Caps::for_current_thread()?;
    caps.update(&[Capability::Fsetid], CapSet::Effective, CapValue::Clear)?;
    caps.apply()?;
    Ok(ScopedFsetid(caps))
}

fn ebadf() -> io::Error {
    io::Error::from_raw_os_error(libc::EBADF)
}

fn stat<F: AsRawDescriptor>(f: &F) -> io::Result<libc::stat64> {
    let mut st = MaybeUninit::<libc::stat64>::zeroed();

    // Safe because this is a constant value and a valid C string.
    let pathname = unsafe { CStr::from_bytes_with_nul_unchecked(EMPTY_CSTR) };

    // Safe because the kernel will only write data in `st` and we check the return
    // value.
    let res = unsafe {
        libc::fstatat64(
            f.as_raw_descriptor(),
            pathname.as_ptr(),
            st.as_mut_ptr(),
            libc::AT_EMPTY_PATH | libc::AT_SYMLINK_NOFOLLOW,
        )
    };
    if res >= 0 {
        // Safe because the kernel guarantees that the struct is now fully initialized.
        Ok(unsafe { st.assume_init() })
    } else {
        Err(io::Error::last_os_error())
    }
}

fn statat<D: AsRawDescriptor>(dir: &D, name: &CStr) -> io::Result<libc::stat64> {
    let mut st = MaybeUninit::<libc::stat64>::zeroed();

    // Safe because the kernel will only write data in `st` and we check the return
    // value.
    let res = unsafe {
        libc::fstatat64(
            dir.as_raw_descriptor(),
            name.as_ptr(),
            st.as_mut_ptr(),
            libc::AT_SYMLINK_NOFOLLOW,
        )
    };
    if res >= 0 {
        // Safe because the kernel guarantees that the struct is now fully initialized.
        Ok(unsafe { st.assume_init() })
    } else {
        Err(io::Error::last_os_error())
    }
}

/// The caching policy that the file system should report to the FUSE client. By default the FUSE
/// protocol uses close-to-open consistency. This means that any cached contents of the file are
/// invalidated the next time that file is opened.
#[derive(Debug, Clone, Eq, PartialEq)]
pub enum CachePolicy {
    /// The client should never cache file data and all I/O should be directly forwarded to the
    /// server. This policy must be selected when file contents may change without the knowledge of
    /// the FUSE client (i.e., the file system does not have exclusive access to the directory).
    Never,

    /// The client is free to choose when and how to cache file data. This is the default policy and
    /// uses close-to-open consistency as described in the enum documentation.
    Auto,

    /// The client should always cache file data. This means that the FUSE client will not
    /// invalidate any cached data that was returned by the file system the last time the file was
    /// opened. This policy should only be selected when the file system has exclusive access to the
    /// directory.
    Always,
}

impl FromStr for CachePolicy {
    type Err = &'static str;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "never" | "Never" | "NEVER" => Ok(CachePolicy::Never),
            "auto" | "Auto" | "AUTO" => Ok(CachePolicy::Auto),
            "always" | "Always" | "ALWAYS" => Ok(CachePolicy::Always),
            _ => Err("invalid cache policy"),
        }
    }
}

impl Default for CachePolicy {
    fn default() -> Self {
        CachePolicy::Auto
    }
}

/// Options that configure the behavior of the file system.
#[derive(Debug, Clone)]
pub struct Config {
    /// How long the FUSE client should consider directory entries to be valid. If the contents of a
    /// directory can only be modified by the FUSE client (i.e., the file system has exclusive
    /// access), then this should be a large value.
    ///
    /// The default value for this option is 5 seconds.
    pub entry_timeout: Duration,

    /// How long the FUSE client should consider file and directory attributes to be valid. If the
    /// attributes of a file or directory can only be modified by the FUSE client (i.e., the file
    /// system has exclusive access), then this should be set to a large value.
    ///
    /// The default value for this option is 5 seconds.
    pub attr_timeout: Duration,

    /// The caching policy the file system should use. See the documentation of `CachePolicy` for
    /// more details.
    pub cache_policy: CachePolicy,

    /// Whether the file system should enabled writeback caching. This can improve performance as it
    /// allows the FUSE client to cache and coalesce multiple writes before sending them to the file
    /// system. However, enabling this option can increase the risk of data corruption if the file
    /// contents can change without the knowledge of the FUSE client (i.e., the server does **NOT**
    /// have exclusive access). Additionally, the file system should have read access to all files
    /// in the directory it is serving as the FUSE client may send read requests even for files
    /// opened with `O_WRONLY`.
    ///
    /// Therefore callers should only enable this option when they can guarantee that: 1) the file
    /// system has exclusive access to the directory and 2) the file system has read permissions for
    /// all files in that directory.
    ///
    /// The default value for this option is `false`.
    pub writeback: bool,

    /// Controls whether security.* xattrs (except for security.selinux) are re-written. When this
    /// is set to true, the server will add a "user.virtiofs" prefix to xattrs in the security
    /// namespace. Setting these xattrs requires CAP_SYS_ADMIN in the namespace where the file
    /// system was mounted and since the server usually runs in an unprivileged user namespace, it's
    /// unlikely to have that capability.
    ///
    /// The default value for this option is `false`.
    pub rewrite_security_xattrs: bool,

    /// Use case-insensitive lookups for directory entries (ASCII only).
    ///
    /// The default value for this option is `false`.
    pub ascii_casefold: bool,
}

impl Default for Config {
    fn default() -> Self {
        Config {
            entry_timeout: Duration::from_secs(5),
            attr_timeout: Duration::from_secs(5),
            cache_policy: Default::default(),
            writeback: false,
            rewrite_security_xattrs: false,
            ascii_casefold: false,
        }
    }
}

/// A file system that simply "passes through" all requests it receives to the underlying file
/// system. To keep the implementation simple it servers the contents of its root directory. Users
/// that wish to serve only a specific directory should set up the environment so that that
/// directory ends up as the root of the file system process. One way to accomplish this is via a
/// combination of mount namespaces and the pivot_root system call.
pub struct PassthroughFs {
    // File descriptors for various points in the file system tree.
    inodes: Mutex<MultikeyBTreeMap<Inode, InodeAltKey, Arc<InodeData>>>,
    next_inode: AtomicU64,

    // File descriptors for open files and directories. Unlike the fds in `inodes`, these _can_ be
    // used for reading and writing data.
    handles: Mutex<BTreeMap<Handle, Arc<HandleData>>>,
    next_handle: AtomicU64,

    // File descriptor pointing to the `/proc` directory. This is used to convert an fd from
    // `inodes` into one that can go into `handles`. This is accomplished by reading the
    // `self/fd/{}` symlink. We keep an open fd here in case the file system tree that we are meant
    // to be serving doesn't have access to `/proc`.
    proc: File,

    // Whether writeback caching is enabled for this directory. This will only be true when
    // `cfg.writeback` is true and `init` was called with `FsOptions::WRITEBACK_CACHE`.
    writeback: AtomicBool,

    // Whether zero message opens are supported by the kernel driver.
    zero_message_open: AtomicBool,

    // Whether zero message opendir is supported by the kernel driver.
    zero_message_opendir: AtomicBool,

    // Used to ensure that only one thread at a time uses chdir(). Since chdir() affects the
    // process-wide CWD, we cannot allow more than one thread to do it at the same time.
    chdir_mutex: Mutex<()>,

    // Used when creating files / directories / nodes. Since the umask is process-wide, we can only
    // allow one thread at a time to change it.
    umask: Mutex<Umask>,

    cfg: Config,
}

impl PassthroughFs {
    pub fn new(cfg: Config) -> io::Result<PassthroughFs> {
        // Safe because this is a constant value and a valid C string.
        let proc_cstr = unsafe { CStr::from_bytes_with_nul_unchecked(PROC_CSTR) };

        // Safe because this doesn't modify any memory and we check the return value.
        let raw_descriptor = unsafe {
            libc::openat(
                libc::AT_FDCWD,
                proc_cstr.as_ptr(),
                libc::O_PATH | libc::O_NOFOLLOW | libc::O_CLOEXEC,
            )
        };
        if raw_descriptor < 0 {
            return Err(io::Error::last_os_error());
        }

        // Safe because we just opened this descriptor.
        let proc = unsafe { File::from_raw_descriptor(raw_descriptor) };

        Ok(PassthroughFs {
            inodes: Mutex::new(MultikeyBTreeMap::new()),
            next_inode: AtomicU64::new(ROOT_ID + 1),

            handles: Mutex::new(BTreeMap::new()),
            next_handle: AtomicU64::new(1),

            proc,

            writeback: AtomicBool::new(false),
            zero_message_open: AtomicBool::new(false),
            zero_message_opendir: AtomicBool::new(false),

            chdir_mutex: Mutex::new(()),
            umask: Mutex::new(Umask),
            cfg,
        })
    }

    pub fn keep_rds(&self) -> Vec<RawDescriptor> {
        vec![self.proc.as_raw_descriptor()]
    }

    fn rewrite_xattr_name<'xattr>(&self, name: &'xattr CStr) -> Cow<'xattr, CStr> {
        if !self.cfg.rewrite_security_xattrs {
            return Cow::Borrowed(name);
        }

        // Does not include nul-terminator.
        let buf = name.to_bytes();
        if !buf.starts_with(SECURITY_XATTR) || buf == SELINUX_XATTR {
            return Cow::Borrowed(name);
        }

        let mut newname = USER_VIRTIOFS_XATTR.to_vec();
        newname.extend_from_slice(buf);

        // The unwrap is safe here because the prefix doesn't contain any interior nul-bytes and the
        // to_bytes() call above will not return a byte slice with any interior nul-bytes either.
        Cow::Owned(CString::new(newname).expect("Failed to re-write xattr name"))
    }

    fn find_inode(&self, inode: Inode) -> io::Result<Arc<InodeData>> {
        self.inodes
            .lock()
            .get(&inode)
            .map(Arc::clone)
            .ok_or_else(ebadf)
    }

    fn find_handle(&self, handle: Handle, inode: Inode) -> io::Result<Arc<HandleData>> {
        self.handles
            .lock()
            .get(&handle)
            .filter(|hd| hd.inode == inode)
            .map(Arc::clone)
            .ok_or_else(ebadf)
    }

    fn open_fd(&self, fd: RawDescriptor, flags: i32) -> io::Result<File> {
        let pathname = CString::new(format!("self/fd/{}", fd))
            .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

        // Safe because this doesn't modify any memory and we check the return value. We don't
        // really check `flags` because if the kernel can't handle poorly specified flags then we
        // have much bigger problems. Also, clear the `O_NOFOLLOW` flag if it is set since we need
        // to follow the `/proc/self/fd` symlink to get the file.
        let raw_descriptor = unsafe {
            libc::openat(
                self.proc.as_raw_descriptor(),
                pathname.as_ptr(),
                (flags | libc::O_CLOEXEC) & !(libc::O_NOFOLLOW | libc::O_DIRECT),
            )
        };
        if raw_descriptor < 0 {
            return Err(io::Error::last_os_error());
        }

        // Safe because we just opened this descriptor.
        Ok(unsafe { File::from_raw_descriptor(raw_descriptor) })
    }

    fn open_inode(&self, inode: &InodeData, mut flags: i32) -> io::Result<File> {
        // When writeback caching is enabled, the kernel may send read requests even if the
        // userspace program opened the file write-only. So we need to ensure that we have opened
        // the file for reading as well as writing.
        let writeback = self.writeback.load(Ordering::Relaxed);
        if writeback && flags & libc::O_ACCMODE == libc::O_WRONLY {
            flags &= !libc::O_ACCMODE;
            flags |= libc::O_RDWR;
        }

        // When writeback caching is enabled the kernel is responsible for handling `O_APPEND`.
        // However, this breaks atomicity as the file may have changed on disk, invalidating the
        // cached copy of the data in the kernel and the offset that the kernel thinks is the end of
        // the file. Just allow this for now as it is the user's responsibility to enable writeback
        // caching only for directories that are not shared. It also means that we need to clear the
        // `O_APPEND` flag.
        if writeback && flags & libc::O_APPEND != 0 {
            flags &= !libc::O_APPEND;
        }

        self.open_fd(inode.as_raw_descriptor(), flags)
    }

    // Creates a new entry for `f` or increases the refcount of the existing entry for `f`.
    fn add_entry(&self, f: File, st: libc::stat64, open_flags: libc::c_int) -> Entry {
        let altkey = InodeAltKey {
            ino: st.st_ino,
            dev: st.st_dev,
        };
        let data = self.inodes.lock().get_alt(&altkey).map(Arc::clone);

        let inode = if let Some(data) = data {
            // Matches with the release store in `forget`.
            data.refcount.fetch_add(1, Ordering::Acquire);
            data.inode
        } else {
            // There is a possible race here where 2 threads end up adding the same file
            // into the inode list.  However, since each of those will get a unique Inode
            // value and unique file descriptors this shouldn't be that much of a problem.
            let inode = self.next_inode.fetch_add(1, Ordering::Relaxed);
            self.inodes.lock().insert(
                inode,
                InodeAltKey {
                    ino: st.st_ino,
                    dev: st.st_dev,
                },
                Arc::new(InodeData {
                    inode,
                    file: Mutex::new((f, open_flags)),
                    refcount: AtomicU64::new(1),
                    filetype: st.st_mode.into(),
                }),
            );

            inode
        };

        Entry {
            inode,
            generation: 0,
            attr: st,
            attr_timeout: self.cfg.attr_timeout,
            entry_timeout: self.cfg.entry_timeout,
        }
    }

    // Performs an ascii case insensitive lookup.
    fn ascii_casefold_lookup(&self, parent: &InodeData, name: &[u8]) -> io::Result<Entry> {
        let mut buf = [0u8; 1024];
        let mut offset = 0;
        loop {
            let mut read_dir = ReadDir::new(parent, offset, &mut buf[..])?;
            if read_dir.remaining() == 0 {
                break;
            }

            while let Some(entry) = read_dir.next() {
                offset = entry.offset as libc::off64_t;
                if name.eq_ignore_ascii_case(entry.name.to_bytes()) {
                    return self.do_lookup(parent, entry.name);
                }
            }
        }
        Err(io::Error::from_raw_os_error(libc::ENOENT))
    }

    fn do_lookup(&self, parent: &InodeData, name: &CStr) -> io::Result<Entry> {
        let st = statat(parent, name)?;

        let mut flags = libc::O_RDONLY | libc::O_NOFOLLOW | libc::O_CLOEXEC;
        match FileType::from(st.st_mode) {
            FileType::Regular => {}
            FileType::Directory => flags |= libc::O_DIRECTORY,
            FileType::Other => flags |= libc::O_PATH,
        }

        // Safe because this doesn't modify any memory and we check the return value.
        let fd = unsafe { libc::openat(parent.as_raw_descriptor(), name.as_ptr(), flags) };
        if fd < 0 {
            return Err(io::Error::last_os_error());
        }

        // Safe because we just opened this fd.
        let f = unsafe { File::from_raw_descriptor(fd) };

        Ok(self.add_entry(f, st, flags))
    }

    fn do_open(&self, inode: Inode, flags: u32) -> io::Result<(Option<Handle>, OpenOptions)> {
        let inode_data = self.find_inode(inode)?;

        let file = Mutex::new(self.open_inode(&inode_data, flags as i32)?);

        let handle = self.next_handle.fetch_add(1, Ordering::Relaxed);
        let data = HandleData { inode, file };

        self.handles.lock().insert(handle, Arc::new(data));

        let mut opts = OpenOptions::empty();
        match self.cfg.cache_policy {
            // We only set the direct I/O option on files.
            CachePolicy::Never => opts.set(
                OpenOptions::DIRECT_IO,
                flags & (libc::O_DIRECTORY as u32) == 0,
            ),
            CachePolicy::Always => {
                opts |= if flags & (libc::O_DIRECTORY as u32) == 0 {
                    OpenOptions::KEEP_CACHE
                } else {
                    OpenOptions::CACHE_DIR
                }
            }
            _ => {}
        };

        Ok((Some(handle), opts))
    }

    fn do_release(&self, inode: Inode, handle: Handle) -> io::Result<()> {
        let mut handles = self.handles.lock();

        if let btree_map::Entry::Occupied(e) = handles.entry(handle) {
            if e.get().inode == inode {
                // We don't need to close the file here because that will happen automatically when
                // the last `Arc` is dropped.
                e.remove();
                return Ok(());
            }
        }

        Err(ebadf())
    }

    fn do_getattr(&self, inode: &InodeData) -> io::Result<(libc::stat64, Duration)> {
        let st = stat(inode)?;

        Ok((st, self.cfg.attr_timeout))
    }

    fn do_unlink(&self, parent: &InodeData, name: &CStr, flags: libc::c_int) -> io::Result<()> {
        // Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe { libc::unlinkat(parent.as_raw_descriptor(), name.as_ptr(), flags) };
        if res == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn do_fsync<F: AsRawDescriptor>(&self, file: &F, datasync: bool) -> io::Result<()> {
        // Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe {
            if datasync {
                libc::fdatasync(file.as_raw_descriptor())
            } else {
                libc::fsync(file.as_raw_descriptor())
            }
        };

        if res == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    // Changes the CWD to `self.proc`, runs `f`, and then changes the CWD back to the root
    // directory. This effectively emulates an *at syscall starting at /proc, which is useful when
    // there is no *at syscall available. Panics if any of the fchdir calls fail or if there is no
    // root inode.
    fn with_proc_chdir<F, T>(&self, f: F) -> T
    where
        F: FnOnce() -> T,
    {
        let root = self.find_inode(ROOT_ID).expect("failed to find root inode");
        let chdir_lock = self.chdir_mutex.lock();

        // Safe because this doesn't modify any memory and we check the return value. Since the
        // fchdir should never fail we just use debug_asserts.
        let proc_cwd = unsafe { libc::fchdir(self.proc.as_raw_descriptor()) };
        debug_assert_eq!(
            proc_cwd,
            0,
            "failed to fchdir to /proc: {}",
            io::Error::last_os_error()
        );

        let res = f();

        // Safe because this doesn't modify any memory and we check the return value. Since the
        // fchdir should never fail we just use debug_asserts.
        let root_cwd = unsafe { libc::fchdir(root.as_raw_descriptor()) };
        debug_assert_eq!(
            root_cwd,
            0,
            "failed to fchdir back to root directory: {}",
            io::Error::last_os_error()
        );

        mem::drop(chdir_lock);
        res
    }

    fn do_getxattr(&self, inode: &InodeData, name: &CStr, value: &mut [u8]) -> io::Result<usize> {
        let res = if inode.filetype == FileType::Other {
            // For non-regular files and directories, we cannot open the fd normally. Instead we
            // emulate an _at syscall by changing the CWD to /proc, running the path based syscall,
            // and then setting the CWD back to the root directory.
            let path = CString::new(format!("self/fd/{}", inode.as_raw_descriptor()))
                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

            // Safe because this will only modify `value` and we check the return value.
            self.with_proc_chdir(|| unsafe {
                libc::getxattr(
                    path.as_ptr(),
                    name.as_ptr(),
                    value.as_mut_ptr() as *mut libc::c_void,
                    value.len() as libc::size_t,
                )
            })
        } else {
            // For regular files and directories, we can just use fgetxattr. Safe because this will
            // only write to `value` and we check the return value.
            unsafe {
                libc::fgetxattr(
                    inode.as_raw_descriptor(),
                    name.as_ptr(),
                    value.as_mut_ptr() as *mut libc::c_void,
                    value.len() as libc::size_t,
                )
            }
        };

        if res < 0 {
            Err(io::Error::last_os_error())
        } else {
            Ok(res as usize)
        }
    }

    fn get_encryption_policy_ex<R: io::Read>(
        &self,
        inode: Inode,
        handle: Handle,
        mut r: R,
    ) -> io::Result<IoctlReply> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            self.find_inode(inode)?
        } else {
            self.find_handle(handle, inode)?
        };

        // Safe because this only has integer fields.
        let mut arg = unsafe { MaybeUninit::<fscrypt_get_policy_ex_arg>::zeroed().assume_init() };
        r.read_exact(arg.policy_size.as_mut_slice())?;

        let policy_size = cmp::min(arg.policy_size, size_of::<fscrypt_policy>() as u64);
        arg.policy_size = policy_size;

        // Safe because the kernel will only write to `arg` and we check the return value.
        let res =
            unsafe { ioctl_with_mut_ptr(&*data, FS_IOC_GET_ENCRYPTION_POLICY_EX(), &mut arg) };
        if res < 0 {
            Ok(IoctlReply::Done(Err(io::Error::last_os_error())))
        } else {
            let len = size_of::<u64>() + arg.policy_size as usize;
            Ok(IoctlReply::Done(Ok(arg.as_slice()[..len].to_vec())))
        }
    }

    fn get_fsxattr(&self, inode: Inode, handle: Handle) -> io::Result<IoctlReply> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            self.find_inode(inode)?
        } else {
            self.find_handle(handle, inode)?
        };

        let mut buf = MaybeUninit::<fsxattr>::zeroed();

        // Safe because the kernel will only write to `buf` and we check the return value.
        let res = unsafe { ioctl_with_mut_ptr(&*data, FS_IOC_FSGETXATTR(), buf.as_mut_ptr()) };
        if res < 0 {
            Ok(IoctlReply::Done(Err(io::Error::last_os_error())))
        } else {
            // Safe because the kernel guarantees that the policy is now initialized.
            let xattr = unsafe { buf.assume_init() };
            Ok(IoctlReply::Done(Ok(xattr.as_slice().to_vec())))
        }
    }

    fn set_fsxattr<R: io::Read>(
        &self,
        inode: Inode,
        handle: Handle,
        r: R,
    ) -> io::Result<IoctlReply> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            self.find_inode(inode)?
        } else {
            self.find_handle(handle, inode)?
        };

        let attr = fsxattr::from_reader(r)?;

        //  Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe { ioctl_with_ptr(&*data, FS_IOC_FSSETXATTR(), &attr) };
        if res < 0 {
            Ok(IoctlReply::Done(Err(io::Error::last_os_error())))
        } else {
            Ok(IoctlReply::Done(Ok(Vec::new())))
        }
    }

    fn get_flags(&self, inode: Inode, handle: Handle) -> io::Result<IoctlReply> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            self.find_inode(inode)?
        } else {
            self.find_handle(handle, inode)?
        };

        // The ioctl encoding is a long but the parameter is actually an int.
        let mut flags: c_int = 0;

        // Safe because the kernel will only write to `flags` and we check the return value.
        let res = unsafe { ioctl_with_mut_ptr(&*data, FS_IOC_GETFLAGS(), &mut flags) };
        if res < 0 {
            Ok(IoctlReply::Done(Err(io::Error::last_os_error())))
        } else {
            Ok(IoctlReply::Done(Ok(flags.to_ne_bytes().to_vec())))
        }
    }

    fn set_flags<R: io::Read>(&self, inode: Inode, handle: Handle, r: R) -> io::Result<IoctlReply> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            self.find_inode(inode)?
        } else {
            self.find_handle(handle, inode)?
        };

        // The ioctl encoding is a long but the parameter is actually an int.
        let flags = c_int::from_reader(r)?;

        // Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe { ioctl_with_ptr(&*data, FS_IOC_SETFLAGS(), &flags) };
        if res < 0 {
            Ok(IoctlReply::Done(Err(io::Error::last_os_error())))
        } else {
            Ok(IoctlReply::Done(Ok(Vec::new())))
        }
    }
}

fn forget_one(
    inodes: &mut MultikeyBTreeMap<Inode, InodeAltKey, Arc<InodeData>>,
    inode: Inode,
    count: u64,
) {
    if let Some(data) = inodes.get(&inode) {
        // Acquiring the write lock on the inode map prevents new lookups from incrementing the
        // refcount but there is the possibility that a previous lookup already acquired a
        // reference to the inode data and is in the process of updating the refcount so we need
        // to loop here until we can decrement successfully.
        loop {
            let refcount = data.refcount.load(Ordering::Relaxed);

            // Saturating sub because it doesn't make sense for a refcount to go below zero and
            // we don't want misbehaving clients to cause integer overflow.
            let new_count = refcount.saturating_sub(count);

            // Synchronizes with the acquire load in `do_lookup`.
            if data
                .refcount
                .compare_exchange_weak(refcount, new_count, Ordering::Release, Ordering::Relaxed)
                .is_ok()
            {
                if new_count == 0 {
                    // We just removed the last refcount for this inode. There's no need for an
                    // acquire fence here because we hold a write lock on the inode map and any
                    // thread that is waiting to do a forget on the same inode will have to wait
                    // until we release the lock. So there's is no other release store for us to
                    // synchronize with before deleting the entry.
                    inodes.remove(&inode);
                }
                break;
            }
        }
    }
}

// Strips any `user.virtiofs.` prefix from `buf`. If buf contains one or more nul-bytes, each
// nul-byte-separated slice is treated as a C string and the prefix is stripped from each one.
fn strip_xattr_prefix(buf: &mut Vec<u8>) {
    fn next_cstr(b: &[u8], start: usize) -> Option<&[u8]> {
        if start >= b.len() {
            return None;
        }

        let end = b[start..]
            .iter()
            .position(|&c| c == b'\0')
            .map(|p| start + p + 1)
            .unwrap_or(b.len());

        Some(&b[start..end])
    }

    let mut pos = 0;
    while let Some(name) = next_cstr(&buf, pos) {
        if !name.starts_with(USER_VIRTIOFS_XATTR) {
            pos += name.len();
            continue;
        }

        let newlen = name.len() - USER_VIRTIOFS_XATTR.len();
        buf.drain(pos..pos + USER_VIRTIOFS_XATTR.len());
        pos += newlen;
    }
}

impl FileSystem for PassthroughFs {
    type Inode = Inode;
    type Handle = Handle;
    type DirIter = ReadDir<Box<[u8]>>;

    fn init(&self, capable: FsOptions) -> io::Result<FsOptions> {
        // Safe because this is a constant value and a valid C string.
        let root = unsafe { CStr::from_bytes_with_nul_unchecked(ROOT_CSTR) };

        let flags = libc::O_DIRECTORY | libc::O_NOFOLLOW | libc::O_CLOEXEC;
        // Safe because this doesn't modify any memory and we check the return value.
        let raw_descriptor = unsafe { libc::openat(libc::AT_FDCWD, root.as_ptr(), flags) };
        if raw_descriptor < 0 {
            return Err(io::Error::last_os_error());
        }

        // Safe because we just opened this descriptor above.
        let f = unsafe { File::from_raw_descriptor(raw_descriptor) };

        let st = stat(&f)?;

        // Safe because this doesn't modify any memory and there is no need to check the return
        // value because this system call always succeeds. We need to clear the umask here because
        // we want the client to be able to set all the bits in the mode.
        unsafe { libc::umask(0o000) };

        let mut inodes = self.inodes.lock();

        // Not sure why the root inode gets a refcount of 2 but that's what libfuse does.
        inodes.insert(
            ROOT_ID,
            InodeAltKey {
                ino: st.st_ino,
                dev: st.st_dev,
            },
            Arc::new(InodeData {
                inode: ROOT_ID,
                file: Mutex::new((f, flags)),
                refcount: AtomicU64::new(2),
                filetype: st.st_mode.into(),
            }),
        );

        let mut opts = FsOptions::DO_READDIRPLUS
            | FsOptions::READDIRPLUS_AUTO
            | FsOptions::EXPORT_SUPPORT
            | FsOptions::DONT_MASK
            | FsOptions::POSIX_ACL;
        if self.cfg.writeback && capable.contains(FsOptions::WRITEBACK_CACHE) {
            opts |= FsOptions::WRITEBACK_CACHE;
            self.writeback.store(true, Ordering::Relaxed);
        }
        if self.cfg.cache_policy == CachePolicy::Always {
            if capable.contains(FsOptions::ZERO_MESSAGE_OPEN) {
                opts |= FsOptions::ZERO_MESSAGE_OPEN;
                self.zero_message_open.store(true, Ordering::Relaxed);
            }
            if capable.contains(FsOptions::ZERO_MESSAGE_OPENDIR) {
                opts |= FsOptions::ZERO_MESSAGE_OPENDIR;
                self.zero_message_opendir.store(true, Ordering::Relaxed);
            }
        }
        Ok(opts)
    }

    fn destroy(&self) {
        self.handles.lock().clear();
        self.inodes.lock().clear();
    }

    fn statfs(&self, _ctx: Context, inode: Inode) -> io::Result<libc::statvfs64> {
        let data = self.find_inode(inode)?;

        let mut out = MaybeUninit::<libc::statvfs64>::zeroed();

        // Safe because this will only modify `out` and we check the return value.
        let res = unsafe { libc::fstatvfs64(data.as_raw_descriptor(), out.as_mut_ptr()) };
        if res == 0 {
            // Safe because the kernel guarantees that `out` has been initialized.
            Ok(unsafe { out.assume_init() })
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn lookup(&self, _ctx: Context, parent: Inode, name: &CStr) -> io::Result<Entry> {
        let data = self.find_inode(parent)?;
        self.do_lookup(&data, name).or_else(|e| {
            if self.cfg.ascii_casefold {
                self.ascii_casefold_lookup(&data, name.to_bytes())
            } else {
                Err(e)
            }
        })
    }

    fn forget(&self, _ctx: Context, inode: Inode, count: u64) {
        let mut inodes = self.inodes.lock();

        forget_one(&mut inodes, inode, count)
    }

    fn batch_forget(&self, _ctx: Context, requests: Vec<(Inode, u64)>) {
        let mut inodes = self.inodes.lock();

        for (inode, count) in requests {
            forget_one(&mut inodes, inode, count)
        }
    }

    fn opendir(
        &self,
        _ctx: Context,
        inode: Inode,
        flags: u32,
    ) -> io::Result<(Option<Handle>, OpenOptions)> {
        if self.zero_message_opendir.load(Ordering::Relaxed) {
            Err(io::Error::from_raw_os_error(libc::ENOSYS))
        } else {
            self.do_open(inode, flags | (libc::O_DIRECTORY as u32))
        }
    }

    fn releasedir(
        &self,
        _ctx: Context,
        inode: Inode,
        _flags: u32,
        handle: Handle,
    ) -> io::Result<()> {
        if self.zero_message_opendir.load(Ordering::Relaxed) {
            Ok(())
        } else {
            self.do_release(inode, handle)
        }
    }

    fn mkdir(
        &self,
        ctx: Context,
        parent: Inode,
        name: &CStr,
        mode: u32,
        umask: u32,
    ) -> io::Result<Entry> {
        let data = self.find_inode(parent)?;

        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;
        let res = {
            let mut um = self.umask.lock();
            let _scoped_umask = um.set(umask);

            // Safe because this doesn't modify any memory and we check the return value.
            unsafe { libc::mkdirat(data.as_raw_descriptor(), name.as_ptr(), mode) }
        };
        if res == 0 {
            self.do_lookup(&data, name)
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn rmdir(&self, _ctx: Context, parent: Inode, name: &CStr) -> io::Result<()> {
        let data = self.find_inode(parent)?;
        self.do_unlink(&data, name, libc::AT_REMOVEDIR)
    }

    fn readdir(
        &self,
        _ctx: Context,
        inode: Inode,
        handle: Handle,
        size: u32,
        offset: u64,
    ) -> io::Result<Self::DirIter> {
        let buf = vec![0; size as usize].into_boxed_slice();

        if self.zero_message_opendir.load(Ordering::Relaxed) {
            let data = self.find_inode(inode)?;
            ReadDir::new(&*data, offset as libc::off64_t, buf)
        } else {
            let data = self.find_handle(handle, inode)?;

            let dir = data.file.lock();

            ReadDir::new(&*dir, offset as libc::off64_t, buf)
        }
    }

    fn open(
        &self,
        _ctx: Context,
        inode: Inode,
        flags: u32,
    ) -> io::Result<(Option<Handle>, OpenOptions)> {
        if self.zero_message_open.load(Ordering::Relaxed) {
            Err(io::Error::from_raw_os_error(libc::ENOSYS))
        } else {
            self.do_open(inode, flags)
        }
    }

    fn release(
        &self,
        _ctx: Context,
        inode: Inode,
        _flags: u32,
        handle: Handle,
        _flush: bool,
        _flock_release: bool,
        _lock_owner: Option<u64>,
    ) -> io::Result<()> {
        if self.zero_message_open.load(Ordering::Relaxed) {
            Ok(())
        } else {
            self.do_release(inode, handle)
        }
    }

    fn chromeos_tmpfile(
        &self,
        ctx: Context,
        parent: Self::Inode,
        mode: u32,
        umask: u32,
    ) -> io::Result<Entry> {
        let data = self.find_inode(parent)?;

        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;

        let tmpflags = libc::O_RDWR | libc::O_TMPFILE | libc::O_CLOEXEC | libc::O_NOFOLLOW;

        // Safe because this is a valid c string.
        let current_dir = unsafe { CStr::from_bytes_with_nul_unchecked(b".\0") };

        let fd = {
            let mut um = self.umask.lock();
            let _scoped_umask = um.set(umask);

            // Safe because this doesn't modify any memory and we check the return value.
            unsafe {
                libc::openat(
                    data.as_raw_descriptor(),
                    current_dir.as_ptr(),
                    tmpflags,
                    mode,
                )
            }
        };
        if fd < 0 {
            return Err(io::Error::last_os_error());
        }

        // Safe because we just opened this fd.
        let tmpfile = unsafe { File::from_raw_descriptor(fd) };

        let st = stat(&tmpfile)?;
        Ok(self.add_entry(tmpfile, st, tmpflags))
    }

    fn create(
        &self,
        ctx: Context,
        parent: Inode,
        name: &CStr,
        mode: u32,
        flags: u32,
        umask: u32,
    ) -> io::Result<(Entry, Option<Handle>, OpenOptions)> {
        let data = self.find_inode(parent)?;

        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;

        let create_flags =
            (flags as i32 | libc::O_CREAT | libc::O_CLOEXEC | libc::O_NOFOLLOW) & !libc::O_DIRECT;

        let fd = {
            let mut um = self.umask.lock();
            let _scoped_umask = um.set(umask);

            // Safe because this doesn't modify any memory and we check the return value. We don't
            // really check `flags` because if the kernel can't handle poorly specified flags then
            // we have much bigger problems.
            unsafe { libc::openat(data.as_raw_descriptor(), name.as_ptr(), create_flags, mode) }
        };
        if fd < 0 {
            return Err(io::Error::last_os_error());
        }

        // Safe because we just opened this fd.
        let file = unsafe { File::from_raw_descriptor(fd) };

        let st = stat(&file)?;
        let entry = self.add_entry(file, st, create_flags);

        let (handle, opts) = if self.zero_message_open.load(Ordering::Relaxed) {
            (None, OpenOptions::KEEP_CACHE)
        } else {
            self.do_open(
                entry.inode,
                flags & !((libc::O_CREAT | libc::O_EXCL | libc::O_NOCTTY) as u32),
            )
            .map_err(|e| {
                // Don't leak the entry.
                self.forget(ctx, entry.inode, 1);
                e
            })?
        };

        Ok((entry, handle, opts))
    }

    fn unlink(&self, _ctx: Context, parent: Inode, name: &CStr) -> io::Result<()> {
        let data = self.find_inode(parent)?;
        self.do_unlink(&data, name, 0)
    }

    fn read<W: io::Write + ZeroCopyWriter>(
        &self,
        _ctx: Context,
        inode: Inode,
        handle: Handle,
        mut w: W,
        size: u32,
        offset: u64,
        _lock_owner: Option<u64>,
        _flags: u32,
    ) -> io::Result<usize> {
        if self.zero_message_open.load(Ordering::Relaxed) {
            let data = self.find_inode(inode)?;

            let mut file = data.file.lock();
            let mut flags = file.1;
            match flags & libc::O_ACCMODE {
                libc::O_WRONLY => {
                    flags &= !libc::O_WRONLY;
                    flags |= libc::O_RDWR;

                    // We need to get a readable handle for this file.
                    let newfile = self.open_fd(file.0.as_raw_descriptor(), libc::O_RDWR)?;
                    *file = (newfile, flags);
                }
                libc::O_RDONLY | libc::O_RDWR => {}
                _ => panic!("Unexpected flags: {:#x}", flags),
            }

            w.write_from(&mut file.0, size as usize, offset)
        } else {
            let data = self.find_handle(handle, inode)?;

            let mut f = data.file.lock();
            w.write_from(&mut f, size as usize, offset)
        }
    }

    fn write<R: io::Read + ZeroCopyReader>(
        &self,
        _ctx: Context,
        inode: Inode,
        handle: Handle,
        mut r: R,
        size: u32,
        offset: u64,
        _lock_owner: Option<u64>,
        _delayed_write: bool,
        flags: u32,
    ) -> io::Result<usize> {
        // When the WRITE_KILL_PRIV flag is set, drop CAP_FSETID so that the kernel will
        // automatically clear the setuid and setgid bits for us.
        let _fsetid = if flags & WRITE_KILL_PRIV != 0 {
            Some(drop_cap_fsetid()?)
        } else {
            None
        };

        if self.zero_message_open.load(Ordering::Relaxed) {
            let data = self.find_inode(inode)?;

            let mut file = data.file.lock();
            let mut flags = file.1;
            match flags & libc::O_ACCMODE {
                libc::O_RDONLY => {
                    flags &= !libc::O_RDONLY;
                    flags |= libc::O_RDWR;

                    // We need to get a writable handle for this file.
                    let newfile = self.open_fd(file.0.as_raw_descriptor(), libc::O_RDWR)?;
                    *file = (newfile, flags);
                }
                libc::O_WRONLY | libc::O_RDWR => {}
                _ => panic!("Unexpected flags: {:#x}", flags),
            }

            r.read_to(&mut file.0, size as usize, offset)
        } else {
            let data = self.find_handle(handle, inode)?;

            let mut f = data.file.lock();
            r.read_to(&mut f, size as usize, offset)
        }
    }

    fn getattr(
        &self,
        _ctx: Context,
        inode: Inode,
        _handle: Option<Handle>,
    ) -> io::Result<(libc::stat64, Duration)> {
        let data = self.find_inode(inode)?;
        self.do_getattr(&data)
    }

    fn setattr(
        &self,
        _ctx: Context,
        inode: Inode,
        attr: libc::stat64,
        handle: Option<Handle>,
        valid: SetattrValid,
    ) -> io::Result<(libc::stat64, Duration)> {
        let inode_data = self.find_inode(inode)?;

        enum Data {
            Handle(Arc<HandleData>, RawDescriptor),
            ProcPath(CString),
        }

        // If we have a handle then use it otherwise get a new fd from the inode.
        let data = if let Some(handle) = handle.filter(|&h| h != 0) {
            let hd = self.find_handle(handle, inode)?;

            let fd = hd.file.lock().as_raw_descriptor();
            Data::Handle(hd, fd)
        } else {
            let pathname = CString::new(format!("self/fd/{}", inode_data.as_raw_descriptor()))
                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
            Data::ProcPath(pathname)
        };

        if valid.contains(SetattrValid::MODE) {
            // Safe because this doesn't modify any memory and we check the return value.
            let res = unsafe {
                match data {
                    Data::Handle(_, fd) => libc::fchmod(fd, attr.st_mode),
                    Data::ProcPath(ref p) => {
                        libc::fchmodat(self.proc.as_raw_descriptor(), p.as_ptr(), attr.st_mode, 0)
                    }
                }
            };
            if res < 0 {
                return Err(io::Error::last_os_error());
            }
        }

        if valid.intersects(SetattrValid::UID | SetattrValid::GID) {
            let uid = if valid.contains(SetattrValid::UID) {
                attr.st_uid
            } else {
                // Cannot use -1 here because these are unsigned values.
                ::std::u32::MAX
            };
            let gid = if valid.contains(SetattrValid::GID) {
                attr.st_gid
            } else {
                // Cannot use -1 here because these are unsigned values.
                ::std::u32::MAX
            };

            // Safe because this is a constant value and a valid C string.
            let empty = unsafe { CStr::from_bytes_with_nul_unchecked(EMPTY_CSTR) };

            // Safe because this doesn't modify any memory and we check the return value.
            let res = unsafe {
                libc::fchownat(
                    inode_data.as_raw_descriptor(),
                    empty.as_ptr(),
                    uid,
                    gid,
                    libc::AT_EMPTY_PATH | libc::AT_SYMLINK_NOFOLLOW,
                )
            };
            if res < 0 {
                return Err(io::Error::last_os_error());
            }
        }

        if valid.contains(SetattrValid::SIZE) {
            // Safe because this doesn't modify any memory and we check the return value.
            let res = match data {
                Data::Handle(_, fd) => unsafe { libc::ftruncate64(fd, attr.st_size) },
                _ => {
                    // There is no `ftruncateat` so we need to get a new fd and truncate it.
                    let f = self.open_inode(&inode_data, libc::O_NONBLOCK | libc::O_RDWR)?;
                    unsafe { libc::ftruncate64(f.as_raw_descriptor(), attr.st_size) }
                }
            };
            if res < 0 {
                return Err(io::Error::last_os_error());
            }
        }

        if valid.intersects(SetattrValid::ATIME | SetattrValid::MTIME) {
            let mut tvs = [
                libc::timespec {
                    tv_sec: 0,
                    tv_nsec: libc::UTIME_OMIT,
                },
                libc::timespec {
                    tv_sec: 0,
                    tv_nsec: libc::UTIME_OMIT,
                },
            ];

            if valid.contains(SetattrValid::ATIME_NOW) {
                tvs[0].tv_nsec = libc::UTIME_NOW;
            } else if valid.contains(SetattrValid::ATIME) {
                tvs[0].tv_sec = attr.st_atime;
                tvs[0].tv_nsec = attr.st_atime_nsec;
            }

            if valid.contains(SetattrValid::MTIME_NOW) {
                tvs[1].tv_nsec = libc::UTIME_NOW;
            } else if valid.contains(SetattrValid::MTIME) {
                tvs[1].tv_sec = attr.st_mtime;
                tvs[1].tv_nsec = attr.st_mtime_nsec;
            }

            // Safe because this doesn't modify any memory and we check the return value.
            let res = match data {
                Data::Handle(_, fd) => unsafe { libc::futimens(fd, tvs.as_ptr()) },
                Data::ProcPath(ref p) => unsafe {
                    libc::utimensat(self.proc.as_raw_descriptor(), p.as_ptr(), tvs.as_ptr(), 0)
                },
            };
            if res < 0 {
                return Err(io::Error::last_os_error());
            }
        }

        self.do_getattr(&inode_data)
    }

    fn rename(
        &self,
        _ctx: Context,
        olddir: Inode,
        oldname: &CStr,
        newdir: Inode,
        newname: &CStr,
        flags: u32,
    ) -> io::Result<()> {
        let old_inode = self.find_inode(olddir)?;
        let new_inode = self.find_inode(newdir)?;

        // Safe because this doesn't modify any memory and we check the return value.
        // TODO: Switch to libc::renameat2 once https://github.com/rust-lang/libc/pull/1508 lands
        // and we have glibc 2.28.
        let res = unsafe {
            libc::syscall(
                libc::SYS_renameat2,
                old_inode.as_raw_descriptor(),
                oldname.as_ptr(),
                new_inode.as_raw_descriptor(),
                newname.as_ptr(),
                flags,
            )
        };
        if res == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn mknod(
        &self,
        ctx: Context,
        parent: Inode,
        name: &CStr,
        mode: u32,
        rdev: u32,
        umask: u32,
    ) -> io::Result<Entry> {
        let data = self.find_inode(parent)?;

        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;

        let res = {
            let mut um = self.umask.lock();
            let _scoped_umask = um.set(umask);

            // Safe because this doesn't modify any memory and we check the return value.
            unsafe {
                libc::mknodat(
                    data.as_raw_descriptor(),
                    name.as_ptr(),
                    mode as libc::mode_t,
                    rdev as libc::dev_t,
                )
            }
        };

        if res < 0 {
            Err(io::Error::last_os_error())
        } else {
            self.do_lookup(&data, name)
        }
    }

    fn link(
        &self,
        _ctx: Context,
        inode: Inode,
        newparent: Inode,
        newname: &CStr,
    ) -> io::Result<Entry> {
        let data = self.find_inode(inode)?;
        let new_inode = self.find_inode(newparent)?;

        let path = CString::new(format!("self/fd/{}", data.as_raw_descriptor()))
            .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

        // Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe {
            libc::linkat(
                self.proc.as_raw_descriptor(),
                path.as_ptr(),
                new_inode.as_raw_descriptor(),
                newname.as_ptr(),
                libc::AT_SYMLINK_FOLLOW,
            )
        };
        if res == 0 {
            self.do_lookup(&new_inode, newname)
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn symlink(
        &self,
        ctx: Context,
        linkname: &CStr,
        parent: Inode,
        name: &CStr,
    ) -> io::Result<Entry> {
        let data = self.find_inode(parent)?;

        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;

        // Safe because this doesn't modify any memory and we check the return value.
        let res =
            unsafe { libc::symlinkat(linkname.as_ptr(), data.as_raw_descriptor(), name.as_ptr()) };
        if res == 0 {
            self.do_lookup(&data, name)
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn readlink(&self, _ctx: Context, inode: Inode) -> io::Result<Vec<u8>> {
        let data = self.find_inode(inode)?;

        let mut buf = vec![0; libc::PATH_MAX as usize];

        // Safe because this is a constant value and a valid C string.
        let empty = unsafe { CStr::from_bytes_with_nul_unchecked(EMPTY_CSTR) };

        // Safe because this will only modify the contents of `buf` and we check the return value.
        let res = unsafe {
            libc::readlinkat(
                data.as_raw_descriptor(),
                empty.as_ptr(),
                buf.as_mut_ptr() as *mut libc::c_char,
                buf.len(),
            )
        };
        if res < 0 {
            return Err(io::Error::last_os_error());
        }

        buf.resize(res as usize, 0);
        Ok(buf)
    }

    fn flush(
        &self,
        _ctx: Context,
        inode: Inode,
        handle: Handle,
        _lock_owner: u64,
    ) -> io::Result<()> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            self.find_inode(inode)?
        } else {
            self.find_handle(handle, inode)?
        };

        // Since this method is called whenever an fd is closed in the client, we can emulate that
        // behavior by doing the same thing (dup-ing the fd and then immediately closing it). Safe
        // because this doesn't modify any memory and we check the return values.
        unsafe {
            let newfd = libc::fcntl(data.as_raw_descriptor(), libc::F_DUPFD_CLOEXEC, 0);

            if newfd < 0 {
                return Err(io::Error::last_os_error());
            }

            if libc::close(newfd) < 0 {
                Err(io::Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }

    fn fsync(&self, _ctx: Context, inode: Inode, datasync: bool, handle: Handle) -> io::Result<()> {
        if self.zero_message_open.load(Ordering::Relaxed) {
            let data = self.find_inode(inode)?;
            self.do_fsync(&*data, datasync)
        } else {
            let data = self.find_handle(handle, inode)?;

            let file = data.file.lock();
            self.do_fsync(&*file, datasync)
        }
    }

    fn fsyncdir(
        &self,
        _ctx: Context,
        inode: Inode,
        datasync: bool,
        handle: Handle,
    ) -> io::Result<()> {
        if self.zero_message_opendir.load(Ordering::Relaxed) {
            let data = self.find_inode(inode)?;
            self.do_fsync(&*data, datasync)
        } else {
            let data = self.find_handle(handle, inode)?;

            let file = data.file.lock();
            self.do_fsync(&*file, datasync)
        }
    }

    fn access(&self, ctx: Context, inode: Inode, mask: u32) -> io::Result<()> {
        let data = self.find_inode(inode)?;

        let st = stat(&*data)?;
        let mode = mask as i32 & (libc::R_OK | libc::W_OK | libc::X_OK);

        if mode == libc::F_OK {
            // The file exists since we were able to call `stat(2)` on it.
            return Ok(());
        }

        if (mode & libc::R_OK) != 0 {
            if ctx.uid != 0
                && (st.st_uid != ctx.uid || st.st_mode & 0o400 == 0)
                && (st.st_gid != ctx.gid || st.st_mode & 0o040 == 0)
                && st.st_mode & 0o004 == 0
            {
                return Err(io::Error::from_raw_os_error(libc::EACCES));
            }
        }

        if (mode & libc::W_OK) != 0 {
            if ctx.uid != 0
                && (st.st_uid != ctx.uid || st.st_mode & 0o200 == 0)
                && (st.st_gid != ctx.gid || st.st_mode & 0o020 == 0)
                && st.st_mode & 0o002 == 0
            {
                return Err(io::Error::from_raw_os_error(libc::EACCES));
            }
        }

        // root can only execute something if it is executable by one of the owner, the group, or
        // everyone.
        if (mode & libc::X_OK) != 0 {
            if (ctx.uid != 0 || st.st_mode & 0o111 == 0)
                && (st.st_uid != ctx.uid || st.st_mode & 0o100 == 0)
                && (st.st_gid != ctx.gid || st.st_mode & 0o010 == 0)
                && st.st_mode & 0o001 == 0
            {
                return Err(io::Error::from_raw_os_error(libc::EACCES));
            }
        }

        Ok(())
    }

    fn setxattr(
        &self,
        _ctx: Context,
        inode: Inode,
        name: &CStr,
        value: &[u8],
        flags: u32,
    ) -> io::Result<()> {
        // We can't allow the VM to set this xattr because an unprivileged process may use it to set
        // a privileged xattr.
        if self.cfg.rewrite_security_xattrs && name.to_bytes().starts_with(USER_VIRTIOFS_XATTR) {
            return Err(io::Error::from_raw_os_error(libc::EPERM));
        }

        let data = self.find_inode(inode)?;
        let name = self.rewrite_xattr_name(name);

        let res = if data.filetype == FileType::Other {
            // For non-regular files and directories, we cannot open the fd normally. Instead we
            // emulate an _at syscall by changing the CWD to /proc, running the path based syscall,
            // and then setting the CWD back to the root directory.
            let path = CString::new(format!("self/fd/{}", data.as_raw_descriptor()))
                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

            // Safe because this doesn't modify any memory and we check the return value.
            self.with_proc_chdir(|| unsafe {
                libc::setxattr(
                    path.as_ptr(),
                    name.as_ptr(),
                    value.as_ptr() as *const libc::c_void,
                    value.len() as libc::size_t,
                    flags as c_int,
                )
            })
        } else {
            // For regular files and directories, we can just use fsetxattr. Safe because this
            // doesn't modify any memory and we check the return value.
            unsafe {
                libc::fsetxattr(
                    data.as_raw_descriptor(),
                    name.as_ptr(),
                    value.as_ptr() as *const libc::c_void,
                    value.len() as libc::size_t,
                    flags as c_int,
                )
            }
        };

        if res < 0 {
            Err(io::Error::last_os_error())
        } else {
            Ok(())
        }
    }

    fn getxattr(
        &self,
        _ctx: Context,
        inode: Inode,
        name: &CStr,
        size: u32,
    ) -> io::Result<GetxattrReply> {
        // We don't allow the VM to set this xattr so we also pretend there is no value associated
        // with it.
        if self.cfg.rewrite_security_xattrs && name.to_bytes().starts_with(USER_VIRTIOFS_XATTR) {
            return Err(io::Error::from_raw_os_error(libc::ENODATA));
        }

        let data = self.find_inode(inode)?;
        let name = self.rewrite_xattr_name(name);
        let mut buf = vec![0u8; size as usize];

        // Safe because this will only modify the contents of `buf`.
        let res = self.do_getxattr(&data, &name, &mut buf[..])?;
        if size == 0 {
            Ok(GetxattrReply::Count(res as u32))
        } else {
            buf.truncate(res as usize);
            Ok(GetxattrReply::Value(buf))
        }
    }

    fn listxattr(&self, _ctx: Context, inode: Inode, size: u32) -> io::Result<ListxattrReply> {
        let data = self.find_inode(inode)?;

        let mut buf = vec![0u8; size as usize];

        let res = if data.filetype == FileType::Other {
            // For non-regular files and directories, we cannot open the fd normally. Instead we
            // emulate an _at syscall by changing the CWD to /proc, running the path based syscall,
            // and then setting the CWD back to the root directory.
            let path = CString::new(format!("self/fd/{}", data.as_raw_descriptor()))
                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

            // Safe because this will only modify `buf` and we check the return value.
            self.with_proc_chdir(|| unsafe {
                libc::listxattr(
                    path.as_ptr(),
                    buf.as_mut_ptr() as *mut libc::c_char,
                    buf.len() as libc::size_t,
                )
            })
        } else {
            // For regular files and directories, we can just flistxattr. Safe because this will only
            // write to `buf` and we check the return value.
            unsafe {
                libc::flistxattr(
                    data.as_raw_descriptor(),
                    buf.as_mut_ptr() as *mut libc::c_char,
                    buf.len() as libc::size_t,
                )
            }
        };

        if res < 0 {
            return Err(io::Error::last_os_error());
        }

        if size == 0 {
            Ok(ListxattrReply::Count(res as u32))
        } else {
            buf.truncate(res as usize);

            if self.cfg.rewrite_security_xattrs {
                strip_xattr_prefix(&mut buf);
            }
            Ok(ListxattrReply::Names(buf))
        }
    }

    fn removexattr(&self, _ctx: Context, inode: Inode, name: &CStr) -> io::Result<()> {
        // We don't allow the VM to set this xattr so we also pretend there is no value associated
        // with it.
        if self.cfg.rewrite_security_xattrs && name.to_bytes().starts_with(USER_VIRTIOFS_XATTR) {
            return Err(io::Error::from_raw_os_error(libc::ENODATA));
        }

        let data = self.find_inode(inode)?;
        let name = self.rewrite_xattr_name(name);

        let res = if data.filetype == FileType::Other {
            // For non-regular files and directories, we cannot open the fd normally. Instead we
            // emulate an _at syscall by changing the CWD to /proc, running the path based syscall,
            // and then setting the CWD back to the root directory.
            let path = CString::new(format!("self/fd/{}", data.as_raw_descriptor()))
                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

            // Safe because this doesn't modify any memory and we check the return value.
            self.with_proc_chdir(|| unsafe { libc::removexattr(path.as_ptr(), name.as_ptr()) })
        } else {
            // For regular files and directories, we can just use fremovexattr. Safe because this
            // doesn't modify any memory and we check the return value.
            unsafe { libc::fremovexattr(data.as_raw_descriptor(), name.as_ptr()) }
        };

        if res == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn fallocate(
        &self,
        _ctx: Context,
        inode: Inode,
        handle: Handle,
        mode: u32,
        offset: u64,
        length: u64,
    ) -> io::Result<()> {
        let data: Arc<dyn AsRawDescriptor> = if self.zero_message_open.load(Ordering::Relaxed) {
            let data = self.find_inode(inode)?;

            {
                // fallocate needs a writable fd
                let mut file = data.file.lock();
                let mut flags = file.1;
                match flags & libc::O_ACCMODE {
                    libc::O_RDONLY => {
                        flags &= !libc::O_RDONLY;
                        flags |= libc::O_RDWR;

                        // We need to get a writable handle for this file.
                        let newfile = self.open_fd(file.0.as_raw_descriptor(), libc::O_RDWR)?;
                        *file = (newfile, flags);
                    }
                    libc::O_WRONLY | libc::O_RDWR => {}
                    _ => panic!("Unexpected flags: {:#x}", flags),
                }
            }

            data
        } else {
            self.find_handle(handle, inode)?
        };

        let fd = data.as_raw_descriptor();
        // Safe because this doesn't modify any memory and we check the return value.
        let res = unsafe {
            libc::fallocate64(
                fd,
                mode as libc::c_int,
                offset as libc::off64_t,
                length as libc::off64_t,
            )
        };
        if res == 0 {
            Ok(())
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn ioctl<R: io::Read>(
        &self,
        _ctx: Context,
        inode: Inode,
        handle: Handle,
        _flags: IoctlFlags,
        cmd: u32,
        _arg: u64,
        in_size: u32,
        out_size: u32,
        r: R,
    ) -> io::Result<IoctlReply> {
        const GET_ENCRYPTION_POLICY_EX: u32 = FS_IOC_GET_ENCRYPTION_POLICY_EX() as u32;
        const GET_FSXATTR: u32 = FS_IOC_FSGETXATTR() as u32;
        const SET_FSXATTR: u32 = FS_IOC_FSSETXATTR() as u32;
        const GET_FLAGS32: u32 = FS_IOC32_GETFLAGS() as u32;
        const SET_FLAGS32: u32 = FS_IOC32_SETFLAGS() as u32;
        const GET_FLAGS64: u32 = FS_IOC64_GETFLAGS() as u32;
        const SET_FLAGS64: u32 = FS_IOC64_SETFLAGS() as u32;

        match cmd {
            GET_ENCRYPTION_POLICY_EX => self.get_encryption_policy_ex(inode, handle, r),
            GET_FSXATTR => {
                if out_size < size_of::<fsxattr>() as u32 {
                    Err(io::Error::from_raw_os_error(libc::ENOMEM))
                } else {
                    self.get_fsxattr(inode, handle)
                }
            }
            SET_FSXATTR => {
                if in_size < size_of::<fsxattr>() as u32 {
                    Err(io::Error::from_raw_os_error(libc::EINVAL))
                } else {
                    self.set_fsxattr(inode, handle, r)
                }
            }
            GET_FLAGS32 | GET_FLAGS64 => {
                if out_size < size_of::<c_int>() as u32 {
                    Err(io::Error::from_raw_os_error(libc::ENOMEM))
                } else {
                    self.get_flags(inode, handle)
                }
            }
            SET_FLAGS32 | SET_FLAGS64 => {
                if in_size < size_of::<c_int>() as u32 {
                    Err(io::Error::from_raw_os_error(libc::ENOMEM))
                } else {
                    self.set_flags(inode, handle, r)
                }
            }
            _ => Err(io::Error::from_raw_os_error(libc::ENOTTY)),
        }
    }

    fn copy_file_range(
        &self,
        ctx: Context,
        inode_src: Inode,
        handle_src: Handle,
        offset_src: u64,
        inode_dst: Inode,
        handle_dst: Handle,
        offset_dst: u64,
        length: u64,
        flags: u64,
    ) -> io::Result<usize> {
        // We need to change credentials during a write so that the kernel will remove setuid or
        // setgid bits from the file if it was written to by someone other than the owner.
        let (_uid, _gid) = set_creds(ctx.uid, ctx.gid)?;
        let (src_data, dst_data): (Arc<dyn AsRawDescriptor>, Arc<dyn AsRawDescriptor>) =
            if self.zero_message_open.load(Ordering::Relaxed) {
                (self.find_inode(inode_src)?, self.find_inode(inode_dst)?)
            } else {
                (
                    self.find_handle(handle_src, inode_src)?,
                    self.find_handle(handle_dst, inode_dst)?,
                )
            };

        let src = src_data.as_raw_descriptor();
        let dst = dst_data.as_raw_descriptor();

        let res = unsafe {
            libc::syscall(
                libc::SYS_copy_file_range,
                src,
                &offset_src,
                dst,
                &offset_dst,
                length,
                flags,
            )
        };

        if res >= 0 {
            Ok(res as usize)
        } else {
            Err(io::Error::last_os_error())
        }
    }

    fn set_up_mapping<M: Mapper>(
        &self,
        _ctx: Context,
        inode: Self::Inode,
        _handle: Self::Handle,
        file_offset: u64,
        mem_offset: u64,
        size: usize,
        prot: u32,
        mapper: M,
    ) -> io::Result<()> {
        let read = prot & libc::PROT_READ as u32 != 0;
        let write = prot & libc::PROT_WRITE as u32 != 0;
        let mmap_flags = match (read, write) {
            (true, true) => libc::O_RDWR,
            (true, false) => libc::O_RDONLY,
            (false, true) => libc::O_RDWR, // mmap always requires an fd opened for reading.
            (false, false) => return Err(io::Error::from_raw_os_error(libc::EINVAL)),
        };

        let data = self.find_inode(inode)?;

        if self.zero_message_open.load(Ordering::Relaxed) {
            let mut file = data.file.lock();
            let mut open_flags = file.1;
            match (mmap_flags, open_flags & libc::O_ACCMODE) {
                (libc::O_RDONLY, libc::O_WRONLY)
                | (libc::O_RDWR, libc::O_RDONLY)
                | (libc::O_RDWR, libc::O_WRONLY) => {
                    // We have a read-only or write-only fd and we need to upgrade it.
                    open_flags &= !libc::O_ACCMODE;
                    open_flags |= libc::O_RDWR;

                    let newfile = self.open_fd(file.0.as_raw_descriptor(), libc::O_RDWR)?;
                    *file = (newfile, open_flags);
                }
                (libc::O_RDONLY, libc::O_RDONLY)
                | (libc::O_RDONLY, libc::O_RDWR)
                | (libc::O_RDWR, libc::O_RDWR) => {}
                (m, o) => panic!(
                    "Unexpected combination of access flags: ({:#x}, {:#x})",
                    m, o
                ),
            }
            mapper.map(mem_offset, size, &file.0, file_offset, prot)
        } else {
            let file = self.open_inode(&data, mmap_flags | libc::O_NONBLOCK)?;
            mapper.map(mem_offset, size, &file, file_offset, prot)
        }
    }

    fn remove_mapping<M: Mapper>(&self, msgs: &[RemoveMappingOne], mapper: M) -> io::Result<()> {
        for RemoveMappingOne { moffset, len } in msgs {
            mapper.unmap(*moffset, *len)?;
        }
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn rewrite_xattr_names() {
        let cfg = Config {
            rewrite_security_xattrs: true,
            ..Default::default()
        };

        let p = PassthroughFs::new(cfg).expect("Failed to create PassthroughFs");

        // Selinux shouldn't get overwritten.
        let selinux = unsafe { CStr::from_bytes_with_nul_unchecked(b"security.selinux\0") };
        assert_eq!(p.rewrite_xattr_name(selinux).to_bytes(), selinux.to_bytes());

        // user, trusted, and system should not be changed either.
        let user = unsafe { CStr::from_bytes_with_nul_unchecked(b"user.foobar\0") };
        assert_eq!(p.rewrite_xattr_name(user).to_bytes(), user.to_bytes());
        let trusted = unsafe { CStr::from_bytes_with_nul_unchecked(b"trusted.foobar\0") };
        assert_eq!(p.rewrite_xattr_name(trusted).to_bytes(), trusted.to_bytes());
        let system = unsafe { CStr::from_bytes_with_nul_unchecked(b"system.foobar\0") };
        assert_eq!(p.rewrite_xattr_name(system).to_bytes(), system.to_bytes());

        // sehash should be re-written.
        let sehash = unsafe { CStr::from_bytes_with_nul_unchecked(b"security.sehash\0") };
        assert_eq!(
            p.rewrite_xattr_name(sehash).to_bytes(),
            b"user.virtiofs.security.sehash"
        );
    }

    #[test]
    fn strip_xattr_names() {
        let only_nuls = b"\0\0\0\0\0";
        let mut actual = only_nuls.to_vec();
        strip_xattr_prefix(&mut actual);
        assert_eq!(&actual[..], &only_nuls[..]);

        let no_nuls = b"security.sehashuser.virtiofs";
        let mut actual = no_nuls.to_vec();
        strip_xattr_prefix(&mut actual);
        assert_eq!(&actual[..], &no_nuls[..]);

        let empty = b"";
        let mut actual = empty.to_vec();
        strip_xattr_prefix(&mut actual);
        assert_eq!(&actual[..], &empty[..]);

        let no_strippable_names = b"security.selinux\0user.foobar\0system.test\0";
        let mut actual = no_strippable_names.to_vec();
        strip_xattr_prefix(&mut actual);
        assert_eq!(&actual[..], &no_strippable_names[..]);

        let only_strippable_names = b"user.virtiofs.security.sehash\0user.virtiofs.security.wtf\0";
        let mut actual = only_strippable_names.to_vec();
        strip_xattr_prefix(&mut actual);
        assert_eq!(&actual[..], b"security.sehash\0security.wtf\0");

        let mixed_names = b"user.virtiofs.security.sehash\0security.selinux\0user.virtiofs.security.wtf\0user.foobar\0";
        let mut actual = mixed_names.to_vec();
        strip_xattr_prefix(&mut actual);
        let expected = b"security.sehash\0security.selinux\0security.wtf\0user.foobar\0";
        assert_eq!(&actual[..], &expected[..]);

        let no_nul_with_prefix = b"user.virtiofs.security.sehash";
        let mut actual = no_nul_with_prefix.to_vec();
        strip_xattr_prefix(&mut actual);
        assert_eq!(&actual[..], b"security.sehash");
    }
}