chore(API): Infrastructure to use endpoints with fd. passing

Cargo.lock

@@ -1828,6 +1828,7 @@ dependencies = [
  "test_bin",
  "thiserror",
  "toml",
+ "uds",
  "zerocopy",
  "zeroize",
 ]
@@ -1923,6 +1924,7 @@ dependencies = [
  "tempfile",
  "thiserror",
  "typenum",
+ "uds",
  "zerocopy",
  "zeroize",
 ]
@@ -2396,6 +2398,15 @@ version = "1.17.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "42ff0bf0c66b8238c6f3b578df37d0b7848e55df8577b3f74f92a69acceeb825"
 
+[[package]]
+name = "uds"
+version = "0.4.2"
+source = "git+https://github.com/rosenpass/uds#b47934fe52422e559f7278938875f9105f91c5a2"
+dependencies = [
+ "libc",
+ "mio",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.12"

Cargo.toml

@@ -69,6 +69,7 @@ hex-literal = { version = "0.4.1" }
 hex = { version = "0.4.3" }
 heck = { version = "0.5.0"  }
 libc = { version = "0.2" }
+uds = { git = "https://github.com/rosenpass/uds" }
 
 #Dev dependencies
 serial_test = "3.1.1"

rosenpass/Cargo.toml

@@ -55,6 +55,7 @@ hex = { workspace = true, optional = true }
 heck = { workspace = true, optional = true }
 command-fds = { workspace = true, optional = true }
 rustix = { workspace = true }
+uds = { workspace = true, optional = true, features = ["mio_1xx"] }
 
 [build-dependencies]
 anyhow = { workspace = true }
@@ -71,6 +72,6 @@ tempfile = { workspace = true }
 experiment_memfd_secret = ["rosenpass-wireguard-broker/experiment_memfd_secret"]
 experiment_broker_api = ["rosenpass-wireguard-broker/experiment_broker_api", "command-fds"]
 experiment_libcrux = ["rosenpass-ciphers/experiment_libcrux"]
-experiment_api = ["hex-literal"]
+experiment_api = ["hex-literal", "uds", "rosenpass-util/experiment_file_descriptor_passing"]
 internal_testing  = []
 internal_bin_gen_ipc_msg_types = ["hex", "heck"]

rosenpass/src/api/api_handler.rs

@@ -1,3 +1,5 @@
+use std::{borrow::BorrowMut, collections::VecDeque, os::fd::OwnedFd};
+
 use rosenpass_to::{ops::copy_slice, To};
 
 use crate::app_server::AppServer;
@@ -30,6 +32,7 @@ where
     fn ping(
         &mut self,
         req: &super::PingRequest,
+        _req_fds: &mut VecDeque<OwnedFd>,
         res: &mut super::PingResponse,
     ) -> anyhow::Result<()> {
         let (req, res) = (&req.payload, &mut res.payload);

rosenpass/src/api/boilerplate/payload.rs

@@ -6,6 +6,7 @@ use super::{Message, RawMsgType, RequestMsgType, ResponseMsgType};
 /// Size required to fit any message in binary form
 pub const MAX_REQUEST_LEN: usize = 2500; // TODO fix this
 pub const MAX_RESPONSE_LEN: usize = 2500; // TODO fix this
+pub const MAX_REQUEST_FDS: usize = 2;
 
 #[repr(packed)]
 #[derive(Debug, Copy, Clone, Hash, AsBytes, FromBytes, FromZeroes, PartialEq, Eq)]

rosenpass/src/api/boilerplate/server.rs

@@ -1,24 +1,35 @@
+use super::{ByteSliceRefExt, Message, PingRequest, PingResponse, RequestRef, RequestResponsePair};
+use std::{collections::VecDeque, os::fd::OwnedFd};
 use zerocopy::{ByteSlice, ByteSliceMut};
 
-use super::{ByteSliceRefExt, Message, PingRequest, PingResponse, RequestRef, RequestResponsePair};
-
 pub trait Server {
-    fn ping(&mut self, req: &PingRequest, res: &mut PingResponse) -> anyhow::Result<()>;
+    fn ping(
+        &mut self,
+        req: &PingRequest,
+        req_fds: &mut VecDeque<OwnedFd>,
+        res: &mut PingResponse,
+    ) -> anyhow::Result<()>;
 
     fn dispatch<ReqBuf, ResBuf>(
         &mut self,
         p: &mut RequestResponsePair<ReqBuf, ResBuf>,
+        req_fds: &mut VecDeque<OwnedFd>,
     ) -> anyhow::Result<()>
     where
         ReqBuf: ByteSlice,
         ResBuf: ByteSliceMut,
     {
         match p {
-            RequestResponsePair::Ping((req, res)) => self.ping(req, res),
+            RequestResponsePair::Ping((req, res)) => self.ping(req, req_fds, res),
         }
     }
 
-    fn handle_message<ReqBuf, ResBuf>(&mut self, req: ReqBuf, res: ResBuf) -> anyhow::Result<usize>
+    fn handle_message<ReqBuf, ResBuf>(
+        &mut self,
+        req: ReqBuf,
+        req_fds: &mut VecDeque<OwnedFd>,
+        res: ResBuf,
+    ) -> anyhow::Result<usize>
     where
         ReqBuf: ByteSlice,
         ResBuf: ByteSliceMut,
@@ -32,7 +43,7 @@ pub trait Server {
                 RequestResponsePair::Ping((req, res))
             }
         };
-        self.dispatch(&mut pair)?;
+        self.dispatch(&mut pair, req_fds)?;
 
         let res_len = pair.response().bytes().len();
         Ok(res_len)

rosenpass/src/api/mio/connection.rs

@@ -1,7 +1,10 @@
 use std::borrow::{Borrow, BorrowMut};
+use std::collections::VecDeque;
+use std::os::fd::OwnedFd;
 
 use mio::net::UnixStream;
 use rosenpass_secret_memory::Secret;
+use rosenpass_util::mio::ReadWithFileDescriptors;
 use rosenpass_util::{
     io::{IoResultKindHintExt, TryIoResultKindHintExt},
     length_prefix_encoding::{
@@ -12,6 +15,7 @@ use rosenpass_util::{
 };
 use zeroize::Zeroize;
 
+use crate::api::MAX_REQUEST_FDS;
 use crate::{api::Server, app_server::AppServer};
 
 use super::super::{ApiHandler, ApiHandlerContext};
@@ -39,11 +43,13 @@ impl<const N: usize> BorrowMut<[u8]> for SecretBuffer<N> {
 // TODO: Unfortunately, zerocopy is quite particular about alignment, hence the 4096
 type ReadBuffer = LengthPrefixDecoder<SecretBuffer<4096>>;
 type WriteBuffer = LengthPrefixEncoder<SecretBuffer<4096>>;
+type ReadFdBuffer = VecDeque<OwnedFd>;
 
 #[derive(Debug)]
 struct MioConnectionBuffers {
     read_buffer: ReadBuffer,
     write_buffer: WriteBuffer,
+    read_fd_buffer: ReadFdBuffer,
 }
 
 #[derive(Debug)]
@@ -65,9 +71,11 @@ impl MioConnection {
         let invalid_read = false;
         let read_buffer = LengthPrefixDecoder::new(SecretBuffer::new());
         let write_buffer = LengthPrefixEncoder::from_buffer(SecretBuffer::new());
+        let read_fd_buffer = VecDeque::new();
         let buffers = Some(MioConnectionBuffers {
             read_buffer,
             write_buffer,
+            read_fd_buffer,
         });
         let api_state = ApiHandler::new();
         Ok(Self {
@@ -106,20 +114,22 @@ pub trait MioConnectionContext {
     }
 
     fn handle_incoming_message(&mut self) -> anyhow::Result<Option<()>> {
-        self.with_buffers_stolen(|this, read_buf, write_buf| {
+        self.with_buffers_stolen(|this, bufs| {
             // Acquire request & response. Caller is responsible to make sure
             // that read buffer holds a message and that write buffer is cleared.
             // Hence the unwraps and assertions
-            assert!(write_buf.exhausted());
-            let req = read_buf.message().unwrap().unwrap();
-            let res = write_buf.buffer_bytes_mut();
+            assert!(bufs.write_buffer.exhausted());
+            let req = bufs.read_buffer.message().unwrap().unwrap();
+            let req_fds = &mut bufs.read_fd_buffer;
+            let res = bufs.write_buffer.buffer_bytes_mut();
 
             // Call API handler
             // Transitive trait implementations: MioConnectionContext -> ApiHandlerContext -> as ApiServer
-            let response_len = this.handle_message(req, res)?;
+            let response_len = this.handle_message(req, req_fds, res)?;
 
-            write_buf.restart_write_with_new_message(response_len)?;
-            read_buf.zeroize(); // clear for new message to read
+            bufs.write_buffer
+                .restart_write_with_new_message(response_len)?;
+            bufs.read_buffer.zeroize(); // clear for new message to read
 
             Ok(Some(()))
         })
@@ -130,20 +140,22 @@ pub trait MioConnectionContext {
             return Ok(Some(()));
         }
 
-        self.with_buffers_stolen(|this, _read_buf, write_buf| {
         use lpe_encoder::WriteToIoReturn as Ret;
         use std::io::ErrorKind as K;
 
         loop {
-                match write_buf
-                    .write_to_stdio(&this.mio_connection_mut().io)
-                    .io_err_kind_hint()
-                {
+            let conn = self.mio_connection_mut();
+            let bufs = conn.buffers.as_mut().unwrap();
+
+            let sock = &conn.io;
+            let write_buf = &mut bufs.write_buffer;
+
+            match write_buf.write_to_stdio(sock).io_err_kind_hint() {
                 // Done
                 Ok(Ret { done: true, .. }) => {
                     write_buf.zeroize(); // clear for new message to write
                     break Ok(Some(()));
-                    },
+                }
 
                 // Would block
                 Ok(Ret {
@@ -159,7 +171,6 @@ pub trait MioConnectionContext {
                 Err((e, _ek)) => Err(e)?,
             }
         }
-        })
     }
 
     fn recv(&mut self) -> anyhow::Result<Option<()>> {
@@ -167,13 +178,24 @@ pub trait MioConnectionContext {
             return Ok(None);
         }
 
-        self.with_buffers_stolen(|this, read_buf, _write_buf| {
         use lpe_decoder::{ReadFromIoError as E, ReadFromIoReturn as Ret};
         use std::io::ErrorKind as K;
 
         loop {
+            let conn = self.mio_connection_mut();
+            let bufs = conn.buffers.as_mut().unwrap();
+
+            let read_buf = &mut bufs.read_buffer;
+            let read_fd_buf = &mut bufs.read_fd_buffer;
+
+            let sock = &conn.io;
+            let fd_passing_sock = ReadWithFileDescriptors::<MAX_REQUEST_FDS, UnixStream, _, _>::new(
+                sock,
+                read_fd_buf,
+            );
+
             match read_buf
-                    .read_from_stdio(&this.mio_connection_mut().io)
+                .read_from_stdio(fd_passing_sock)
                 .try_io_err_kind_hint()
             {
                 // We actually received a proper message
@@ -193,7 +215,7 @@ pub trait MioConnectionContext {
                     // open connections.
                     // Just leaving the API connections dangling for now.
                     // This should be fixed for non-experimental use of the API.
-                        this.mio_connection_mut().invalid_read = true;
+                    conn.invalid_read = true;
                     break Ok(None);
                 }
 
@@ -206,10 +228,18 @@ pub trait MioConnectionContext {
                 Err((_, Some(K::Interrupted))) => continue,
 
                 // Other IO Error (just pass on to the caller)
-                    Err((E::IoError(e), _)) => Err(e)?,
+                Err((E::IoError(e), _)) => {
+                    log::warn!(
+                        "IO error while trying to read message from API socket. \
+                            The connection is broken. Stopping to process messages of the client.\n\
+                            Error: {e:?}"
+                    );
+                    // TODO: Same as above
+                    conn.invalid_read = true;
+                    break Err(e.into());
+                }
             };
         }
-        })
     }
 }
 
@@ -224,32 +254,23 @@ trait MioConnectionContextPrivate: MioConnectionContext {
         let _ = opt.insert(buffers);
     }
 
-    fn with_buffers_stolen<R, F: FnOnce(&mut Self, &mut ReadBuffer, &mut WriteBuffer) -> R>(
+    fn with_buffers_stolen<R, F: FnOnce(&mut Self, &mut MioConnectionBuffers) -> R>(
         &mut self,
         f: F,
     ) -> R {
         let mut bufs = self.steal_buffers();
-        let res = f(self, &mut bufs.read_buffer, &mut bufs.write_buffer);
+        let res = f(self, &mut bufs);
         self.return_buffers(bufs);
         res
     }
 
-    fn both_buffers_mut(&mut self) -> (&mut ReadBuffer, &mut WriteBuffer) {
-        let bufs = self.mio_connection_mut().buffers.as_mut().unwrap();
-        let MioConnectionBuffers {
-            ref mut read_buffer,
-            ref mut write_buffer,
-        } = bufs;
-        (read_buffer, write_buffer)
-    }
-
-    #[allow(dead_code)]
-    fn read_buf_mut(&mut self) -> &mut ReadBuffer {
-        self.both_buffers_mut().0
-    }
-
     fn write_buf_mut(&mut self) -> &mut WriteBuffer {
-        self.both_buffers_mut().1
+        self.mio_connection_mut()
+            .buffers
+            .as_mut()
+            .unwrap()
+            .write_buffer
+            .borrow_mut()
     }
 }
 

util/Cargo.toml

@@ -22,3 +22,8 @@ zerocopy = { workspace = true }
 thiserror = { workspace = true }
 mio = { workspace = true }
 tempfile = { workspace = true }
+uds = { workspace = true, optional = true, features = ["mio_1xx"] }
+
+
+[features]
+experiment_file_descriptor_passing = ["uds"]

util/src/controlflow.rs

@@ -0,0 +1,50 @@
+#[macro_export]
+macro_rules! repeat {
+    ($times:expr, $body:expr) => {
+        for _ in 0..($times) {
+            $body
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! return_if {
+    ($cond:expr) => {
+        if $cond {
+            return;
+        }
+    };
+    ($cond:expr, $val:expr) => {
+        if $cond {
+            return $val;
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! break_if {
+    ($cond:expr) => {
+        if $cond {
+            break;
+        }
+    };
+    ($cond:expr, $val:expr) => {
+        if $cond {
+            break $val;
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! continue_if {
+    ($cond:expr) => {
+        if $cond {
+            continue;
+        }
+    };
+    ($cond:expr, $val:expr) => {
+        if $cond {
+            break $val;
+        }
+    };
+}

util/src/fd.rs

@@ -25,6 +25,18 @@ pub fn claim_fd(fd: RawFd) -> rustix::io::Result<OwnedFd> {
     Ok(new)
 }
 
+/// Prepare a file descriptor for use in Rust code.
+///
+/// Checks if the file descriptor is valid.
+///
+/// Unlike [claim_fd], this will reuse the same file descriptor identifier instead of masking it.
+pub fn claim_fd_inplace(fd: RawFd) -> rustix::io::Result<OwnedFd> {
+    let mut new = unsafe { OwnedFd::from_raw_fd(fd) };
+    let tmp = clone_fd_cloexec(&new)?;
+    clone_fd_to_cloexec(tmp, &mut new)?;
+    Ok(new)
+}
+
 pub fn mask_fd(fd: RawFd) -> rustix::io::Result<()> {
     // Safety: because the OwnedFd resulting from OwnedFd::from_raw_fd is wrapped in a Forgetting,
     // it never gets dropped, meaning that fd is never closed and thus outlives the OwnedFd
@@ -58,6 +70,47 @@ pub fn open_nullfd() -> rustix::io::Result<OwnedFd> {
     open("/dev/null", OFlags::CLOEXEC, Mode::empty())
 }
 
+/// Convert low level errors into std::io::Error
+pub trait IntoStdioErr {
+    type Target;
+    fn into_stdio_err(self) -> Self::Target;
+}
+
+impl IntoStdioErr for rustix::io::Errno {
+    type Target = std::io::Error;
+
+    fn into_stdio_err(self) -> Self::Target {
+        std::io::Error::from_raw_os_error(self.raw_os_error())
+    }
+}
+
+impl<T> IntoStdioErr for rustix::io::Result<T> {
+    type Target = std::io::Result<T>;
+
+    fn into_stdio_err(self) -> Self::Target {
+        self.map_err(IntoStdioErr::into_stdio_err)
+    }
+}
+
+/// Read and write directly from a file descriptor
+pub struct FdIo<Fd: AsFd>(pub Fd);
+
+impl<Fd: AsFd> std::io::Read for FdIo<Fd> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        rustix::io::read(&self.0, buf).into_stdio_err()
+    }
+}
+
+impl<Fd: AsFd> std::io::Write for FdIo<Fd> {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        rustix::io::write(&self.0, buf).into_stdio_err()
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        Ok(())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

util/src/mio/mod.rs

@@ -0,0 +1,12 @@
+#[allow(clippy::module_inception)]
+mod mio;
+pub use mio::*;
+
+#[cfg(feature = "experiment_file_descriptor_passing")]
+mod uds_recv_fd;
+#[cfg(feature = "experiment_file_descriptor_passing")]
+mod uds_send_fd;
+#[cfg(feature = "experiment_file_descriptor_passing")]
+pub use uds_recv_fd::*;
+#[cfg(feature = "experiment_file_descriptor_passing")]
+pub use uds_send_fd::*;

util/src/mio/uds_recv_fd.rs

@@ -0,0 +1,123 @@
+use std::{
+    borrow::{Borrow, BorrowMut},
+    collections::VecDeque,
+    io::Read,
+    marker::PhantomData,
+    os::fd::OwnedFd,
+};
+use uds::UnixStreamExt as FdPassingExt;
+
+use crate::fd::{claim_fd_inplace, IntoStdioErr};
+
+pub struct ReadWithFileDescriptors<const MAX_FDS: usize, Sock, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    BorrowSock: Borrow<Sock>,
+    BorrowFds: BorrowMut<VecDeque<OwnedFd>>,
+{
+    socket: BorrowSock,
+    fds: BorrowFds,
+    _sock_dummy: PhantomData<Sock>,
+}
+
+impl<const MAX_FDS: usize, Sock, BorrowSock, BorrowFds>
+    ReadWithFileDescriptors<MAX_FDS, Sock, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    BorrowSock: Borrow<Sock>,
+    BorrowFds: BorrowMut<VecDeque<OwnedFd>>,
+{
+    pub fn new(socket: BorrowSock, fds: BorrowFds) -> Self {
+        let _sock_dummy = PhantomData;
+        Self {
+            socket,
+            fds,
+            _sock_dummy,
+        }
+    }
+
+    pub fn into_parts(self) -> (BorrowSock, BorrowFds) {
+        let Self { socket, fds, .. } = self;
+        (socket, fds)
+    }
+
+    pub fn socket(&self) -> &Sock {
+        self.socket.borrow()
+    }
+
+    pub fn fds(&self) -> &VecDeque<OwnedFd> {
+        self.fds.borrow()
+    }
+
+    pub fn fds_mut(&mut self) -> &mut VecDeque<OwnedFd> {
+        self.fds.borrow_mut()
+    }
+}
+
+impl<const MAX_FDS: usize, Sock, BorrowSock, BorrowFds>
+    ReadWithFileDescriptors<MAX_FDS, Sock, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    BorrowSock: BorrowMut<Sock>,
+    BorrowFds: BorrowMut<VecDeque<OwnedFd>>,
+{
+    pub fn socket_mut(&mut self) -> &mut Sock {
+        self.socket.borrow_mut()
+    }
+}
+
+impl<const MAX_FDS: usize, Sock, BorrowSock, BorrowFds> Read
+    for ReadWithFileDescriptors<MAX_FDS, Sock, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    BorrowSock: Borrow<Sock>,
+    BorrowFds: BorrowMut<VecDeque<OwnedFd>>,
+{
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        // Calculate space for additional file descriptors
+        let have_fds_before_read = self.fds().len();
+        let free_fd_slots = MAX_FDS.saturating_sub(have_fds_before_read);
+
+        // Allocate a buffer for file descriptors
+        let mut fd_buf = [0; MAX_FDS];
+        let fd_buf = &mut fd_buf[..free_fd_slots];
+
+        // Read from the unix socket
+        let (bytes_read, fds_read) = self.socket.borrow().recv_fds(buf, fd_buf)?;
+        let fd_buf = &fd_buf[..fds_read];
+
+        // Process the file descriptors
+        let mut fd_iter = fd_buf.iter();
+
+        // Try claiming all the file descriptors
+        let mut claim_fd_result = Ok(bytes_read);
+        self.fds_mut().reserve(fd_buf.len());
+        for fd in fd_iter.by_ref() {
+            match claim_fd_inplace(*fd) {
+                Ok(owned) => self.fds_mut().push_back(owned),
+                Err(e) => {
+                    // Abort on error and pass to error handler
+                    // Note that claim_fd_inplace is responsible for closing this particular
+                    // file descriptor if claiming it fails
+                    claim_fd_result = Err(e.into_stdio_err());
+                    break;
+                }
+            }
+        }
+
+        // Return if we where able to claim all file descriptors
+        if claim_fd_result.is_ok() {
+            return claim_fd_result;
+        };
+
+        // An error occurred while claiming fds
+        self.fds_mut().truncate(have_fds_before_read); // Close fds successfully claimed
+
+        // Close the remaining fds
+        for fd in fd_iter {
+            unsafe { rustix::io::close(*fd) };
+        }
+
+        claim_fd_result
+    }
+}

util/src/mio/uds_send_fd.rs

@@ -0,0 +1,121 @@
+use rustix::fd::{AsFd, AsRawFd};
+use std::{
+    borrow::{Borrow, BorrowMut},
+    cmp::min,
+    collections::VecDeque,
+    io::Write,
+    marker::PhantomData,
+};
+use uds::UnixStreamExt as FdPassingExt;
+
+use crate::{repeat, return_if};
+
+pub struct WriteWithFileDescriptors<Sock, Fd, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    Fd: AsFd,
+    BorrowSock: Borrow<Sock>,
+    BorrowFds: BorrowMut<VecDeque<Fd>>,
+{
+    socket: BorrowSock,
+    fds: BorrowFds,
+    _sock_dummy: PhantomData<Sock>,
+    _fd_dummy: PhantomData<Fd>,
+}
+
+impl<Sock, Fd, BorrowSock, BorrowFds> WriteWithFileDescriptors<Sock, Fd, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    Fd: AsFd,
+    BorrowSock: Borrow<Sock>,
+    BorrowFds: BorrowMut<VecDeque<Fd>>,
+{
+    pub fn new(socket: BorrowSock, fds: BorrowFds) -> Self {
+        let _sock_dummy = PhantomData;
+        let _fd_dummy = PhantomData;
+        Self {
+            socket,
+            fds,
+            _sock_dummy,
+            _fd_dummy,
+        }
+    }
+
+    pub fn into_parts(self) -> (BorrowSock, BorrowFds) {
+        let Self { socket, fds, .. } = self;
+        (socket, fds)
+    }
+
+    pub fn socket(&self) -> &Sock {
+        self.socket.borrow()
+    }
+
+    pub fn fds(&self) -> &VecDeque<Fd> {
+        self.fds.borrow()
+    }
+
+    pub fn fds_mut(&mut self) -> &mut VecDeque<Fd> {
+        self.fds.borrow_mut()
+    }
+}
+
+impl<Sock, Fd, BorrowSock, BorrowFds> WriteWithFileDescriptors<Sock, Fd, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    Fd: AsFd,
+    BorrowSock: BorrowMut<Sock>,
+    BorrowFds: BorrowMut<VecDeque<Fd>>,
+{
+    pub fn socket_mut(&mut self) -> &mut Sock {
+        self.socket.borrow_mut()
+    }
+}
+
+impl<Sock, Fd, BorrowSock, BorrowFds> Write
+    for WriteWithFileDescriptors<Sock, Fd, BorrowSock, BorrowFds>
+where
+    Sock: FdPassingExt,
+    Fd: AsFd,
+    BorrowSock: Borrow<Sock>,
+    BorrowFds: BorrowMut<VecDeque<Fd>>,
+{
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        // At least one byte of real data should be sent when sending ancillary data. -- unix(7)
+        return_if!(buf.is_empty(), Ok(0));
+
+        // The kernel constant SCM_MAX_FD defines a limit on the number of file descriptors
+        // in the array.  Attempting to  send  an  array  larger  than  this  limit  causes
+        // sendmsg(2)  to fail with the error EINVAL.  SCM_MAX_FD has the value 253 (or 255
+        // before Linux 2.6.38).
+        // -- unix(7)
+        const SCM_MAX_FD: usize = 253;
+        let buf = match self.fds().len() <= SCM_MAX_FD {
+            false => &buf[..1], // Force caller to immediately call write() again to send its data
+            true => buf,
+        };
+
+        // Allocate the buffer for the file descriptor array
+        let fd_no = min(SCM_MAX_FD, self.fds().len());
+        let mut fd_buf = [0; SCM_MAX_FD]; // My kingdom for alloca(3)
+        let fd_buf = &mut fd_buf[..fd_no];
+
+        // Fill the file descriptor array
+        for (raw, fancy) in fd_buf.iter_mut().zip(self.fds().iter()) {
+            *raw = fancy.as_fd().as_raw_fd();
+        }
+
+        // Send data and file descriptors
+        let bytes_written = self.socket().send_fds(buf, fd_buf)?;
+
+        // Drop the file descriptors from the Deque
+        repeat!(fd_no, {
+            self.fds_mut().pop_front();
+        });
+
+        Ok(bytes_written)
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        Ok(())
+    }
+}