sys_util/src/sock_ctrl_msg.rs - platform/external/crosvm - Git at Google

 // Copyright 2017 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.

 //! Used to send and receive messages with file descriptors on sockets that accept control messages
 //! (e.g. Unix domain sockets).

 use std::fs::File;
 use std::mem::size_of;
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use std::os::unix::net::{UnixDatagram, UnixStream};
 use std::ptr::{copy_nonoverlapping, null_mut, write_unaligned};

 use libc::{
     c_long, c_void, cmsghdr, iovec, msghdr, recvmsg, sendmsg, MSG_NOSIGNAL, SCM_RIGHTS, SOL_SOCKET,
 };

 use data_model::VolatileSlice;

 use crate::net::UnixSeqpacket;
 use crate::{Error, Result};

 // Each of the following macros performs the same function as their C counterparts. They are each
 // macros because they are used to size statically allocated arrays.

 macro_rules! CMSG_ALIGN {
     ($len:expr) => {
         (($len) + size_of::<c_long>() - 1) & !(size_of::<c_long>() - 1)
     };
 }

 macro_rules! CMSG_SPACE {
     ($len:expr) => {
         size_of::<cmsghdr>() + CMSG_ALIGN!($len)
     };
 }

 macro_rules! CMSG_LEN {
     ($len:expr) => {
         size_of::<cmsghdr>() + ($len)
     };
 }

 // This function (macro in the C version) is not used in any compile time constant slots, so is just
 // an ordinary function. The returned pointer is hard coded to be RawFd because that's all that this
 // module supports.
 #[allow(non_snake_case)]
 #[inline(always)]
 fn CMSG_DATA(cmsg_buffer: *mut cmsghdr) -> *mut RawFd {
     // Essentially returns a pointer to just past the header.
     cmsg_buffer.wrapping_offset(1) as *mut RawFd
 }

 // This function is like CMSG_NEXT, but safer because it reads only from references, although it
 // does some pointer arithmetic on cmsg_ptr.
 #[allow(clippy::cast_ptr_alignment)]
 fn get_next_cmsg(msghdr: &msghdr, cmsg: &cmsghdr, cmsg_ptr: *mut cmsghdr) -> *mut cmsghdr {
     let next_cmsg = (cmsg_ptr as *mut u8).wrapping_add(CMSG_ALIGN!(cmsg.cmsg_len)) as *mut cmsghdr;
     if next_cmsg
         .wrapping_offset(1)
         .wrapping_sub(msghdr.msg_control as usize) as usize
         > msghdr.msg_controllen
     {
         null_mut()
     } else {
         next_cmsg
     }
 }

 const CMSG_BUFFER_INLINE_CAPACITY: usize = CMSG_SPACE!(size_of::<RawFd>() * 32);

 enum CmsgBuffer {
     Inline([u64; (CMSG_BUFFER_INLINE_CAPACITY + 7) / 8]),
     Heap(Box<[cmsghdr]>),
 }

 impl CmsgBuffer {
     fn with_capacity(capacity: usize) -> CmsgBuffer {
         let cap_in_cmsghdr_units =
             (capacity.checked_add(size_of::<cmsghdr>()).unwrap() - 1) / size_of::<cmsghdr>();
         if capacity <= CMSG_BUFFER_INLINE_CAPACITY {
             CmsgBuffer::Inline([0u64; (CMSG_BUFFER_INLINE_CAPACITY + 7) / 8])
         } else {
             CmsgBuffer::Heap(
                 vec![
                     cmsghdr {
                         cmsg_len: 0,
                         cmsg_level: 0,
                         cmsg_type: 0,
                     };
                     cap_in_cmsghdr_units
                 ]
                 .into_boxed_slice(),
             )
         }
     }

     fn as_mut_ptr(&mut self) -> *mut cmsghdr {
         match self {
             CmsgBuffer::Inline(a) => a.as_mut_ptr() as *mut cmsghdr,
             CmsgBuffer::Heap(a) => a.as_mut_ptr(),
         }
     }
 }

 fn raw_sendmsg<D: IntoIovec>(fd: RawFd, out_data: D, out_fds: &[RawFd]) -> Result<usize> {
     let cmsg_capacity = CMSG_SPACE!(size_of::<RawFd>() * out_fds.len());
     let mut cmsg_buffer = CmsgBuffer::with_capacity(cmsg_capacity);

     let mut iovec = iovec {
         iov_base: out_data.as_ptr() as *mut c_void,
         iov_len: out_data.size(),
     };

     let mut msg = msghdr {
         msg_name: null_mut(),
         msg_namelen: 0,
         msg_iov: &mut iovec as *mut iovec,
         msg_iovlen: 1,
         msg_control: null_mut(),
         msg_controllen: 0,
         msg_flags: 0,
     };

     if !out_fds.is_empty() {
         let cmsg = cmsghdr {
             cmsg_len: CMSG_LEN!(size_of::<RawFd>() * out_fds.len()),
             cmsg_level: SOL_SOCKET,
             cmsg_type: SCM_RIGHTS,
         };
         unsafe {
             // Safe because cmsg_buffer was allocated to be large enough to contain cmsghdr.
             write_unaligned(cmsg_buffer.as_mut_ptr() as *mut cmsghdr, cmsg);
             // Safe because the cmsg_buffer was allocated to be large enough to hold out_fds.len()
             // file descriptors.
             copy_nonoverlapping(
                 out_fds.as_ptr(),
                 CMSG_DATA(cmsg_buffer.as_mut_ptr()),
                 out_fds.len(),
             );
         }

         msg.msg_control = cmsg_buffer.as_mut_ptr() as *mut c_void;
         msg.msg_controllen = cmsg_capacity;
     }

     // Safe because the msghdr was properly constructed from valid (or null) pointers of the
     // indicated length and we check the return value.
     let write_count = unsafe { sendmsg(fd, &msg, MSG_NOSIGNAL) };

     if write_count == -1 {
         Err(Error::last())
     } else {
         Ok(write_count as usize)
     }
 }

 fn raw_recvmsg(fd: RawFd, in_data: &mut [u8], in_fds: &mut [RawFd]) -> Result<(usize, usize)> {
     let cmsg_capacity = CMSG_SPACE!(size_of::<RawFd>() * in_fds.len());
     let mut cmsg_buffer = CmsgBuffer::with_capacity(cmsg_capacity);

     let mut iovec = iovec {
         iov_base: in_data.as_mut_ptr() as *mut c_void,
         iov_len: in_data.len(),
     };

     let mut msg = msghdr {
         msg_name: null_mut(),
         msg_namelen: 0,
         msg_iov: &mut iovec as *mut iovec,
         msg_iovlen: 1,
         msg_control: null_mut(),
         msg_controllen: 0,
         msg_flags: 0,
     };

     if !in_fds.is_empty() {
         msg.msg_control = cmsg_buffer.as_mut_ptr() as *mut c_void;
         msg.msg_controllen = cmsg_capacity;
     }

     // Safe because the msghdr was properly constructed from valid (or null) pointers of the
     // indicated length and we check the return value.
     let total_read = unsafe { recvmsg(fd, &mut msg, 0) };

     if total_read == -1 {
         return Err(Error::last());
     }

     if total_read == 0 && msg.msg_controllen < size_of::<cmsghdr>() {
         return Ok((0, 0));
     }

     let mut cmsg_ptr = msg.msg_control as *mut cmsghdr;
     let mut in_fds_count = 0;
     while !cmsg_ptr.is_null() {
         // Safe because we checked that cmsg_ptr was non-null, and the loop is constructed such that
         // that only happens when there is at least sizeof(cmsghdr) space after the pointer to read.
         let cmsg = unsafe { (cmsg_ptr as *mut cmsghdr).read_unaligned() };

         if cmsg.cmsg_level == SOL_SOCKET && cmsg.cmsg_type == SCM_RIGHTS {
             let fd_count = (cmsg.cmsg_len - CMSG_LEN!(0)) / size_of::<RawFd>();
             unsafe {
                 copy_nonoverlapping(
                     CMSG_DATA(cmsg_ptr),
                     in_fds[in_fds_count..(in_fds_count + fd_count)].as_mut_ptr(),
                     fd_count,
                 );
             }
             in_fds_count += fd_count;
         }

         cmsg_ptr = get_next_cmsg(&msg, &cmsg, cmsg_ptr);
     }

     Ok((total_read as usize, in_fds_count))
 }

 /// Trait for file descriptors can send and receive socket control messages via `sendmsg` and
 /// `recvmsg`.
 pub trait ScmSocket {
     /// Gets the file descriptor of this socket.
     fn socket_fd(&self) -> RawFd;

     /// Sends the given data and file descriptor over the socket.
     ///
     /// On success, returns the number of bytes sent.
     ///
     /// # Arguments
     ///
     /// * `buf` - A buffer of data to send on the `socket`.
     /// * `fd` - A file descriptors to be sent.
     fn send_with_fd<D: IntoIovec>(&self, buf: D, fd: RawFd) -> Result<usize> {
         self.send_with_fds(buf, &[fd])
     }

     /// Sends the given data and file descriptors over the socket.
     ///
     /// On success, returns the number of bytes sent.
     ///
     /// # Arguments
     ///
     /// * `buf` - A buffer of data to send on the `socket`.
     /// * `fds` - A list of file descriptors to be sent.
     fn send_with_fds<D: IntoIovec>(&self, buf: D, fd: &[RawFd]) -> Result<usize> {
         raw_sendmsg(self.socket_fd(), buf, fd)
     }

     /// Receives data and potentially a file descriptor from the socket.
     ///
     /// On success, returns the number of bytes and an optional file descriptor.
     ///
     /// # Arguments
     ///
     /// * `buf` - A buffer to receive data from the socket.vm
     fn recv_with_fd(&self, buf: &mut [u8]) -> Result<(usize, Option<File>)> {
         let mut fd = [0];
         let (read_count, fd_count) = self.recv_with_fds(buf, &mut fd)?;
         let file = if fd_count == 0 {
             None
         } else {
             // Safe because the first fd from recv_with_fds is owned by us and valid because this
             // branch was taken.
             Some(unsafe { File::from_raw_fd(fd[0]) })
         };
         Ok((read_count, file))
     }

     /// Receives data and file descriptors from the socket.
     ///
     /// On success, returns the number of bytes and file descriptors received as a tuple
     /// `(bytes count, files count)`.
     ///
     /// # Arguments
     ///
     /// * `buf` - A buffer to receive data from the socket.
     /// * `fds` - A slice of `RawFd`s to put the received file descriptors into. On success, the
     ///           number of valid file descriptors is indicated by the second element of the
     ///           returned tuple. The caller owns these file descriptors, but they will not be
     ///           closed on drop like a `File`-like type would be. It is recommended that each valid
     ///           file descriptor gets wrapped in a drop type that closes it after this returns.
     fn recv_with_fds(&self, buf: &mut [u8], fds: &mut [RawFd]) -> Result<(usize, usize)> {
         raw_recvmsg(self.socket_fd(), buf, fds)
     }
 }

 impl ScmSocket for UnixDatagram {
     fn socket_fd(&self) -> RawFd {
         self.as_raw_fd()
     }
 }

 impl ScmSocket for UnixStream {
     fn socket_fd(&self) -> RawFd {
         self.as_raw_fd()
     }
 }
 impl ScmSocket for UnixSeqpacket {
     fn socket_fd(&self) -> RawFd {
         self.as_raw_fd()
     }
 }

 /// Trait for types that can be converted into an `iovec` that can be referenced by a syscall for
 /// the lifetime of this object.
 ///
 /// This trait is unsafe because interfaces that use this trait depend on the base pointer and size
 /// being accurate.
 pub unsafe trait IntoIovec {
     /// Gets the base pointer of this `iovec`.
     fn as_ptr(&self) -> *const c_void;

     /// Gets the size in bytes of this `iovec`.
     fn size(&self) -> usize;
 }

 // Safe because this slice can not have another mutable reference and it's pointer and size are
 // guaranteed to be valid.
 unsafe impl<'a> IntoIovec for &'a [u8] {
     // Clippy false positive: https://github.com/rust-lang/rust-clippy/issues/3480
     #[allow(clippy::useless_asref)]
     fn as_ptr(&self) -> *const c_void {
         self.as_ref().as_ptr() as *const c_void
     }

     fn size(&self) -> usize {
         self.len()
     }
 }

 // Safe because volatile slices are only ever accessed with other volatile interfaces and the
 // pointer and size are guaranteed to be accurate.
 unsafe impl<'a> IntoIovec for VolatileSlice<'a> {
     fn as_ptr(&self) -> *const c_void {
         self.as_ptr() as *const c_void
     }

     fn size(&self) -> usize {
         self.size() as usize
     }
 }

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

     use std::io::Write;
     use std::mem::size_of;
     use std::os::raw::c_long;
     use std::os::unix::net::UnixDatagram;
     use std::slice::from_raw_parts;

     use libc::cmsghdr;

     use crate::EventFd;

     #[test]
     fn buffer_len() {
         assert_eq!(CMSG_SPACE!(0 * size_of::<RawFd>()), size_of::<cmsghdr>());
         assert_eq!(
             CMSG_SPACE!(1 * size_of::<RawFd>()),
             size_of::<cmsghdr>() + size_of::<c_long>()
         );
         if size_of::<RawFd>() == 4 {
             assert_eq!(
                 CMSG_SPACE!(2 * size_of::<RawFd>()),
                 size_of::<cmsghdr>() + size_of::<c_long>()
             );
             assert_eq!(
                 CMSG_SPACE!(3 * size_of::<RawFd>()),
                 size_of::<cmsghdr>() + size_of::<c_long>() * 2
             );
             assert_eq!(
                 CMSG_SPACE!(4 * size_of::<RawFd>()),
                 size_of::<cmsghdr>() + size_of::<c_long>() * 2
             );
         } else if size_of::<RawFd>() == 8 {
             assert_eq!(
                 CMSG_SPACE!(2 * size_of::<RawFd>()),
                 size_of::<cmsghdr>() + size_of::<c_long>() * 2
             );
             assert_eq!(
                 CMSG_SPACE!(3 * size_of::<RawFd>()),
                 size_of::<cmsghdr>() + size_of::<c_long>() * 3
             );
             assert_eq!(
                 CMSG_SPACE!(4 * size_of::<RawFd>()),
                 size_of::<cmsghdr>() + size_of::<c_long>() * 4
             );
         }
     }

     #[test]
     fn send_recv_no_fd() {
         let (s1, s2) = UnixDatagram::pair().expect("failed to create socket pair");

         let write_count = s1
             .send_with_fds([1u8, 1, 2, 21, 34, 55].as_ref(), &[])
             .expect("failed to send data");

         assert_eq!(write_count, 6);

         let mut buf = [0; 6];
         let mut files = [0; 1];
         let (read_count, file_count) = s2
             .recv_with_fds(&mut buf[..], &mut files)
             .expect("failed to recv data");

         assert_eq!(read_count, 6);
         assert_eq!(file_count, 0);
         assert_eq!(buf, [1, 1, 2, 21, 34, 55]);
     }

     #[test]
     fn send_recv_only_fd() {
         let (s1, s2) = UnixDatagram::pair().expect("failed to create socket pair");

         let evt = EventFd::new().expect("failed to create eventfd");
         let write_count = s1
             .send_with_fd([].as_ref(), evt.as_raw_fd())
             .expect("failed to send fd");

         assert_eq!(write_count, 0);

         let (read_count, file_opt) = s2.recv_with_fd(&mut []).expect("failed to recv fd");

         let mut file = file_opt.unwrap();

         assert_eq!(read_count, 0);
         assert!(file.as_raw_fd() >= 0);
         assert_ne!(file.as_raw_fd(), s1.as_raw_fd());
         assert_ne!(file.as_raw_fd(), s2.as_raw_fd());
         assert_ne!(file.as_raw_fd(), evt.as_raw_fd());

         file.write(unsafe { from_raw_parts(&1203u64 as *const u64 as *const u8, 8) })
             .expect("failed to write to sent fd");

         assert_eq!(evt.read().expect("failed to read from eventfd"), 1203);
     }

     #[test]
     fn send_recv_with_fd() {
         let (s1, s2) = UnixDatagram::pair().expect("failed to create socket pair");

         let evt = EventFd::new().expect("failed to create eventfd");
         let write_count = s1
             .send_with_fds([237].as_ref(), &[evt.as_raw_fd()])
             .expect("failed to send fd");

         assert_eq!(write_count, 1);

         let mut files = [0; 2];
         let mut buf = [0u8];
         let (read_count, file_count) = s2
             .recv_with_fds(&mut buf, &mut files)
             .expect("failed to recv fd");

         assert_eq!(read_count, 1);
         assert_eq!(buf[0], 237);
         assert_eq!(file_count, 1);
         assert!(files[0] >= 0);
         assert_ne!(files[0], s1.as_raw_fd());
         assert_ne!(files[0], s2.as_raw_fd());
         assert_ne!(files[0], evt.as_raw_fd());

         let mut file = unsafe { File::from_raw_fd(files[0]) };

         file.write(unsafe { from_raw_parts(&1203u64 as *const u64 as *const u8, 8) })
             .expect("failed to write to sent fd");

         assert_eq!(evt.read().expect("failed to read from eventfd"), 1203);
     }
 }
	// Copyright 2017 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.

	//! Used to send and receive messages with file descriptors on sockets that accept control messages
	//! (e.g. Unix domain sockets).

	use std::fs::File;
	use std::mem::size_of;
	use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
	use std::os::unix::net::{UnixDatagram, UnixStream};
	use std::ptr::{copy_nonoverlapping, null_mut, write_unaligned};

	use libc::{
	c_long, c_void, cmsghdr, iovec, msghdr, recvmsg, sendmsg, MSG_NOSIGNAL, SCM_RIGHTS, SOL_SOCKET,
	};

	use data_model::VolatileSlice;

	use crate::net::UnixSeqpacket;
	use crate::{Error, Result};

	// Each of the following macros performs the same function as their C counterparts. They are each
	// macros because they are used to size statically allocated arrays.

	macro_rules! CMSG_ALIGN {
	($len:expr) => {
	(($len) + size_of::<c_long>() - 1) & !(size_of::<c_long>() - 1)
	};
	}

	macro_rules! CMSG_SPACE {
	($len:expr) => {
	size_of::<cmsghdr>() + CMSG_ALIGN!($len)
	};
	}

	macro_rules! CMSG_LEN {
	($len:expr) => {
	size_of::<cmsghdr>() + ($len)
	};
	}

	// This function (macro in the C version) is not used in any compile time constant slots, so is just
	// an ordinary function. The returned pointer is hard coded to be RawFd because that's all that this
	// module supports.
	#[allow(non_snake_case)]
	#[inline(always)]
	fn CMSG_DATA(cmsg_buffer: mut cmsghdr) -> mut RawFd {
	// Essentially returns a pointer to just past the header.
	cmsg_buffer.wrapping_offset(1) as *mut RawFd
	}

	// This function is like CMSG_NEXT, but safer because it reads only from references, although it
	// does some pointer arithmetic on cmsg_ptr.
	#[allow(clippy::cast_ptr_alignment)]
	fn get_next_cmsg(msghdr: &msghdr, cmsg: &cmsghdr, cmsg_ptr: mut cmsghdr) -> mut cmsghdr {
	let next_cmsg = (cmsg_ptr as mut u8).wrapping_add(CMSG_ALIGN!(cmsg.cmsg_len)) as mut cmsghdr;
	if next_cmsg
	.wrapping_offset(1)
	.wrapping_sub(msghdr.msg_control as usize) as usize
	> msghdr.msg_controllen
	{
	null_mut()
	} else {
	next_cmsg
	}
	}

	const CMSG_BUFFER_INLINE_CAPACITY: usize = CMSG_SPACE!(size_of::<RawFd>() * 32);

	enum CmsgBuffer {
	Inline([u64; (CMSG_BUFFER_INLINE_CAPACITY + 7) / 8]),
	Heap(Box<[cmsghdr]>),
	}

	impl CmsgBuffer {
	fn with_capacity(capacity: usize) -> CmsgBuffer {
	let cap_in_cmsghdr_units =
	(capacity.checked_add(size_of::<cmsghdr>()).unwrap() - 1) / size_of::<cmsghdr>();
	if capacity <= CMSG_BUFFER_INLINE_CAPACITY {
	CmsgBuffer::Inline([0u64; (CMSG_BUFFER_INLINE_CAPACITY + 7) / 8])
	} else {
	CmsgBuffer::Heap(
	vec![
	cmsghdr {
	cmsg_len: 0,
	cmsg_level: 0,
	cmsg_type: 0,
	};
	cap_in_cmsghdr_units
	]
	.into_boxed_slice(),
	)
	}
	}

	fn as_mut_ptr(&mut self) -> *mut cmsghdr {
	match self {
	CmsgBuffer::Inline(a) => a.as_mut_ptr() as *mut cmsghdr,
	CmsgBuffer::Heap(a) => a.as_mut_ptr(),
	}
	}
	}

	fn raw_sendmsg<D: IntoIovec>(fd: RawFd, out_data: D, out_fds: &[RawFd]) -> Result<usize> {
	let cmsg_capacity = CMSG_SPACE!(size_of::<RawFd>() * out_fds.len());
	let mut cmsg_buffer = CmsgBuffer::with_capacity(cmsg_capacity);

	let mut iovec = iovec {
	iov_base: out_data.as_ptr() as *mut c_void,
	iov_len: out_data.size(),
	};

	let mut msg = msghdr {
	msg_name: null_mut(),
	msg_namelen: 0,
	msg_iov: &mut iovec as *mut iovec,
	msg_iovlen: 1,
	msg_control: null_mut(),
	msg_controllen: 0,
	msg_flags: 0,
	};

	if !out_fds.is_empty() {
	let cmsg = cmsghdr {
	cmsg_len: CMSG_LEN!(size_of::<RawFd>() * out_fds.len()),
	cmsg_level: SOL_SOCKET,
	cmsg_type: SCM_RIGHTS,
	};
	unsafe {
	// Safe because cmsg_buffer was allocated to be large enough to contain cmsghdr.
	write_unaligned(cmsg_buffer.as_mut_ptr() as *mut cmsghdr, cmsg);
	// Safe because the cmsg_buffer was allocated to be large enough to hold out_fds.len()
	// file descriptors.
	copy_nonoverlapping(
	out_fds.as_ptr(),
	CMSG_DATA(cmsg_buffer.as_mut_ptr()),
	out_fds.len(),
	);
	}

	msg.msg_control = cmsg_buffer.as_mut_ptr() as *mut c_void;
	msg.msg_controllen = cmsg_capacity;
	}

	// Safe because the msghdr was properly constructed from valid (or null) pointers of the
	// indicated length and we check the return value.
	let write_count = unsafe { sendmsg(fd, &msg, MSG_NOSIGNAL) };

	if write_count == -1 {
	Err(Error::last())
	} else {
	Ok(write_count as usize)
	}
	}

	fn raw_recvmsg(fd: RawFd, in_data: &mut [u8], in_fds: &mut [RawFd]) -> Result<(usize, usize)> {
	let cmsg_capacity = CMSG_SPACE!(size_of::<RawFd>() * in_fds.len());
	let mut cmsg_buffer = CmsgBuffer::with_capacity(cmsg_capacity);

	let mut iovec = iovec {
	iov_base: in_data.as_mut_ptr() as *mut c_void,
	iov_len: in_data.len(),
	};

	let mut msg = msghdr {
	msg_name: null_mut(),
	msg_namelen: 0,
	msg_iov: &mut iovec as *mut iovec,
	msg_iovlen: 1,
	msg_control: null_mut(),
	msg_controllen: 0,
	msg_flags: 0,
	};

	if !in_fds.is_empty() {
	msg.msg_control = cmsg_buffer.as_mut_ptr() as *mut c_void;
	msg.msg_controllen = cmsg_capacity;
	}

	// Safe because the msghdr was properly constructed from valid (or null) pointers of the
	// indicated length and we check the return value.
	let total_read = unsafe { recvmsg(fd, &mut msg, 0) };

	if total_read == -1 {
	return Err(Error::last());
	}

	if total_read == 0 && msg.msg_controllen < size_of::<cmsghdr>() {
	return Ok((0, 0));
	}

	let mut cmsg_ptr = msg.msg_control as *mut cmsghdr;
	let mut in_fds_count = 0;
	while !cmsg_ptr.is_null() {
	// Safe because we checked that cmsg_ptr was non-null, and the loop is constructed such that
	// that only happens when there is at least sizeof(cmsghdr) space after the pointer to read.
	let cmsg = unsafe { (cmsg_ptr as *mut cmsghdr).read_unaligned() };

	if cmsg.cmsg_level == SOL_SOCKET && cmsg.cmsg_type == SCM_RIGHTS {
	let fd_count = (cmsg.cmsg_len - CMSG_LEN!(0)) / size_of::<RawFd>();
	unsafe {
	copy_nonoverlapping(
	CMSG_DATA(cmsg_ptr),
	in_fds[in_fds_count..(in_fds_count + fd_count)].as_mut_ptr(),
	fd_count,
	);
	}
	in_fds_count += fd_count;
	}

	cmsg_ptr = get_next_cmsg(&msg, &cmsg, cmsg_ptr);
	}

	Ok((total_read as usize, in_fds_count))
	}

	/// Trait for file descriptors can send and receive socket control messages via `sendmsg` and
	/// `recvmsg`.
	pub trait ScmSocket {
	/// Gets the file descriptor of this socket.
	fn socket_fd(&self) -> RawFd;

	/// Sends the given data and file descriptor over the socket.
	///
	/// On success, returns the number of bytes sent.
	///
	/// # Arguments
	///
	/// * `buf` - A buffer of data to send on the `socket`.
	/// * `fd` - A file descriptors to be sent.
	fn send_with_fd<D: IntoIovec>(&self, buf: D, fd: RawFd) -> Result<usize> {
	self.send_with_fds(buf, &[fd])
	}

	/// Sends the given data and file descriptors over the socket.
	///
	/// On success, returns the number of bytes sent.
	///
	/// # Arguments
	///
	/// * `buf` - A buffer of data to send on the `socket`.
	/// * `fds` - A list of file descriptors to be sent.
	fn send_with_fds<D: IntoIovec>(&self, buf: D, fd: &[RawFd]) -> Result<usize> {
	raw_sendmsg(self.socket_fd(), buf, fd)
	}

	/// Receives data and potentially a file descriptor from the socket.
	///
	/// On success, returns the number of bytes and an optional file descriptor.
	///
	/// # Arguments
	///
	/// * `buf` - A buffer to receive data from the socket.vm
	fn recv_with_fd(&self, buf: &mut [u8]) -> Result<(usize, Option<File>)> {
	let mut fd = [0];
	let (read_count, fd_count) = self.recv_with_fds(buf, &mut fd)?;
	let file = if fd_count == 0 {
	None
	} else {
	// Safe because the first fd from recv_with_fds is owned by us and valid because this
	// branch was taken.
	Some(unsafe { File::from_raw_fd(fd[0]) })
	};
	Ok((read_count, file))
	}

	/// Receives data and file descriptors from the socket.
	///
	/// On success, returns the number of bytes and file descriptors received as a tuple
	/// `(bytes count, files count)`.
	///
	/// # Arguments
	///
	/// * `buf` - A buffer to receive data from the socket.
	/// * `fds` - A slice of `RawFd`s to put the received file descriptors into. On success, the
	/// number of valid file descriptors is indicated by the second element of the
	/// returned tuple. The caller owns these file descriptors, but they will not be
	/// closed on drop like a `File`-like type would be. It is recommended that each valid
	/// file descriptor gets wrapped in a drop type that closes it after this returns.
	fn recv_with_fds(&self, buf: &mut [u8], fds: &mut [RawFd]) -> Result<(usize, usize)> {
	raw_recvmsg(self.socket_fd(), buf, fds)
	}
	}

	impl ScmSocket for UnixDatagram {
	fn socket_fd(&self) -> RawFd {
	self.as_raw_fd()
	}
	}

	impl ScmSocket for UnixStream {
	fn socket_fd(&self) -> RawFd {
	self.as_raw_fd()
	}
	}
	impl ScmSocket for UnixSeqpacket {
	fn socket_fd(&self) -> RawFd {
	self.as_raw_fd()
	}
	}

	/// Trait for types that can be converted into an `iovec` that can be referenced by a syscall for
	/// the lifetime of this object.
	///
	/// This trait is unsafe because interfaces that use this trait depend on the base pointer and size
	/// being accurate.
	pub unsafe trait IntoIovec {
	/// Gets the base pointer of this `iovec`.
	fn as_ptr(&self) -> *const c_void;

	/// Gets the size in bytes of this `iovec`.
	fn size(&self) -> usize;
	}

	// Safe because this slice can not have another mutable reference and it's pointer and size are
	// guaranteed to be valid.
	unsafe impl<'a> IntoIovec for &'a [u8] {
	// Clippy false positive: https://github.com/rust-lang/rust-clippy/issues/3480
	#[allow(clippy::useless_asref)]
	fn as_ptr(&self) -> *const c_void {
	self.as_ref().as_ptr() as *const c_void
	}

	fn size(&self) -> usize {
	self.len()
	}
	}

	// Safe because volatile slices are only ever accessed with other volatile interfaces and the
	// pointer and size are guaranteed to be accurate.
	unsafe impl<'a> IntoIovec for VolatileSlice<'a> {
	fn as_ptr(&self) -> *const c_void {
	self.as_ptr() as *const c_void
	}

	fn size(&self) -> usize {
	self.size() as usize
	}
	}

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

	use std::io::Write;
	use std::mem::size_of;
	use std::os::raw::c_long;
	use std::os::unix::net::UnixDatagram;
	use std::slice::from_raw_parts;

	use libc::cmsghdr;

	use crate::EventFd;

	#[test]
	fn buffer_len() {
	assert_eq!(CMSG_SPACE!(0 * size_of::<RawFd>()), size_of::<cmsghdr>());
	assert_eq!(
	CMSG_SPACE!(1 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>()
	);
	if size_of::<RawFd>() == 4 {
	assert_eq!(
	CMSG_SPACE!(2 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>()
	);
	assert_eq!(
	CMSG_SPACE!(3 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>() * 2
	);
	assert_eq!(
	CMSG_SPACE!(4 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>() * 2
	);
	} else if size_of::<RawFd>() == 8 {
	assert_eq!(
	CMSG_SPACE!(2 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>() * 2
	);
	assert_eq!(
	CMSG_SPACE!(3 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>() * 3
	);
	assert_eq!(
	CMSG_SPACE!(4 * size_of::<RawFd>()),
	size_of::<cmsghdr>() + size_of::<c_long>() * 4
	);
	}
	}

	#[test]
	fn send_recv_no_fd() {
	let (s1, s2) = UnixDatagram::pair().expect("failed to create socket pair");

	let write_count = s1
	.send_with_fds([1u8, 1, 2, 21, 34, 55].as_ref(), &[])
	.expect("failed to send data");

	assert_eq!(write_count, 6);

	let mut buf = [0; 6];
	let mut files = [0; 1];
	let (read_count, file_count) = s2
	.recv_with_fds(&mut buf[..], &mut files)
	.expect("failed to recv data");

	assert_eq!(read_count, 6);
	assert_eq!(file_count, 0);
	assert_eq!(buf, [1, 1, 2, 21, 34, 55]);
	}

	#[test]
	fn send_recv_only_fd() {
	let (s1, s2) = UnixDatagram::pair().expect("failed to create socket pair");

	let evt = EventFd::new().expect("failed to create eventfd");
	let write_count = s1
	.send_with_fd([].as_ref(), evt.as_raw_fd())
	.expect("failed to send fd");

	assert_eq!(write_count, 0);

	let (read_count, file_opt) = s2.recv_with_fd(&mut []).expect("failed to recv fd");

	let mut file = file_opt.unwrap();

	assert_eq!(read_count, 0);
	assert!(file.as_raw_fd() >= 0);
	assert_ne!(file.as_raw_fd(), s1.as_raw_fd());
	assert_ne!(file.as_raw_fd(), s2.as_raw_fd());
	assert_ne!(file.as_raw_fd(), evt.as_raw_fd());

	file.write(unsafe { from_raw_parts(&1203u64 as const u64 as const u8, 8) })
	.expect("failed to write to sent fd");

	assert_eq!(evt.read().expect("failed to read from eventfd"), 1203);
	}

	#[test]
	fn send_recv_with_fd() {
	let (s1, s2) = UnixDatagram::pair().expect("failed to create socket pair");

	let evt = EventFd::new().expect("failed to create eventfd");
	let write_count = s1
	.send_with_fds([237].as_ref(), &[evt.as_raw_fd()])
	.expect("failed to send fd");

	assert_eq!(write_count, 1);

	let mut files = [0; 2];
	let mut buf = [0u8];
	let (read_count, file_count) = s2
	.recv_with_fds(&mut buf, &mut files)
	.expect("failed to recv fd");

	assert_eq!(read_count, 1);
	assert_eq!(buf[0], 237);
	assert_eq!(file_count, 1);
	assert!(files[0] >= 0);
	assert_ne!(files[0], s1.as_raw_fd());
	assert_ne!(files[0], s2.as_raw_fd());
	assert_ne!(files[0], evt.as_raw_fd());

	let mut file = unsafe { File::from_raw_fd(files[0]) };

	file.write(unsafe { from_raw_parts(&1203u64 as const u64 as const u8, 8) })
	.expect("failed to write to sent fd");

	assert_eq!(evt.read().expect("failed to read from eventfd"), 1203);
	}
	}