Fix passing multiple file descriptors / control messages via sendmsg

CHANGELOG.md

@@ -12,6 +12,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Made `preadv` take immutable slice of IoVec.
   ([#914](https://github.com/nix-rust/nix/pull/914))
+- Fixed passing multiple file descriptors over Unix Sockets.
+  ([#918](https://github.com/nix-rust/nix/pull/918))
 
 ### Removed
 

src/sys/socket/mod.rs

@@ -287,18 +287,37 @@ impl fmt::Debug for Ipv6MembershipRequest {
     }
 }
 
-/// Copy the in-memory representation of src into the byte slice dst,
-/// updating the slice to point to the remainder of dst only. Unsafe
-/// because it exposes all bytes in src, which may be UB if some of them
-/// are uninitialized (including padding).
-unsafe fn copy_bytes<'a, 'b, T: ?Sized>(src: &T, dst: &'a mut &'b mut [u8]) {
+/// Copy the in-memory representation of `src` into the byte slice `dst`.
+///
+/// Returns the remainder of `dst`.
+///
+/// Panics when `dst` is too small for `src` (more precisely, panics if
+/// `mem::size_of_val(src) >= dst.len()`).
+///
+/// Unsafe because it transmutes `src` to raw bytes, which is only safe for some
+/// types `T`. Refer to the [Rustonomicon] for details.
+///
+/// [Rustonomicon]: https://doc.rust-lang.org/nomicon/transmutes.html
+unsafe fn copy_bytes<'a, T: ?Sized>(src: &T, dst: &'a mut [u8]) -> &'a mut [u8] {
     let srclen = mem::size_of_val(src);
-    let mut tmpdst = &mut [][..];
-    mem::swap(&mut tmpdst, dst);
-    let (target, mut remainder) = tmpdst.split_at_mut(srclen);
-    // Safe because the mutable borrow of dst guarantees that src does not alias it.
-    ptr::copy_nonoverlapping(src as *const T as *const u8, target.as_mut_ptr(), srclen);
-    mem::swap(dst, &mut remainder);
+    ptr::copy_nonoverlapping(
+        src as *const T as *const u8,
+        dst[..srclen].as_mut_ptr(),
+        srclen
+    );
+
+    &mut dst[srclen..]
+}
+
+/// Fills `dst` with `len` zero bytes and returns the remainder of the slice.
+///
+/// Panics when `len >= dst.len()`.
+fn pad_bytes(len: usize, dst: &mut [u8]) -> &mut [u8] {
+    for pad in &mut dst[..len] {
+        *pad = 0;
+    }
+
+    &mut dst[len..]
 }
 
 cfg_if! {
@@ -434,6 +453,11 @@ pub enum ControlMessage<'a> {
     ///
     /// See the description in the "Ancillary messages" section of the
     /// [unix(7) man page](http://man7.org/linux/man-pages/man7/unix.7.html).
+    ///
+    /// Using multiple `ScmRights` messages for a single `sendmsg` call isn't recommended since it
+    /// causes platform-dependent behaviour: It might swallow all but the first `ScmRights` message
+    /// or fail with `EINVAL`. Instead, you can put all fds to be passed into a single `ScmRights`
+    /// message.
     ScmRights(&'a [RawFd]),
     /// A message of type `SCM_TIMESTAMP`, containing the time the
     /// packet was received by the kernel.
@@ -545,7 +569,7 @@ impl<'a> ControlMessage<'a> {
 
     // Unsafe: start and end of buffer must be cmsg_align'd. Updates
     // the provided slice; panics if the buffer is too small.
-    unsafe fn encode_into<'b>(&self, buf: &mut &'b mut [u8]) {
+    unsafe fn encode_into(&self, buf: &mut [u8]) {
         match *self {
             ControlMessage::ScmRights(fds) => {
                 let cmsg = cmsghdr {
@@ -554,17 +578,16 @@ impl<'a> ControlMessage<'a> {
                     cmsg_type: libc::SCM_RIGHTS,
                     ..mem::uninitialized()
                 };
-                copy_bytes(&cmsg, buf);
+                let buf = copy_bytes(&cmsg, buf);
 
                 let padlen = cmsg_align(mem::size_of_val(&cmsg)) -
                     mem::size_of_val(&cmsg);
+                let buf = pad_bytes(padlen, buf);
 
-                let mut tmpbuf = &mut [][..];
-                mem::swap(&mut tmpbuf, buf);
-                let (_padding, mut remainder) = tmpbuf.split_at_mut(padlen);
-                mem::swap(buf, &mut remainder);
+                let buf = copy_bytes(fds, buf);
 
-                copy_bytes(fds, buf);
+                let padlen = self.space() - self.len();
+                pad_bytes(padlen, buf);
             },
             ControlMessage::ScmTimestamp(t) => {
                 let cmsg = cmsghdr {
@@ -573,21 +596,28 @@ impl<'a> ControlMessage<'a> {
                     cmsg_type: libc::SCM_TIMESTAMP,
                     ..mem::uninitialized()
                 };
-                copy_bytes(&cmsg, buf);
+                let buf = copy_bytes(&cmsg, buf);
 
                 let padlen = cmsg_align(mem::size_of_val(&cmsg)) -
                     mem::size_of_val(&cmsg);
+                let buf = pad_bytes(padlen, buf);
 
-                let mut tmpbuf = &mut [][..];
-                mem::swap(&mut tmpbuf, buf);
-                let (_padding, mut remainder) = tmpbuf.split_at_mut(padlen);
-                mem::swap(buf, &mut remainder);
+                let buf = copy_bytes(t, buf);
 
-                copy_bytes(t, buf);
+                let padlen = self.space() - self.len();
+                pad_bytes(padlen, buf);
             },
             ControlMessage::Unknown(UnknownCmsg(orig_cmsg, bytes)) => {
-                copy_bytes(orig_cmsg, buf);
-                copy_bytes(bytes, buf);
+                let buf = copy_bytes(orig_cmsg, buf);
+
+                let padlen = cmsg_align(mem::size_of_val(&orig_cmsg)) -
+                    mem::size_of_val(&orig_cmsg);
+                let buf = pad_bytes(padlen, buf);
+
+                let buf = copy_bytes(bytes, buf);
+
+                let padlen = self.space() - self.len();
+                pad_bytes(padlen, buf);
             }
         }
     }
@@ -600,23 +630,25 @@ impl<'a> ControlMessage<'a> {
 ///
 /// Allocates if cmsgs is nonempty.
 pub fn sendmsg<'a>(fd: RawFd, iov: &[IoVec<&'a [u8]>], cmsgs: &[ControlMessage<'a>], flags: MsgFlags, addr: Option<&'a SockAddr>) -> Result<usize> {
-    let mut len = 0;
     let mut capacity = 0;
     for cmsg in cmsgs {
-        len += cmsg.len();
         capacity += cmsg.space();
     }
     // Note that the resulting vector claims to have length == capacity,
     // so it's presently uninitialized.
     let mut cmsg_buffer = unsafe {
-        let mut vec = Vec::<u8>::with_capacity(len);
-        vec.set_len(len);
+        let mut vec = Vec::<u8>::with_capacity(capacity);
+        vec.set_len(capacity);
         vec
     };
     {
-        let mut ptr = &mut cmsg_buffer[..];
+        let mut ofs = 0;
         for cmsg in cmsgs {
-            unsafe { cmsg.encode_into(&mut ptr) };
+            let mut ptr = &mut cmsg_buffer[ofs..];
+            unsafe {
+                cmsg.encode_into(ptr);
+            }
+            ofs += cmsg.space();
         }
     }
 
@@ -669,10 +701,23 @@ pub fn recvmsg<'a, T>(fd: RawFd, iov: &[IoVec<&mut [u8]>], cmsg_buffer: Option<&
     };
     let ret = unsafe { libc::recvmsg(fd, &mut mhdr, flags.bits()) };
 
+    let cmsg_buffer = if msg_controllen > 0 {
+        // got control message(s)
+        debug_assert!(!mhdr.msg_control.is_null());
+        unsafe {
+            // Safe: The pointer is not null and the length is correct as part of `recvmsg`s
+            // contract.
+            slice::from_raw_parts(mhdr.msg_control as *const u8,
+                                  mhdr.msg_controllen as usize)
+        }
+    } else {
+        // No control message, create an empty buffer to avoid creating a slice from a null pointer
+        &[]
+    };
+
     Ok(unsafe { RecvMsg {
         bytes: try!(Errno::result(ret)) as usize,
-        cmsg_buffer: slice::from_raw_parts(mhdr.msg_control as *const u8,
-                                           mhdr.msg_controllen as usize),
+        cmsg_buffer,
         address: sockaddr_storage_to_addr(&address,
                                           mhdr.msg_namelen as usize).ok(),
         flags: MsgFlags::from_bits_truncate(mhdr.msg_flags),

test/sys/test_socket.rs

@@ -167,6 +167,51 @@ pub fn test_scm_rights() {
     close(w).unwrap();
 }
 
+/// Tests that passing multiple fds using a single `ControlMessage` works.
+#[test]
+fn test_scm_rights_single_cmsg_multiple_fds() {
+    use std::os::unix::net::UnixDatagram;
+    use std::os::unix::io::{RawFd, AsRawFd};
+    use std::thread;
+    use nix::sys::socket::{CmsgSpace, ControlMessage, MsgFlags, sendmsg, recvmsg};
+    use nix::sys::uio::IoVec;
+    use libc;
+
+    let (send, receive) = UnixDatagram::pair().unwrap();
+    let thread = thread::spawn(move || {
+        let mut buf = [0u8; 8];
+        let iovec = [IoVec::from_mut_slice(&mut buf)];
+        let mut space = CmsgSpace::<[RawFd; 2]>::new();
+        let msg = recvmsg(
+            receive.as_raw_fd(),
+            &iovec,
+            Some(&mut space),
+            MsgFlags::empty()
+        ).unwrap();
+        assert!(!msg.flags.intersects(MsgFlags::MSG_TRUNC | MsgFlags::MSG_CTRUNC));
+
+        let mut cmsgs = msg.cmsgs();
+        match cmsgs.next() {
+            Some(ControlMessage::ScmRights(fds)) => {
+                assert_eq!(fds.len(), 2,
+                           "unexpected fd count (expected 2 fds, got {})",
+                           fds.len());
+            },
+            _ => panic!(),
+        }
+        assert!(cmsgs.next().is_none(), "unexpected control msg");
+
+        assert_eq!(iovec[0].as_slice(), [1u8, 2u8, 3u8, 4u8, 5u8, 6u8, 7u8, 8u8]);
+    });
+
+    let slice = [1u8, 2u8, 3u8, 4u8, 5u8, 6u8, 7u8, 8u8];
+    let iov = [IoVec::from_slice(&slice)];
+    let fds = [libc::STDIN_FILENO, libc::STDOUT_FILENO];    // pass stdin and stdout
+    let cmsg = [ControlMessage::ScmRights(&fds)];
+    sendmsg(send.as_raw_fd(), &iov, &cmsg, MsgFlags::empty(), None).unwrap();
+    thread.join().unwrap();
+}
+
 // Verify `sendmsg` builds a valid `msghdr` when passing an empty
 // `cmsgs` argument.  This should result in a msghdr with a nullptr
 // msg_control field and a msg_controllen of 0 when calling into the