initialize msg_name with null pointer when msg_name is empty (nix-rus…

…t#2530)

* Use as_mut_ptr() to initialize msg_name in pack_mhdr_to_receive

The msg_name field points to a caller-allocated buffer that is used to
return the source address if the socket is unconnected. The caller
should set msg_namelen to the size of this buffer before this call; upon
return from a successful call, msg_namelen will contain the length of
the returned address. If the application does not need to know the
source address, msg_name can be specified as NULL.

In case we use () msgname_len gets initialized with 0, but a dangling
pointer to the array with msg_name. This works for the first iteration
somehow, but after that kernel sets msgname_len to a non-zero and second
invocation with the same MultiHeader fails

Fixes nix-rust#2506

* CI doesn't check for rustfmt but I'm tired of picking stuff

src/sys/socket/mod.rs

@@ -39,10 +39,20 @@ pub use self::addr::{SockaddrLike, SockaddrStorage};
 pub use self::addr::{AddressFamily, UnixAddr};
 #[cfg(not(solarish))]
 pub use self::addr::{AddressFamily, UnixAddr};
-#[cfg(not(any(solarish, target_os = "haiku", target_os = "hurd", target_os = "redox")))]
+#[cfg(not(any(
+    solarish,
+    target_os = "haiku",
+    target_os = "hurd",
+    target_os = "redox"
+)))]
 #[cfg(feature = "net")]
 pub use self::addr::{LinkAddr, SockaddrIn, SockaddrIn6};
-#[cfg(any(solarish, target_os = "haiku", target_os = "hurd", target_os = "redox"))]
+#[cfg(any(
+    solarish,
+    target_os = "haiku",
+    target_os = "hurd",
+    target_os = "redox"
+))]
 #[cfg(feature = "net")]
 pub use self::addr::{SockaddrIn, SockaddrIn6};
 
@@ -851,17 +861,17 @@ pub enum ControlMessageOwned {
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
     Ipv6HopLimit(i32),
 
-    /// Retrieve the DSCP (ToS) header field of the incoming IPv4 packet. 
+    /// Retrieve the DSCP (ToS) header field of the incoming IPv4 packet.
     #[cfg(any(linux_android, target_os = "freebsd"))]
     #[cfg(feature = "net")]
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
     Ipv4Tos(u8),
 
-    /// Retrieve the DSCP (Traffic Class) header field of the incoming IPv6 packet. 
+    /// Retrieve the DSCP (Traffic Class) header field of the incoming IPv6 packet.
     #[cfg(any(linux_android, target_os = "freebsd"))]
     #[cfg(feature = "net")]
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
-    Ipv6TClass(i32), 
+    Ipv6TClass(i32),
 
     /// UDP Generic Receive Offload (GRO) allows receiving multiple UDP
     /// packets from a single sender.
@@ -1119,7 +1129,7 @@ impl ControlMessageOwned {
             (_, _) => {
                 let sl = unsafe { std::slice::from_raw_parts(p, len) };
                 let ucmsg = UnknownCmsg {
-                    cmsg_header: *header, 
+                    cmsg_header: *header,
                     data_bytes: Vec::<u8>::from(sl),
                 };
                 ControlMessageOwned::Unknown(ucmsg)
@@ -2059,7 +2069,10 @@ unsafe fn pack_mhdr_to_receive<S>(
     let mut mhdr = mem::MaybeUninit::<msghdr>::zeroed();
     let p = mhdr.as_mut_ptr();
     unsafe {
-        (*p).msg_name = address as *mut c_void;
+        // it is important to use as_mut_ptr() here since S can be
+        // a zero sized type representing by a dangling pointer.
+        // as_mut_ptr() handles this case and uses a null pointer instead
+        (*p).msg_name = (*address).as_mut_ptr() as *mut c_void;
         (*p).msg_namelen = S::size();
         (*p).msg_iov = iov_buffer as *mut iovec;
         (*p).msg_iovlen = iov_buffer_len as _;
@@ -2517,4 +2530,3 @@ pub fn shutdown(df: RawFd, how: Shutdown) -> Result<()> {
         Errno::result(shutdown(df, how)).map(drop)
     }
 }
-