Workaround XNU bug in getifaddrs netmasks

src/ifaddrs.rs

@@ -4,6 +4,7 @@
 //! of interfaces and their associated addresses.
 
 use cfg_if::cfg_if;
+use std::convert::TryFrom;
 use std::ffi;
 use std::iter::Iterator;
 use std::mem;
@@ -42,11 +43,46 @@ cfg_if! {
     }
 }
 
+/// Workaround a bug in XNU where netmasks will always have the wrong size in
+/// the sa_len field due to the kernel ignoring trailing zeroes in the structure
+/// when setting the field. See https://github.com/nix-rust/nix/issues/1709#issuecomment-1199304470
+///
+/// To fix this, we stack-allocate a new sockaddr_storage, zero it out, and
+/// memcpy sa_len of the netmask to that new storage. Finally, we reset the
+/// ss_len field to sizeof(sockaddr_storage). This is supposedly valid as all
+/// members of the sockaddr_storage are "ok" with being zeroed out (there are
+/// no pointers).
+#[cfg(any(target_os = "ios", target_os = "macos"))]
+unsafe fn workaround_xnu_bug(info: &libc::ifaddrs) -> Option<SockaddrStorage> {
+    let src_sock = info.ifa_netmask;
+    if src_sock.is_null() {
+        return None;
+    }
+
+    let mut dst_sock = mem::MaybeUninit::<libc::sockaddr_storage>::zeroed();
+
+    // memcpy only sa_len bytes, assume the rest is zero
+    std::ptr::copy_nonoverlapping(src_sock as *const u8, dst_sock.as_mut_ptr() as *mut u8, (*src_sock).sa_len.into());
+
+    // Initialize ss_len to sizeof(libc::sockaddr_storage).
+    (*dst_sock.as_mut_ptr()).ss_len = u8::try_from(mem::size_of::<libc::sockaddr_storage>()).unwrap();
+    let dst_sock = dst_sock.assume_init();
+
+    let dst_sock_ptr = &dst_sock as *const libc::sockaddr_storage as *const libc::sockaddr;
+
+    SockaddrStorage::from_raw(dst_sock_ptr, None)
+}
+
 impl InterfaceAddress {
     /// Create an `InterfaceAddress` from the libc struct.
     fn from_libc_ifaddrs(info: &libc::ifaddrs) -> InterfaceAddress {
         let ifname = unsafe { ffi::CStr::from_ptr(info.ifa_name) };
         let address = unsafe { SockaddrStorage::from_raw(info.ifa_addr, None) };
+        #[cfg(any(target_os = "ios", target_os = "macos"))]
+        let netmask = unsafe {
+            workaround_xnu_bug(info)
+        };
+        #[cfg(not(any(target_os = "ios", target_os = "macos")))]
         let netmask = unsafe { SockaddrStorage::from_raw(info.ifa_netmask, None) };
         let mut addr = InterfaceAddress {
             interface_name: ifname.to_string_lossy().to_string(),
@@ -144,4 +180,26 @@ mod tests {
     fn test_getifaddrs() {
         let _ = getifaddrs();
     }
+
+    // Ensures getting the netmask works, and in particular that
+    // `workaround_xnu_bug` works properly.
+    #[test]
+    fn test_getifaddrs_netmask_correct() {
+        let addrs = getifaddrs().unwrap();
+        for iface in addrs {
+            let sock = if let Some(sock) = iface.netmask {
+                sock
+            } else {
+                continue
+            };
+            if sock.family() == Some(crate::sys::socket::AddressFamily::Inet) {
+                let _ = sock.as_sockaddr_in().unwrap();
+                return;
+            } else if sock.family() == Some(crate::sys::socket::AddressFamily::Inet6) {
+                let _ = sock.as_sockaddr_in6().unwrap();
+                return;
+            }
+        }
+        panic!("No address?");
+    }
 }