Error in decoding length of AncillaryData (forget to remove the header offset to the data length)

[ETKNeil]

If you look at your encoding you will see that you encode the length of the payload as header + data length https://github.com/kotauskas/interprocess/blob/master/src/os/unix/udsocket/ancillary.rs#L94

            let mut cmsg_len = size_of::<cmsghdr>();  // here 16 or 12
            let cmsg_level_bytes = SOL_SOCKET.to_ne_bytes();
            let cmsg_type_bytes;

            match i {
                AncillaryData::FileDescriptors(fds) => {
                    cmsg_type_bytes = SCM_RIGHTS.to_ne_bytes();
                    cmsg_len += fds.len() * 4; // plus data size
                    // #[cfg(target_pointer_width = "64")]
                    // this was here, I don't even remember why, but that
                    // wouldn't compile on a 32-bit machine
                    let cmsg_len_bytes = cmsg_len.to_ne_bytes(); 
                    buffer.extend_from_slice(&cmsg_len_bytes); // header size + data size

But when you actually decode it you are forgetting that part https://github.com/kotauskas/interprocess/blob/master/src/os/unix/udsocket/ancillary.rs#L303

        // The first field is the length, which is a size_t -> full size as header size + data size
        #[cfg(target_pointer_width = "64")]
        let element_size = {
            if bytes.len() - self.i < 8 {
                self.i = end;
                return None;
            }
            u64_from_slice(&bytes[self.i..self.i + 8]) as usize
        };
        #[cfg(target_pointer_width = "32")]
        let element_size = {
            if bytes.len() - self.i < 4 {
                self.i = end;
                return None;
            }
            u32_from_slice(&bytes[self.i..self.i + 4]) as usize
        };
     // SNIP
        // Update the counter before returning. -> you are adding header size + full size= header_size + data size so its already off by 16 or 12 here
        self.i += element_offset // cmsg_size, cmsg_level and cmsg_type
                + element_size; // data size
      // SNIP
                // We're reading one or multiple descriptors from the ancillary data payload.
                // All descriptors are 4 bytes in size — leftover bytes are discarded thanks
                // to integer division rules
               // here you are dividing header size + data size by 4 which means that in best case you add 3 more descriptors in worse you add 4 more (since 64 bits is 16bits for header size)
                let amount_of_descriptors = element_size / 4;

The fix is relatively simple just add this here https://github.com/kotauskas/interprocess/blob/master/src/os/unix/udsocket/ancillary.rs#L319

        };
        
        // You should never have an empty payload, if it is then return
        let element_size=match element_size.checked_sub(std::mem::size_of::<cmsghdr>()){
            None => {
                self.i = end;
                return None;
            }
            Some(i) if i==0 => {
                self.i = end;
                return None;
            }
            Some(i) => {
                i
            }
        };
        
        // The cmsg_level field is always SOL_SOCKET — we don't need it, let's get the

this remove the header size offset of the element_size (which I would suggest to rename to elements_size) thus getting only the data size here. Commit for reference: https://github.com/ETKNeil/interprocess/commit/b1de6339d6ecdfc6fb0122a7be2d973898739039