Linux: Cleanup: Trim unsafe blocks in recv()

This requires some shuffling around of declarations, to faciliate
untangling the actually unsafe operations from those not affecting
safety. (As far as reasonably possible...)

Also added a new assertion to make sure that the trimmed `unsafe` blocks
really do not rely on any conditions being upheld outside.

Author: antrik

platform/linux/mod.rs

@@ -733,14 +733,18 @@ enum BlockingMode {
 
 fn recv(fd: c_int, blocking_mode: BlockingMode)
         -> Result<(Vec<u8>, Vec<OpaqueUnixChannel>, Vec<UnixSharedMemory>),UnixError> {
+
+    let (mut channels, mut shared_memory_regions) = (Vec::new(), Vec::new());
+
+    // First fragments begins with a header recording the total data length.
+    //
+    // We use this to determine whether we already got the entire message,
+    // or need to receive additional fragments -- and if so, how much.
+    let mut total_size = 0usize;
+    let mut main_data_buffer;
     unsafe {
-        // First fragment begins with a header recording the total data length.
-        //
-        // We use this to determine whether we already got the entire message,
-        // or need to receive additional fragments -- and if so, how much.
-        let mut total_size = 0usize;
         // Allocate a buffer without initialising the memory.
-        let mut main_data_buffer = Vec::with_capacity(UnixSender::get_max_fragment_size());
+        main_data_buffer = Vec::with_capacity(UnixSender::get_max_fragment_size());
         main_data_buffer.set_len(UnixSender::get_max_fragment_size());
 
         let iovec = &mut [
@@ -765,7 +769,6 @@ fn recv(fd: c_int, blocking_mode: BlockingMode)
         } else {
             (cmsg.cmsg_len() - mem::size_of::<cmsghdr>()) / mem::size_of::<c_int>()
         };
-        let (mut channels, mut shared_memory_regions) = (Vec::new(), Vec::new());
         for index in 0..channel_length {
             let fd = *cmsg_fds.offset(index as isize);
             if is_socket(fd) {
@@ -774,30 +777,35 @@ fn recv(fd: c_int, blocking_mode: BlockingMode)
             }
             shared_memory_regions.push(UnixSharedMemory::from_fd(fd));
         }
+    }
 
-        if total_size == main_data_buffer.len() {
-            // Fast path: no fragments.
-            return Ok((main_data_buffer, channels, shared_memory_regions))
-        }
+    if total_size == main_data_buffer.len() {
+        // Fast path: no fragments.
+        return Ok((main_data_buffer, channels, shared_memory_regions))
+    }
 
-        // Reassemble fragments.
-        //
-        // The initial fragment carries the receive end of a dedicated channel
-        // through which all the remaining fragments will be coming in.
-        let dedicated_rx = channels.pop().unwrap().to_receiver();
-
-        // Extend the buffer to hold the entire message, without initialising the memory.
-        let len = main_data_buffer.len();
-        main_data_buffer.reserve(total_size - len);
-
-        // Receive followup fragments directly into the main buffer.
-        while main_data_buffer.len() < total_size {
-            let write_pos = main_data_buffer.len();
-            let end_pos = cmp::min(write_pos + UnixSender::fragment_size(*SYSTEM_SENDBUF_SIZE),
-                                   total_size);
+    // Reassemble fragments.
+    //
+    // The initial fragment carries the receive end of a dedicated channel
+    // through which all the remaining fragments will be coming in.
+    let dedicated_rx = channels.pop().unwrap().to_receiver();
+
+    // Extend the buffer to hold the entire message, without initialising the memory.
+    let len = main_data_buffer.len();
+    main_data_buffer.reserve(total_size - len);
+
+    // Receive followup fragments directly into the main buffer.
+    while main_data_buffer.len() < total_size {
+        let write_pos = main_data_buffer.len();
+        let end_pos = cmp::min(write_pos + UnixSender::fragment_size(*SYSTEM_SENDBUF_SIZE),
+                               total_size);
+        let result = unsafe {
             assert!(end_pos <= main_data_buffer.capacity());
             main_data_buffer.set_len(end_pos);
 
+            // Integer underflow could make the following code unsound...
+            assert!(end_pos >= write_pos);
+
             // Note: we always use blocking mode for followup fragments,
             // to make sure that once we start receiving a multi-fragment message,
             // we don't abort in the middle of it...
@@ -806,16 +814,17 @@ fn recv(fd: c_int, blocking_mode: BlockingMode)
                                     end_pos - write_pos,
                                     0);
             main_data_buffer.set_len(write_pos + cmp::max(result, 0) as usize);
+            result
+        };
 
-            if result == 0 {
-                return Err(UnixError(libc::ECONNRESET))
-            } else if result < 0 {
-                return Err(UnixError::last())
-            };
-        }
-
-        Ok((main_data_buffer, channels, shared_memory_regions))
+        if result == 0 {
+            return Err(UnixError(libc::ECONNRESET))
+        } else if result < 0 {
+            return Err(UnixError::last())
+        };
     }
+
+    Ok((main_data_buffer, channels, shared_memory_regions))
 }
 
 #[cfg(target_os="android")]