add testing for overflow in read_segment_table

.github/workflows/ci.yml

@@ -86,6 +86,12 @@ jobs:
       - name: Test no default features
         run: cargo miri test --package capnp --package capnpc-test --no-default-features
 
+      - name: Test default features 32-bit
+        run: cargo miri test --package capnp --package capnpc-test --target i686-unknown-linux-gnu
+
+      - name: Test no default features 32-bit
+        run: cargo miri test --package capnp --package capnpc-test --no-default-features --target i686-unknown-linux-gnu
+
       - name: Test sync_reader
         run: cargo miri test --package capnp --package capnpc-test --features sync_reader
 

capnp/src/serialize.rs

@@ -233,12 +233,13 @@ impl SegmentLengthsBuilder {
     /// the segment with ID `n`. If the segment overflows the total word count, then this returns
     /// a MessageSizeOverflow error.
     pub fn try_push_segment(&mut self, length_in_words: usize) -> Result<()> {
-        self.segment_indices
-            .push((self.total_words, self.total_words + length_in_words));
-        self.total_words = self
+        let new_total_words = self
             .total_words
             .checked_add(length_in_words)
             .ok_or_else(|| Error::from_kind(ErrorKind::MessageSizeOverflow))?;
+        self.segment_indices
+            .push((self.total_words, new_total_words));
+        self.total_words = new_total_words;
         Ok(())
     }
 
@@ -885,6 +886,17 @@ pub mod test {
         buf.clear();
     }
 
+    #[test]
+    fn test_read_segment_table_overflow() {
+        let mut buf = vec![];
+
+        buf.extend([1, 0, 0, 0]); // 2 segments
+        buf.extend([0xff, 0xff, 0xff, 0xff]); // 2^32 - 1 words
+        buf.extend([2, 0, 0, 0]); // 2 words
+        buf.extend([0, 0, 0, 0]); // padding
+        assert!(read_segment_table(&mut &buf[..], message::ReaderOptions::new()).is_err());
+    }
+
     #[test]
     fn test_write_segment_table() {
         let mut buf = vec![];