Fix cbor cid serde

dag-cbor/Cargo.toml

@@ -15,3 +15,4 @@ thiserror = "1.0.14"
 [dev-dependencies]
 libipld-macro = { path = "../macro" }
 multihash = "0.10.1"
+hex = "0.4.2"

dag-cbor/src/decode.rs

@@ -105,8 +105,17 @@ pub fn read_link<R: Read>(r: &mut R) -> Result<Cid> {
         return Err(CborError::UnknownTag);
     }
     let len = read_u8(r)?;
+    if len == 0 {
+        return Err(CborError::LengthOutOfRange);
+    }
     let bytes = read_bytes(r, len as usize)?;
-    Ok(Cid::try_from(bytes)?)
+    if bytes[0] != 0 {
+        return Err(CborError::InvalidCidPrefix(bytes[0]));
+    }
+
+    // skip the first byte per
+    // https://github.com/ipld/specs/blob/master/block-layer/codecs/dag-cbor.md#links
+    Ok(Cid::try_from(&bytes[1..])?)
 }
 
 pub trait ReadCbor: Sized {

dag-cbor/src/encode.rs

@@ -218,8 +218,11 @@ impl WriteCbor for Cid {
     #[inline]
     fn write_cbor<W: Write>(&self, w: &mut W) -> Result<()> {
         write_tag(w, 42)?;
+        // insert zero byte per https://github.com/ipld/specs/blob/master/block-layer/codecs/dag-cbor.md#links
         let bytes = self.to_bytes();
-        bytes.as_slice().write_cbor(w)?;
+        write_u64(w, 2, (bytes.len() + 1) as u64)?;
+        w.write_all(&[0])?;
+        w.write_all(&bytes)?;
         Ok(())
     }
 }

dag-cbor/src/lib.rs

@@ -39,7 +39,7 @@ pub enum CborError {
     /// Number larger than u64.
     #[error("Number larger than u64.")]
     NumberOutOfRange,
-    /// Length larger than usize.
+    /// Length larger than usize or too small, for example zero length cid field.
     #[error("Length out of range.")]
     LengthOutOfRange,
     /// Unexpected cbor code.
@@ -60,6 +60,9 @@ pub enum CborError {
     /// Utf8 error.
     #[error("{0}")]
     Utf8(#[from] std::str::Utf8Error),
+    /// The byte before Cid was not multibase identity prefix.
+    #[error("Invalid Cid prefix: {0}")]
+    InvalidCidPrefix(u8),
     /// Cid error.
     #[error("{0}")]
     Cid(#[from] cid::Error),

dag-cbor/tests/roundtrip.rs

@@ -0,0 +1,47 @@
+use dag_cbor::{CborError, DagCborCodec};
+use libipld_base::codec::Codec;
+
+#[test]
+fn roundtrip_with_cid() {
+    // generated with go-ipfs
+    // $ echo foobar > file1
+    // $ ipfs add foobar
+    // QmRgutAxd8t7oGkSm4wmeuByG6M51wcTso6cubDdQtuEfL
+    // $ echo -n '{ "foo": { "/": "QmRgutAxd8t7oGkSm4wmeuByG6M51wcTso6cubDdQtuEfL" } }' \
+    //   | ipfs dag put
+    // bafyreibvjvcv745gig4mvqs4hctx4zfkono4rjejm2ta6gtyzkqxfjeily
+    // $ ipfs block get bafyreibvjvcv745gig4mvqs4hctx4zfkono4rjejm2ta6gtyzkqxfjeily \
+    //   | xxd -ps | paste -s --delimiters=
+
+    let input =
+        "a163666f6fd82a582300122031c3d57080d8463a3c63b2923df5a1d40ad7a73eae5a14af584213e5f504ac33";
+    let input = hex::decode(input).unwrap();
+
+    let ipld = DagCborCodec::decode(&input).unwrap();
+    let bytes = DagCborCodec::encode(&ipld).unwrap().to_vec();
+
+    assert_eq!(input, bytes);
+}
+
+#[test]
+fn invalid_cid_prefix() {
+    let input =
+        "a163666f6fd82a582301122031c3d57080d8463a3c63b2923df5a1d40ad7a73eae5a14af584213e5f504ac33";
+    let input = hex::decode(input).unwrap();
+
+    match DagCborCodec::decode(&input).unwrap_err() {
+        CborError::InvalidCidPrefix(1) => {}
+        x => panic!("unexpected error: {:?}", x),
+    }
+}
+
+#[test]
+fn zero_length_cid() {
+    let input = "a163666f6fd82a5800";
+    let input = hex::decode(input).unwrap();
+
+    match DagCborCodec::decode(&input).unwrap_err() {
+        CborError::LengthOutOfRange => {}
+        x => panic!("unexpected error: {:?}", x),
+    }
+}