Optimise SSZ encoding and decoding

src/fixed_vector.rs

@@ -2,7 +2,9 @@ use crate::tree_hash::vec_tree_hash_root;
 use crate::Error;
 use serde::Deserialize;
 use serde_derive::Serialize;
+use std::any::TypeId;
 use std::marker::PhantomData;
+use std::mem;
 use std::ops::{Deref, DerefMut, Index, IndexMut};
 use std::slice::SliceIndex;
 use tree_hash::Hash256;
@@ -283,7 +285,7 @@ impl<T, N: Unsigned> ssz::TryFromIter<T> for FixedVector<T, N> {
 
 impl<T, N: Unsigned> ssz::Decode for FixedVector<T, N>
 where
-    T: ssz::Decode,
+    T: ssz::Decode + 'static,
 {
     fn is_ssz_fixed_len() -> bool {
         T::is_ssz_fixed_len()
@@ -305,6 +307,24 @@ where
                 len: 0,
                 expected: 1,
             })
+        } else if TypeId::of::<T>() == TypeId::of::<u8>() {
+            if bytes.len() != fixed_len {
+                return Err(ssz::DecodeError::BytesInvalid(format!(
+                    "FixedVector of {} items has {} items",
+                    fixed_len,
+                    bytes.len(),
+                )));
+            }
+
+            // Safety: We've verified T is u8, so Vec<T> *is* Vec<u8>.
+            let vec_u8 = bytes.to_vec();
+            let vec_t = unsafe { mem::transmute::<Vec<u8>, Vec<T>>(vec_u8) };
+            Self::new(vec_t).map_err(|e| {
+                ssz::DecodeError::BytesInvalid(format!(
+                    "Wrong number of FixedVector elements: {:?}",
+                    e
+                ))
+            })
         } else if T::is_ssz_fixed_len() {
             let num_items = bytes
                 .len()
@@ -314,17 +334,24 @@ where
             if num_items != fixed_len {
                 return Err(ssz::DecodeError::BytesInvalid(format!(
                     "FixedVector of {} items has {} items",
-                    num_items, fixed_len
+                    fixed_len, num_items
                 )));
             }
 
-            let vec = bytes.chunks(T::ssz_fixed_len()).try_fold(
-                Vec::with_capacity(num_items),
-                |mut vec, chunk| {
-                    vec.push(T::from_ssz_bytes(chunk)?);
-                    Ok(vec)
-                },
-            )?;
+            // Check that we have a whole number of items and that it is safe to use chunks_exact
+            if !bytes.len().is_multiple_of(T::ssz_fixed_len()) {
+                return Err(ssz::DecodeError::BytesInvalid(format!(
+                    "FixedVector of {} items has {} bytes",
+                    num_items,
+                    bytes.len()
+                )));
+            }
+
+            let mut vec = Vec::with_capacity(num_items);
+            for chunk in bytes.chunks_exact(T::ssz_fixed_len()) {
+                vec.push(T::from_ssz_bytes(chunk)?);
+            }
+
             Self::new(vec).map_err(|e| {
                 ssz::DecodeError::BytesInvalid(format!(
                     "Wrong number of FixedVector elements: {:?}",
@@ -479,6 +506,56 @@ mod test {
         ssz_round_trip::<FixedVector<u16, U8>>(vec![0; 8].try_into().unwrap());
     }
 
+    // Test byte decoding (we have a specialised code path with unsafe code that NEEDS coverage).
+    #[test]
+    fn ssz_round_trip_u8_len_1024() {
+        ssz_round_trip::<FixedVector<u8, U1024>>(vec![42; 1024].try_into().unwrap());
+        ssz_round_trip::<FixedVector<u8, U1024>>(vec![0; 1024].try_into().unwrap());
+    }
+
+    // bool is layout equivalent to u8 but must not use the same unsafe codepath because not all u8
+    // values are valid bools.
+    #[test]
+    fn ssz_round_trip_bool_len_1024() {
+        assert_eq!(mem::size_of::<bool>(), 1);
+        assert_eq!(mem::align_of::<bool>(), 1);
+        ssz_round_trip::<FixedVector<bool, U1024>>(vec![true; 1024].try_into().unwrap());
+        ssz_round_trip::<FixedVector<bool, U1024>>(vec![false; 1024].try_into().unwrap());
+    }
+
+    // Decoding a u8 vector as a vector of bools must fail, if we aren't careful we could trigger UB.
+    #[test]
+    fn ssz_u8_to_bool_len_1024() {
+        let list_u8 = FixedVector::<u8, U8>::new(vec![0, 1, 2, 3, 4, 5, 6, 7]).unwrap();
+        FixedVector::<bool, U8>::from_ssz_bytes(&list_u8.as_ssz_bytes()).unwrap_err();
+    }
+
+    #[test]
+    fn ssz_u8_len_1024_too_long() {
+        assert_eq!(
+            FixedVector::<u8, U1024>::from_ssz_bytes(&vec![42; 1025]).unwrap_err(),
+            ssz::DecodeError::BytesInvalid("FixedVector of 1024 items has 1025 items".into())
+        );
+    }
+
+    #[test]
+    fn ssz_u64_len_1024_too_long() {
+        assert_eq!(
+            FixedVector::<u64, U1024>::from_ssz_bytes(&vec![42; 8 * 1025]).unwrap_err(),
+            ssz::DecodeError::BytesInvalid("FixedVector of 1024 items has 1025 items".into())
+        );
+    }
+
+    // Decoding an input with invalid trailing bytes MUST fail.
+    #[test]
+    fn ssz_bytes_u32_trailing() {
+        let bytes = [1, 0, 0, 0, 2, 0, 0, 0, 1];
+        assert_eq!(
+            FixedVector::<u32, U2>::from_ssz_bytes(&bytes).unwrap_err(),
+            ssz::DecodeError::BytesInvalid("FixedVector of 2 items has 9 bytes".into())
+        );
+    }
+
     #[test]
     fn tree_hash_u8() {
         let fixed: FixedVector<u8, U0> = FixedVector::try_from(vec![]).unwrap();

src/variable_list.rs

@@ -2,7 +2,9 @@ use crate::tree_hash::vec_tree_hash_root;
 use crate::Error;
 use serde::Deserialize;
 use serde_derive::Serialize;
+use std::any::TypeId;
 use std::marker::PhantomData;
+use std::mem;
 use std::ops::{Deref, DerefMut, Index, IndexMut};
 use std::slice::SliceIndex;
 use tree_hash::Hash256;
@@ -288,7 +290,7 @@ impl<T, N: Unsigned> ssz::TryFromIter<T> for VariableList<T, N> {
 
 impl<T, N> ssz::Decode for VariableList<T, N>
 where
-    T: ssz::Decode,
+    T: ssz::Decode + 'static,
     N: Unsigned,
 {
     fn is_ssz_fixed_len() -> bool {
@@ -302,6 +304,26 @@ where
             return Ok(Self::default());
         }
 
+        if TypeId::of::<T>() == TypeId::of::<u8>() {
+            if bytes.len() > max_len {
+                return Err(ssz::DecodeError::BytesInvalid(format!(
+                    "VariableList of {} items exceeds maximum of {}",
+                    bytes.len(),
+                    max_len
+                )));
+            }
+
+            // Safety: We've verified T is u8, so Vec<T> *is* Vec<u8>.
+            let vec_u8 = bytes.to_vec();
+            let vec_t = unsafe { mem::transmute::<Vec<u8>, Vec<T>>(vec_u8) };
+            return Self::new(vec_t).map_err(|e| {
+                ssz::DecodeError::BytesInvalid(format!(
+                    "Wrong number of VariableList elements: {:?}",
+                    e
+                ))
+            });
+        }
+
         if T::is_ssz_fixed_len() {
             let num_items = bytes
                 .len()
@@ -315,20 +337,28 @@ where
                 )));
             }
 
-            bytes.chunks(T::ssz_fixed_len()).try_fold(
-                Vec::with_capacity(num_items),
-                |mut vec, chunk| {
-                    vec.push(T::from_ssz_bytes(chunk)?);
-                    Ok(vec)
-                },
-            )
+            // Check that we have a whole number of items and that it is safe to use chunks_exact
+            if !bytes.len().is_multiple_of(T::ssz_fixed_len()) {
+                return Err(ssz::DecodeError::BytesInvalid(format!(
+                    "VariableList of {} items has {} bytes",
+                    num_items,
+                    bytes.len()
+                )));
+            }
+
+            let mut vec = Vec::with_capacity(num_items);
+            for chunk in bytes.chunks_exact(T::ssz_fixed_len()) {
+                vec.push(T::from_ssz_bytes(chunk)?);
+            }
+            Self::new(vec).map_err(|e| {
+                ssz::DecodeError::BytesInvalid(format!(
+                    "Wrong number of VariableList elements: {:?}",
+                    e
+                ))
+            })
         } else {
             ssz::decode_list_of_variable_length_items(bytes, Some(max_len))
-        }?
-        .try_into()
-        .map_err(|e| {
-            ssz::DecodeError::BytesInvalid(format!("VariableList::try_from failed: {e:?}"))
-        })
+        }
     }
 }
 
@@ -452,17 +482,60 @@ mod test {
         assert_eq!(<VariableList<u16, U2> as Encode>::ssz_fixed_len(), 4);
     }
 
-    fn round_trip<T: Encode + Decode + std::fmt::Debug + PartialEq>(item: T) {
+    fn ssz_round_trip<T: Encode + Decode + std::fmt::Debug + PartialEq>(item: T) {
         let encoded = &item.as_ssz_bytes();
         assert_eq!(item.ssz_bytes_len(), encoded.len());
         assert_eq!(T::from_ssz_bytes(encoded), Ok(item));
     }
 
     #[test]
     fn u16_len_8() {
-        round_trip::<VariableList<u16, U8>>(vec![42; 8].try_into().unwrap());
-        round_trip::<VariableList<u16, U8>>(vec![0; 8].try_into().unwrap());
-        round_trip::<VariableList<u16, U8>>(vec![].try_into().unwrap());
+        ssz_round_trip::<VariableList<u16, U8>>(vec![42; 8].try_into().unwrap());
+        ssz_round_trip::<VariableList<u16, U8>>(vec![0; 8].try_into().unwrap());
+        ssz_round_trip::<VariableList<u16, U8>>(vec![].try_into().unwrap());
+    }
+
+    #[test]
+    fn ssz_round_trip_u8_len_1024() {
+        ssz_round_trip::<VariableList<u8, U1024>>(vec![42; 1024].try_into().unwrap());
+        ssz_round_trip::<VariableList<u8, U1024>>(vec![0; 1024].try_into().unwrap());
+    }
+
+    // bool is layout equivalent to u8 but must not use the same unsafe codepath because not all u8
+    // values are valid bools.
+    #[test]
+    fn ssz_round_trip_bool_len_1024() {
+        assert_eq!(mem::size_of::<bool>(), 1);
+        assert_eq!(mem::align_of::<bool>(), 1);
+        ssz_round_trip::<VariableList<bool, U1024>>(vec![true; 1024].try_into().unwrap());
+        ssz_round_trip::<VariableList<bool, U1024>>(vec![false; 1024].try_into().unwrap());
+    }
+
+    // Decoding a u8 list as a list of bools must fail, if we aren't careful we could trigger UB.
+    #[test]
+    fn ssz_u8_to_bool_len_1024() {
+        let list_u8 = VariableList::<u8, U8>::new(vec![0, 1, 2, 3, 4, 5, 6, 7]).unwrap();
+        VariableList::<bool, U8>::from_ssz_bytes(&list_u8.as_ssz_bytes()).unwrap_err();
+    }
+
+    #[test]
+    fn ssz_u8_len_1024_too_long() {
+        assert_eq!(
+            VariableList::<u8, U1024>::from_ssz_bytes(&vec![42; 1025]).unwrap_err(),
+            ssz::DecodeError::BytesInvalid(
+                "VariableList of 1025 items exceeds maximum of 1024".into()
+            )
+        );
+    }
+
+    #[test]
+    fn ssz_u64_len_1024_too_long() {
+        assert_eq!(
+            VariableList::<u64, U1024>::from_ssz_bytes(&vec![42; 8 * 1025]).unwrap_err(),
+            ssz::DecodeError::BytesInvalid(
+                "VariableList of 1025 items exceeds maximum of 1024".into()
+            )
+        );
     }
 
     #[test]
@@ -473,6 +546,21 @@ mod test {
         assert_eq!(VariableList::from_ssz_bytes(&[]).unwrap(), empty_list);
     }
 
+    #[test]
+    fn ssz_bytes_u32_trailing() {
+        let bytes = [1, 0, 0, 0, 2, 0];
+        assert_eq!(
+            VariableList::<u32, U2>::from_ssz_bytes(&bytes).unwrap_err(),
+            ssz::DecodeError::BytesInvalid("VariableList of 1 items has 6 bytes".into())
+        );
+
+        let bytes = [1, 0, 0, 0, 2, 0, 0, 0, 3];
+        assert_eq!(
+            VariableList::<u32, U2>::from_ssz_bytes(&bytes).unwrap_err(),
+            ssz::DecodeError::BytesInvalid("VariableList of 2 items has 9 bytes".into())
+        );
+    }
+
     fn root_with_length(bytes: &[u8], len: usize) -> Hash256 {
         let root = merkle_root(bytes, 0);
         tree_hash::mix_in_length(&root, len)