Add static_assert to prevent VARINT(<signed value>)

Using VARINT with signed types is dangerous because negative values will appear
to serialize correctly, but then deserialize as positive values mod 128.

This commit changes the VARINT macro to trigger an error by default if called
with an signed value, and updates broken uses of VARINT to pass a special flag
that lets them keep working with no change in behavior.

1 files changed, 33 insertions, 9 deletions

diff --git a/src/serialize.h b/src/serialize.h
index c454ba16b7..91da6b0f80 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -296,9 +296,31 @@ uint64_t ReadCompactSize(Stream& is)
  * 2^32:           [0x8E 0xFE 0xFE 0xFF 0x00]
  */
 
-template<typename I>
+/**
+ * Mode for encoding VarInts.
+ *
+ * Currently there is no support for signed encodings. The default mode will not
+ * compile with signed values, and the legacy "nonnegative signed" mode will
+ * accept signed values, but improperly encode and decode them if they are
+ * negative. In the future, the DEFAULT mode could be extended to support
+ * negative numbers in a backwards compatible way, and additional modes could be
+ * added to support different varint formats (e.g. zigzag encoding).
+ */
+enum class VarIntMode { DEFAULT, NONNEGATIVE_SIGNED };
+
+template <VarIntMode Mode, typename I>
+struct CheckVarIntMode {
+    constexpr CheckVarIntMode()
+    {
+        static_assert(Mode != VarIntMode::DEFAULT || std::is_unsigned<I>::value, "Unsigned type required with mode DEFAULT.");
+        static_assert(Mode != VarIntMode::NONNEGATIVE_SIGNED || std::is_signed<I>::value, "Signed type required with mode NONNEGATIVE_SIGNED.");
+    }
+};
+
+template<VarIntMode Mode, typename I>
 inline unsigned int GetSizeOfVarInt(I n)
 {
+    CheckVarIntMode<Mode, I>();
     int nRet = 0;
     while(true) {
         nRet++;
@@ -312,9 +334,10 @@ inline unsigned int GetSizeOfVarInt(I n)
 template<typename I>
 inline void WriteVarInt(CSizeComputer& os, I n);
 
-template<typename Stream, typename I>
+template<typename Stream, VarIntMode Mode, typename I>
 void WriteVarInt(Stream& os, I n)
 {
+    CheckVarIntMode<Mode, I>();
     unsigned char tmp[(sizeof(n)*8+6)/7];
     int len=0;
     while(true) {
@@ -329,9 +352,10 @@ void WriteVarInt(Stream& os, I n)
     } while(len--);
 }
 
-template<typename Stream, typename I>
+template<typename Stream, VarIntMode Mode, typename I>
 I ReadVarInt(Stream& is)
 {
+    CheckVarIntMode<Mode, I>();
     I n = 0;
     while(true) {
         unsigned char chData = ser_readdata8(is);
@@ -351,7 +375,7 @@ I ReadVarInt(Stream& is)
 }
 
 #define FLATDATA(obj) CFlatData((char*)&(obj), (char*)&(obj) + sizeof(obj))
-#define VARINT(obj) WrapVarInt(REF(obj))
+#define VARINT(obj, ...) WrapVarInt<__VA_ARGS__>(REF(obj))
 #define COMPACTSIZE(obj) CCompactSize(REF(obj))
 #define LIMITED_STRING(obj,n) LimitedString< n >(REF(obj))
 
@@ -395,7 +419,7 @@ public:
     }
 };
 
-template<typename I>
+template<VarIntMode Mode, typename I>
 class CVarInt
 {
 protected:
@@ -405,12 +429,12 @@ public:
 
     template<typename Stream>
     void Serialize(Stream &s) const {
-        WriteVarInt<Stream,I>(s, n);
+        WriteVarInt<Stream,Mode,I>(s, n);
     }
 
     template<typename Stream>
     void Unserialize(Stream& s) {
-        n = ReadVarInt<Stream,I>(s);
+        n = ReadVarInt<Stream,Mode,I>(s);
     }
 };
 
@@ -461,8 +485,8 @@ public:
     }
 };
 
-template<typename I>
-CVarInt<I> WrapVarInt(I& n) { return CVarInt<I>(n); }
+template<VarIntMode Mode=VarIntMode::DEFAULT, typename I>
+CVarInt<Mode, I> WrapVarInt(I& n) { return CVarInt<Mode, I>{n}; }
 
 /**
  * Forward declarations