fix: g2.Serialize sporadic failure (#4626)

There was a long-standing bug that only occurred occasionally. Memory
sanitizer found this nicely. The crux is that g2 uses field2 which is
NOT the same size as uint256_t. Simplified the code to just either be
normal serialization or a buffer of all set bits.

---------

Co-authored-by: ludamad <adam@aztecprotocol.com>

barretenberg/cpp/dockerfiles/interactive/Dockerfile.msan.ubuntu

@@ -35,8 +35,8 @@ RUN cmake -G Ninja -S llvm-project/runtimes -B llvm-project/build \
 RUN ninja -C llvm-project/build cxx cxxabi
 RUN ninja -C llvm-project/build install-cxx install-cxxabi
 
-ENV MSAN_CFLAGS="-std=c++20 -fsanitize=memory -nostdinc++ -I/opt/include -I/opt/include/c++/v1"
-ENV MSAN_LFLAGS="-fsanitize=memory -stdlib=libc++ -L/opt/lib -lc++abi -Wl,-rpath,/opt/lib"
+ENV MSAN_CFLAGS="-std=c++20 -fsanitize=memory -fsanitize-memory-track-origins -nostdinc++ -I/opt/include -I/opt/include/c++/v1"
+ENV MSAN_LFLAGS="-fsanitize=memory -fsanitize-memory-track-origins -stdlib=libc++ -L/opt/lib -lc++abi -Wl,-rpath,/opt/lib"
 
 WORKDIR /usr/src/barretenberg/cpp
 

barretenberg/cpp/src/barretenberg/ecc/groups/affine_element.hpp

@@ -2,6 +2,7 @@
 #include "barretenberg/ecc/curves/bn254/fq2.hpp"
 #include "barretenberg/numeric/uint256/uint256.hpp"
 #include "barretenberg/serialize/msgpack.hpp"
+#include <cstring>
 #include <type_traits>
 #include <vector>
 
@@ -21,7 +22,7 @@ template <typename Fq_, typename Fr_, typename Params> class alignas(64) affine_
     affine_element() noexcept = default;
     ~affine_element() noexcept = default;
 
-    constexpr affine_element(const Fq& a, const Fq& b) noexcept;
+    constexpr affine_element(const Fq& x, const Fq& y) noexcept;
 
     constexpr affine_element(const affine_element& other) noexcept = default;
 
@@ -103,13 +104,9 @@ template <typename Fq_, typename Fr_, typename Params> class alignas(64) affine_
     static void serialize_to_buffer(const affine_element& value, uint8_t* buffer)
     {
         if (value.is_point_at_infinity()) {
-            if constexpr (Fq::modulus.get_msb() == 255) {
-                write(buffer, uint256_t(0));
-                write(buffer, Fq::modulus);
-            } else {
-                write(buffer, uint256_t(0));
-                write(buffer, uint256_t(1) << 255);
-            }
+            // if we are infinity, just set all buffer bits to 1
+            // we only need this case because the below gets mangled converting from montgomery for infinity points
+            memset(buffer, 255, sizeof(Fq) * 2);
         } else {
             Fq::serialize_to_buffer(value.y, buffer);
             Fq::serialize_to_buffer(value.x, buffer + sizeof(Fq));
@@ -130,38 +127,17 @@ template <typename Fq_, typename Fr_, typename Params> class alignas(64) affine_
      */
     static affine_element serialize_from_buffer(uint8_t* buffer)
     {
-        affine_element result;
-
-        // need to read a raw uint256_t to avoid reductions so we can check whether the point is the point at infinity
-        auto raw_x = from_buffer<uint256_t>(buffer + sizeof(Fq));
-
-        if constexpr (Fq::modulus.get_msb() == 255) {
-            if (raw_x == Fq::modulus) {
-                result.y = Fq::zero();
-                result.x.data[0] = raw_x.data[0];
-                result.x.data[1] = raw_x.data[1];
-                result.x.data[2] = raw_x.data[2];
-                result.x.data[3] = raw_x.data[3];
-            } else {
-                result.y = Fq::serialize_from_buffer(buffer);
-                result.x = Fq(raw_x);
-            }
-        } else {
-            if (raw_x.get_msb() == 255) {
-                result.y = Fq::zero();
-                result.x = Fq::zero();
-                result.self_set_infinity();
-            } else {
-                // conditional here to avoid reading the same data twice in case of a field of prime order
-                if constexpr (std::is_same<Fq, fq2>::value) {
-                    result.y = Fq::serialize_from_buffer(buffer);
-                    result.x = Fq::serialize_from_buffer(buffer + sizeof(Fq));
-                } else {
-                    result.y = Fq::serialize_from_buffer(buffer);
-                    result.x = Fq(raw_x);
-                }
-            }
+        // Does the buffer consist entirely of set bits? If so, we have a point at infinity
+        // Note that if it isn't, this loop should end early.
+        // We only need this case because the below gets mangled converting to montgomery for infinity points
+        bool is_point_at_infinity =
+            std::all_of(buffer, buffer + sizeof(Fq) * 2, [](uint8_t val) { return val == 255; });
+        if (is_point_at_infinity) {
+            return affine_element::infinity();
         }
+        affine_element result;
+        result.y = Fq::serialize_from_buffer(buffer);
+        result.x = Fq::serialize_from_buffer(buffer + sizeof(Fq));
         return result;
     }
 

barretenberg/cpp/src/barretenberg/ecc/groups/affine_element_impl.hpp

@@ -5,9 +5,9 @@
 
 namespace bb::group_elements {
 template <class Fq, class Fr, class T>
-constexpr affine_element<Fq, Fr, T>::affine_element(const Fq& a, const Fq& b) noexcept
-    : x(a)
-    , y(b)
+constexpr affine_element<Fq, Fr, T>::affine_element(const Fq& x, const Fq& y) noexcept
+    : x(x)
+    , y(y)
 {}
 
 template <class Fq, class Fr, class T>