Reject a matrix file whose modulus does not fit in Scalar

src/mathicgb/SparseMatrix.cpp

@@ -498,6 +498,13 @@ SparseMatrix::Scalar SparseMatrix::read(FILE* file) {
   const auto rowCount = readOne<uint32>(file);
   [[maybe_unused]] const auto colCount = readOne<uint32>(file);
   const auto modulus = readOne<uint32>(file);
+  if (modulus > std::numeric_limits<Scalar>::max()) {
+    std::ostringstream err;
+    err << "The matrix file has modulus " << modulus
+      << ", which does not fit in the " << 8 * sizeof(Scalar)
+      << " bits that this file format stores coefficients in.";
+    mathic::reportError(err.str());
+  }
   const auto entryCount64 = readOne<uint64>(file);
   if (entryCount64 > std::numeric_limits<size_t>::max())
     throw std::bad_alloc();
@@ -545,7 +552,7 @@ SparseMatrix::Scalar SparseMatrix::read(FILE* file) {
   }
 
   MATHICGB_ASSERT(mBlock.mPreviousBlock == 0); // still only one block
-  return modulus;
+  return static_cast<Scalar>(modulus);
 }
 
 void SparseMatrix::writePBM(FILE* file) {

src/test/SparseMatrix.cpp

@@ -7,7 +7,9 @@
 #include "mathicgb/PolyRing.hpp"
 #include "mathicgb/io-util.hpp"
 #include "mathicgb/MathicIO.hpp"
+#include "mathicgb/CFile.hpp"
 #include <gtest/gtest.h>
+#include <cstdio>
 #include <memory>
 #include <string>
 
@@ -88,3 +90,30 @@ TEST(SparseMatrix, toRow) {
   mat.rowToPolynomial(2, monomials, p);
   ASSERT_EQ(*parsePoly(*ring, "20a3+40a1"), p);
 }
+
+TEST(SparseMatrix, ReadRejectsAnOversizedModulus) {
+  const char* const fileName = "SparseMatrix-modulus-test.tmp";
+  SparseMatrix mat;
+  mat.appendEntry(0, 5);
+  mat.rowDone();
+
+  // write() takes a Scalar, so the only way to get an out-of-range modulus
+  // into a file is to patch the field, which sits after two uint32s.
+  {
+    CFile file(fileName, "wb");
+    mat.write(101, file.handle());
+  }
+  {
+    CFile file(fileName, "r+b");
+    const uint32 modulus = 65637; // 65536 + 101, so it truncates to 101
+    ASSERT_EQ(0, std::fseek(file.handle(), 2 * sizeof(uint32), SEEK_SET));
+    ASSERT_EQ(1, std::fwrite(&modulus, sizeof(modulus), 1, file.handle()));
+  }
+
+  SparseMatrix read;
+  {
+    CFile file(fileName, "rb");
+    ASSERT_THROW(read.read(file.handle()), mathic::MathicException);
+  }
+  ASSERT_EQ(0, std::remove(fileName));
+}