Cast the other side of the expression to unsigned.

src/google/protobuf/arena_unittest.cc

@@ -16,6 +16,7 @@
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
+#include <limits>
 #include <memory>
 #include <new>  // IWYU pragma: keep for operator new
 #include <string>
@@ -72,6 +73,7 @@ using proto2_unittest::TestRepeatedString;
 using ::testing::AnyOf;
 using ::testing::ElementsAre;
 using ::testing::ElementsAreArray;
+using ::testing::Gt;
 using ::testing::HasSubstr;
 using ::testing::Optional;
 using ::testing::Pointee;
@@ -1786,6 +1788,34 @@ TEST(ArenaTest, StartingBlockSize) {
   EXPECT_EQ(custom_arena.SpaceAllocated(), options.start_block_size);
 }
 
+TEST(ArenaTest, VeryLargeAllocIn32BitMode) {
+  if (sizeof(size_t) != 4) {
+    GTEST_SKIP() << "Only care about 32-bit mode.";
+  }
+
+  static absl::optional<size_t> large_allocation_size;
+
+  // Make sure we don't have bugs allocating buffers greater that ptrdiff_t max.
+  // It will likely fail, but it should fail in the allocator.
+
+  ArenaOptions options;
+  options.block_alloc = [](size_t n) {
+    if (n > size_t{std::numeric_limits<ptrdiff_t>::max()}) {
+      large_allocation_size = n;
+      // Just fake it. Doesn't matter.
+      n = 1000000;
+    }
+    return ::operator new(n);
+  };
+  options.block_dealloc = [](void* ptr, size_t) { ::operator delete(ptr); };
+
+  Arena arena(options);
+  void* ptr = arena.AllocateAligned(size_t{1} << 31);
+  EXPECT_NE(ptr, nullptr);
+  EXPECT_THAT(large_allocation_size,
+              Optional(Gt(size_t{std::numeric_limits<ptrdiff_t>::max()})));
+}
+
 TEST(ArenaTest, BlockSizeDoubling) {
   Arena arena;
   EXPECT_EQ(0, arena.SpaceUsed());

src/google/protobuf/serial_arena.h

@@ -203,6 +203,7 @@ class PROTOBUF_EXPORT SerialArena {
     ABSL_DCHECK_GE(limit_, ptr());
     char* ret = ptr();
     if (ABSL_PREDICT_FALSE(limit_ - ret < static_cast<ptrdiff_t>(n))) {
+      // if (ABSL_PREDICT_FALSE(static_cast<size_t>(limit_ - ret) < n)) {
       return false;
     }
     internal::UnpoisonMemoryRegion(ret, n);