Fix 1-byte buffer overrun

Summary: Fix 1-byte buffer overrun when parsing the string "-".

Reviewed By: mhlakhani

Differential Revision: D22695995

fbshipit-source-id: 9d1834c068fc3889a514b77d53d626eb6f72b6c6

folly/Conv.cpp

@@ -407,7 +407,11 @@ Expected<Tgt, ConversionCode> str_to_floating(StringPiece* src) noexcept {
     negative = true;
     ++b;
     --size;
+    if (size == 0) {
+      return makeUnexpected(ConversionCode::STRING_TO_FLOAT_ERROR);
+    }
   }
+  assert(size > 0);
 
   result = 0.0;
 

folly/test/ConvTest.cpp

@@ -1201,6 +1201,10 @@ TEST(Conv, TryStringToFloat) {
   auto rv2 = folly::tryTo<float>("3.14");
   EXPECT_TRUE(rv2.hasValue());
   EXPECT_NEAR(rv2.value(), 3.14, 1e-5);
+  // No trailing '\0' to expose 1-byte buffer over-read
+  char x = '-';
+  auto rv3 = folly::tryTo<float>(folly::StringPiece(&x, 1));
+  EXPECT_FALSE(rv3.hasValue());
 }
 
 TEST(Conv, TryStringToDouble) {