libbinder: readCString: no ubsan sub-overflow

Bug: 131859347
Test: fuzzer
Change-Id: I95a0f59684a172925f1eab97ff21e5d14bc79cc8
Merged-In: I95a0f59684a172925f1eab97ff21e5d14bc79cc8
(cherry picked from commit d0d4b584fc294d2c124385644099852918416344)

diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index 460bbe2..13555fd 100644 (file)
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -2013,8 +2013,8 @@ status_t Parcel::readUtf8FromUtf16(std::unique_ptr<std::string>* str) const {
 
 const char* Parcel::readCString() const
 {
-    const size_t avail = mDataSize-mDataPos;
-    if (avail > 0) {
+    if (mDataPos < mDataSize) {
+        const size_t avail = mDataSize-mDataPos;
         const char* str = reinterpret_cast<const char*>(mData+mDataPos);
         // is the string's trailing NUL within the parcel's valid bounds?
         const char* eos = reinterpret_cast<const char*>(memchr(str, 0, avail));