FormatString: avoid an UB when we need to grow the buffer

src/common/angleutils.cpp

@@ -19,8 +19,15 @@ const uintptr_t DirtyPointer = std::numeric_limits<uintptr_t>::max();
 
 size_t FormatStringIntoVector(const char *fmt, va_list vararg, std::vector<char>& outBuffer)
 {
+    // The state of the va_list passed to vsnprintf is undefined after the call, do a copy in case
+    // we need to grow the buffer.
+    va_list varargCopy;
+    va_copy(varargCopy, vararg);
+
     // Attempt to just print to the current buffer
-    int len = vsnprintf(&(outBuffer.front()), outBuffer.size(), fmt, vararg);
+    int len = vsnprintf(&(outBuffer.front()), outBuffer.size(), fmt, varargCopy);
+    va_end(varargCopy);
+
     if (len < 0 || static_cast<size_t>(len) >= outBuffer.size())
     {
         // Buffer was not large enough, calculate the required size and resize the buffer
@@ -28,7 +35,9 @@ size_t FormatStringIntoVector(const char *fmt, va_list vararg, std::vector<char>
         outBuffer.resize(len + 1);
 
         // Print again
-        len = vsnprintf(&(outBuffer.front()), outBuffer.size(), fmt, vararg);
+        va_copy(varargCopy, vararg);
+        len = vsnprintf(&(outBuffer.front()), outBuffer.size(), fmt, varargCopy);
+        va_end(varargCopy);
     }
     ASSERT(len >= 0);
     return static_cast<size_t>(len);