Ensure library file starts with 'lib/' prefix.

Test: ApkParsing_test.cpp
Change-Id: I3a1ac8c10b315e49ec668ff357aafe861cd2d0fe

libs/androidfw/ApkParsing.cpp

@@ -56,6 +56,11 @@ const char* ValidLibraryPathLastSlash(const char* fileName, bool suppress64Bit,
         return nullptr;
     }
 
+    // Make sure file starts with 'lib/' prefix.
+    if (strncmp(fileName, APK_LIB.data(), APK_LIB_LEN) != 0) {
+        return nullptr;
+    }
+
     // Make sure there aren't subdirectories by checking if the next / after lib/ is the last slash
     if (memchr(fileName + APK_LIB_LEN, '/', fileNameLen - APK_LIB_LEN) != lastSlash) {
         return nullptr;

libs/androidfw/tests/ApkParsing_test.cpp

@@ -74,4 +74,10 @@ TEST(ApkParsingTest, InvalidFileAtRoot) {
   auto lastSlash = util::ValidLibraryPathLastSlash(path, false, false);
   ASSERT_THAT(lastSlash, IsNull());
 }
+
+TEST(ApkParsingTest, InvalidPrefix) {
+  const char* path = "assets/libhello.so";
+  auto lastSlash = util::ValidLibraryPathLastSlash(path, false, false);
+  ASSERT_THAT(lastSlash, IsNull());
+}
 }