feat(string): allow limiting the length of obfuscated string via an environment variable

Author: dzamlo

Plugin.cpp

@@ -45,7 +45,9 @@ bool addPassWithName(FunctionPassManager &FPM, StringRef &passName) {
 
 bool addPassWithName(ModulePassManager &MPM, StringRef &passName) {
   if (passName == "string-encryption") {
-    MPM.addPass(StringObfuscatorPass());
+    size_t maxSize = 0;
+    getEnvVar(EnvVarPrefix + "MAX_STRING_LENGTH").getAsInteger(10, maxSize);
+    MPM.addPass(StringObfuscatorPass{maxSize});
   } else {
     return false;
   }

README.md

@@ -55,6 +55,9 @@ Or you can run the string encryption pass with:
 
 Refer to the llvm::PassBuilder documentation for more information on each insertion point.
 
+You can limit the length of strings that are obfuscated with the `LLVM_OBF_MAX_STRING_LENGTH` environment variable. A 
+value of 0 (the default) means no limit.
+
 ### With opt
 
 [`opt`](https://llvm.org/docs/CommandGuide/opt.html) can be used to apply specific passes from LLRM-IR you

string/StringObfuscation.cpp

@@ -28,6 +28,9 @@ ConstantDataArray *StringObfuscatorPass::encodeStringDataArray(LLVMContext &ctx,
                                                                const char *str,
                                                                size_t size,
                                                                uint8_t key) {
+  if (this->maxSize != 0 && size > this->maxSize)
+    return nullptr;
+
   // Encode the data
   char *encodedStr = (char *)malloc(size);
   for (unsigned int i = 0; i < size; i++) {
@@ -78,7 +81,7 @@ void StringObfuscatorPass::encodeStructString(LLVMContext &ctx,
   }
 }
 
-StringObfuscatorPass::StringObfuscatorPass() {}
+StringObfuscatorPass::StringObfuscatorPass(size_t maxSize) : maxSize{maxSize} {}
 
 bool StringObfuscatorPass::encodeAllStrings(Module &M) {
   auto &ctx = M.getContext();

string/StringObfuscation.h

@@ -30,11 +30,13 @@ struct GlobalStringVariable {
   }
 };
 
+
 namespace llvm {
 struct StringObfuscatorPass : public PassInfoMixin<StringObfuscatorPass> {
   std::vector<GlobalStringVariable> globalStrings;
+  size_t maxSize;
 
-  StringObfuscatorPass();
+  StringObfuscatorPass(size_t maxSize);
   ConstantDataArray *encodeStringDataArray(LLVMContext &ctx, const char *str,
                                            size_t size, uint8_t key);
   void encodeStructString(LLVMContext &ctx, GlobalVariable *gv,