Do not try opaque constants when handling infinite loops

We should not break the correctness of the program, even when the
compiler may be able to remove infinite loops with no side-effects.
As constants get reconstructed with a stack-allocated variable, do
not conceal loop-variant constants when possibly in presence of
infinite loops; this otherwise could lead to stack-overflow issues.

Fixes: open-obfuscator#39.

src/passes/opaque-constants/OpaqueConstants.cpp

@@ -267,6 +267,9 @@ PreservedAnalyses OpaqueConstants::run(Module &M,
     }
 
     for (BasicBlock& BB : F) {
+      // Don't try opaque constants when potentially handling infinite loops.
+      if (is_contained(successors(&BB), &BB))
+        continue;
       Changed |= runOnBasicBlock(BB, inserted);
     }
   }

src/test/passes/opaque-constants/config_all.py

@@ -0,0 +1,12 @@
+import omvll
+from functools import lru_cache
+
+class MyConfig(omvll.ObfuscationConfig):
+    def __init__(self):
+        super().__init__()
+    def obfuscate_constants(self, mod: omvll.Module, func: omvll.Function):
+        return True
+
+@lru_cache(maxsize=1)
+def omvll_get_config() -> omvll.ObfuscationConfig:
+    return MyConfig()

src/test/passes/opaque-constants/opaque-constants-infinite-loop.ll

@@ -0,0 +1,25 @@
+; RUN: env OMVLL_CONFIG=%S/config_all.py clang -target aarch64-linux-android -fno-legacy-pass-manager -fpass-plugin=%libOMVLL -O0 -S -emit-llvm %s -o - | FileCheck %s --check-prefix=O0
+; RUN: env OMVLL_CONFIG=%S/config_all.py clang -target aarch64-linux-android -fno-legacy-pass-manager -fpass-plugin=%libOMVLL -O1 -S -emit-llvm %s -o - | FileCheck %s --check-prefix=O1
+; RUN: env OMVLL_CONFIG=%S/config_all.py clang -target arm64-apple-ios       -fno-legacy-pass-manager -fpass-plugin=%libOMVLL -O0 -S -emit-llvm %s -o - | FileCheck %s --check-prefix=O0
+; RUN: env OMVLL_CONFIG=%S/config_all.py clang -target arm64-apple-ios       -fno-legacy-pass-manager -fpass-plugin=%libOMVLL -O1 -S -emit-llvm %s -o - | FileCheck %s --check-prefix=O1
+
+define void @opaque_infinite_loop() {
+; O0-LABEL: @opaque_infinite_loop
+; O0: loop
+; O0: %loaded_i = load i32, i32* %i, align 4
+; O0-NEXT: %add = add nsw i32 %loaded_i, 1
+; O0-NEXT: store i32 %add, i32* %i, align 4
+; O0-NEXT: br label %loop
+; O1-LABEL: @opaque_infinite_loop
+; O1: loop
+; O1: br label %loop
+entry:
+  %i = alloca i32, align 4
+  store i32 0, i32* %i, align 4
+  br label %loop
+loop:                                                ; preds = %loop, %entry
+  %loaded_i = load i32, i32* %i, align 4
+  %add = add nsw i32 %loaded_i, 1
+  store i32 %add, i32* %i, align 4
+  br label %loop
+}