From 87df7f7c43d746503773aeafd104f566001e87c6 Mon Sep 17 00:00:00 2001
From: Amr Hesham <amr96@programmer.net>
Date: Fri, 7 Apr 2023 01:50:02 +0200
Subject: [PATCH] Explorer: Fuzzer issue around infinite recursion (#2741)

I limited the stack size to 1K we can change it or make it dynamic from CLI later

Closes #2733
---
 explorer/interpreter/action_stack.h           |  2 ++
 explorer/interpreter/interpreter.cpp          |  4 ++++
 .../fail_recursion_stackoverflow.carbon       | 20 +++++++++++++++++++
 3 files changed, 26 insertions(+)
 create mode 100644 explorer/testdata/function/fail_recursion_stackoverflow.carbon

diff --git a/explorer/interpreter/action_stack.h b/explorer/interpreter/action_stack.h
index 383dd8562cc6..4f4b686f9e44 100644
--- a/explorer/interpreter/action_stack.h
+++ b/explorer/interpreter/action_stack.h
@@ -119,6 +119,8 @@ class ActionStack {
 
   void Pop() { todo_.Pop(); }
 
+  auto Count() const -> int { return todo_.Count(); }
+
  private:
   // Pop any ScopeActions from the top of the stack, propagating results as
   // needed, to restore the invariant that todo_.Top() is not a ScopeAction.
diff --git a/explorer/interpreter/interpreter.cpp b/explorer/interpreter/interpreter.cpp
index a17ed744a185..e143c259f22b 100644
--- a/explorer/interpreter/interpreter.cpp
+++ b/explorer/interpreter/interpreter.cpp
@@ -952,6 +952,10 @@ auto Interpreter::CallFunction(const CallExpression& call,
                                Nonnull<const Value*> fun,
                                Nonnull<const Value*> arg,
                                ImplWitnessMap&& witnesses) -> ErrorOr<Success> {
+  constexpr int StackSizeLimit = 1000;
+  if (todo_.Count() > StackSizeLimit) {
+    return ProgramError(call.source_loc()) << "stack overflow";
+  }
   if (trace_stream_->is_enabled()) {
     *trace_stream_ << "calling function: " << *fun << "\n";
   }
diff --git a/explorer/testdata/function/fail_recursion_stackoverflow.carbon b/explorer/testdata/function/fail_recursion_stackoverflow.carbon
new file mode 100644
index 000000000000..153b9718b8a3
--- /dev/null
+++ b/explorer/testdata/function/fail_recursion_stackoverflow.carbon
@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package EmptyIdentifier impl;
+
+fn A() {
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/function/fail_recursion_stackoverflow.carbon:[[@LINE+1]]: stack overflow
+  A();
+}
+
+fn Main() -> i32
+{
+  A();
+  return 0;
+}