From fce8ed15630a4969be7c9761b9b7d3cef0530cc6 Mon Sep 17 00:00:00 2001
From: Diego <96022404+dzfrias@users.noreply.github.com>
Date: Thu, 4 Jul 2024 08:40:36 -0700
Subject: [PATCH] LibWasm: Validate potentially empty `else` branch in `if`
 instruction

---
 Userland/Libraries/LibWasm/AbstractMachine/Validator.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Userland/Libraries/LibWasm/AbstractMachine/Validator.cpp b/Userland/Libraries/LibWasm/AbstractMachine/Validator.cpp
index faa9ae9873e..325bd0dda5c 100644
--- a/Userland/Libraries/LibWasm/AbstractMachine/Validator.cpp
+++ b/Userland/Libraries/LibWasm/AbstractMachine/Validator.cpp
@@ -1939,6 +1939,13 @@ VALIDATE_INSTRUCTION(structured_end)
 
     auto& last_frame = m_frames.last();
 
+    // If this is true, then the `if` had no else. In that case, validate that the
+    // empty else block produces the correct type.
+    if (last_frame.kind == FrameKind::If) {
+        bool is_constant = false;
+        TRY(validate(Instruction(Instructions::structured_else), stack, is_constant));
+    }
+
     auto& results = last_frame.type.results();
     for (size_t i = 1; i <= results.size(); ++i)
         TRY(stack.take(results[results.size() - i]));