Fix function section validation

The validation of type indices in function section must be fully
finished before processing any code. This is because there may be a call
in code[i] referencing function[i + n].

lib/fizzy/parser.cpp

@@ -493,6 +493,12 @@ Module parse(bytes_view input)
         }
     }
 
+    for (const auto type_idx : module.funcsec)
+    {
+        if (type_idx >= module.typesec.size())
+            throw validation_error{"invalid function type index"};
+    }
+
     if (module.tablesec.size() > 1)
         throw validation_error{"too many table sections (at most one is allowed)"};
 
@@ -533,9 +539,7 @@ Module parse(bytes_view input)
     module.codesec.reserve(code_binaries.size());
     for (size_t i = 0; i < code_binaries.size(); ++i)
     {
-        const auto type_idx = module.funcsec[i];
-        if (type_idx >= module.typesec.size())
-            throw validation_error{"invalid function type index"};
+        assert(module.funcsec[i] < module.typesec.size());
         module.codesec.emplace_back(parse_code(code_binaries[i], module));
     }
 

test/unittests/parser_expr_test.cpp

@@ -349,4 +349,16 @@ TEST(parser_expr, call_1arg_1result)
 
     const auto module = parse(wasm);
     ASSERT_EQ(module.codesec.size(), 2);
-}
+}
+
+TEST(parser_expr, call_nonexisting_typeidx)
+{
+    // This creates a wasm module where code[0] has a call instruction calling function[1] which
+    // has invalid type_idx 1.
+    // wat2wasm cannot be used as there is no way to have invalid type_idx in WAT form.
+    const auto wasm = bytes{wasm_prefix} + make_section(1, make_vec({"600000"_bytes})) +
+                      make_section(3, make_vec({"00"_bytes, "01"_bytes})) +
+                      make_section(10, make_vec({"040010010b"_bytes, "02000b"_bytes}));
+
+    EXPECT_THROW_MESSAGE(parse(wasm), validation_error, "invalid function type index");
+}

test/unittests/parser_test.cpp

@@ -436,8 +436,10 @@ TEST(parser, function_section_end_out_of_bounds)
 
 TEST(parser, function_section_without_code)
 {
+    const auto type_section = make_vec({make_functype({}, {})});
     const auto function_section = "0100"_bytes;
-    const auto bin = bytes{wasm_prefix} + make_section(3, function_section);
+    const auto bin =
+        bytes{wasm_prefix} + make_section(1, type_section) + make_section(3, function_section);
     EXPECT_THROW_MESSAGE(parse(bin), parser_error,
         "malformed binary: number of function and code entries must match");
 }