1 //! Generate a Wasm module and the configuration for generating it.
2 
3 use arbitrary::{Arbitrary, Unstructured};
4 use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
5 
6 /// Default module-level configuration for fuzzing Wasmtime.
7 ///
8 /// Internally this uses `wasm-smith`'s own `Config` but we further refine
9 /// the defaults here as well.
10 #[derive(Debug, Clone)]
11 #[expect(missing_docs, reason = "self-describing fields")]
12 pub struct ModuleConfig {
13     pub config: wasm_smith::Config,
14 
15     // These knobs aren't exposed in `wasm-smith` at this time but are exposed
16     // in our `*.wast` testing so keep knobs here so they can be read during
17     // config-to-`wasmtime::Config` translation.
18     pub function_references_enabled: bool,
19     pub component_model_async: bool,
20     pub component_model_async_builtins: bool,
21     pub component_model_async_stackful: bool,
22     pub component_model_error_context: bool,
23     pub component_model_gc: bool,
24     pub legacy_exceptions: bool,
25 }
26 
27 impl<'a> Arbitrary<'a> for ModuleConfig {
28     fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<ModuleConfig> {
29         let mut config = wasm_smith::Config::arbitrary(u)?;
30 
31         // This list is intended to be the definitive source of truth for
32         // what's at least possible to fuzz within Wasmtime. This is a
33         // combination of features in `wasm-smith` where some proposals are
34         // on-by-default (as determined by fuzz input) and others are
35         // off-by-default (as they aren't stage4+). Wasmtime will default-fuzz
36         // proposals that a pre-stage-4 to test our own implementation. Wasmtime
37         // might also unconditionally disable proposals that it doesn't
38         // implement yet which are stage4+. This is intended to be an exhaustive
39         // list of all the wasm proposals that `wasm-smith` supports and the
40         // fuzzing status within Wasmtime too.
41         let _ = config.multi_value_enabled;
42         let _ = config.saturating_float_to_int_enabled;
43         let _ = config.sign_extension_ops_enabled;
44         let _ = config.bulk_memory_enabled;
45         let _ = config.reference_types_enabled;
46         let _ = config.simd_enabled;
47         let _ = config.relaxed_simd_enabled;
48         let _ = config.tail_call_enabled;
49         let _ = config.extended_const_enabled;
50         let _ = config.gc_enabled;
51         let _ = config.exceptions_enabled;
52         config.custom_page_sizes_enabled = u.arbitrary()?;
53         config.wide_arithmetic_enabled = u.arbitrary()?;
54         config.memory64_enabled = u.ratio(1, 20)?;
55         config.threads_enabled = u.ratio(1, 20)?;
56         // Allow multi-memory but make it unlikely
57         if u.ratio(1, 20)? {
58             config.max_memories = config.max_memories.max(2);
59         } else {
60             config.max_memories = 1;
61         }
62         // ... NB: if you add something above this line please be sure to update
63         // `docs/stability-wasm-proposals.md`
64 
65         // We get better differential execution when we disallow traps, so we'll
66         // do that most of the time.
67         config.disallow_traps = u.ratio(9, 10)?;
68 
69         Ok(ModuleConfig {
70             component_model_async: false,
71             component_model_async_builtins: false,
72             component_model_async_stackful: false,
73             component_model_error_context: false,
74             component_model_gc: false,
75             legacy_exceptions: false,
76             function_references_enabled: config.gc_enabled,
77             config,
78         })
79     }
80 }
81 
82 impl ModuleConfig {
83     /// Uses this configuration and the supplied source of data to generate a
84     /// Wasm module.
85     ///
86     /// If a `default_fuel` is provided, the resulting module will be configured
87     /// to ensure termination; as doing so will add an additional global to the
88     /// module, the pooling allocator, if configured, must also have its globals
89     /// limit updated.
90     pub fn generate(
91         &self,
92         input: &mut Unstructured<'_>,
93         default_fuel: Option<u32>,
94     ) -> arbitrary::Result<wasm_smith::Module> {
95         crate::init_fuzzing();
96 
97         // If requested, save `*.{dna,json}` files for recreating this module
98         // in wasm-tools alone.
99         let input_before = if log::log_enabled!(log::Level::Debug) {
100             let len = input.len();
101             Some(input.peek_bytes(len).unwrap().to_vec())
102         } else {
103             None
104         };
105 
106         let mut module = wasm_smith::Module::new(self.config.clone(), input)?;
107 
108         if let Some(before) = input_before {
109             static GEN_CNT: AtomicUsize = AtomicUsize::new(0);
110             let used = before.len() - input.len();
111             let i = GEN_CNT.fetch_add(1, Relaxed);
112             let dna = format!("testcase{i}.dna");
113             let config = format!("testcase{i}.json");
114             log::debug!("writing `{dna}` and `{config}`");
115             std::fs::write(&dna, &before[..used]).unwrap();
116             std::fs::write(&config, serde_json::to_string_pretty(&self.config).unwrap()).unwrap();
117         }
118 
119         if let Some(default_fuel) = default_fuel {
120             module.ensure_termination(default_fuel).unwrap();
121         }
122 
123         Ok(module)
124     }
125 }
126