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