wasmtime_fuzzing/
mutators.rs

1//! Custom fuzz input mutators.
2//!
3//! The functions in this module are intended to be used with [the
4//! `libfuzzer_sys::fuzz_mutator!` macro][fuzz-mutator].
5//!
6//! [fuzz-mutator]: https://docs.rs/libfuzzer-sys/latest/libfuzzer_sys/macro.fuzz_mutator.html
7
8use arbitrary::{Arbitrary, Unstructured};
9use std::sync::Arc;
10
11/// Use [`wasm-mutate`][wasm-mutate] to mutate a fuzz input.
12///
13/// [wasm-mutate]: https://github.com/bytecodealliance/wasm-tools/tree/main/crates/wasm-mutate
14pub fn wasm_mutate(
15    data: &mut [u8],
16    size: usize,
17    max_size: usize,
18    seed: u32,
19    libfuzzer_mutate: fn(data: &mut [u8], size: usize, max_size: usize) -> usize,
20) -> usize {
21    const MUTATION_FUEL: u64 = 100;
22    const MUTATION_ITERS: usize = 100;
23
24    let wasm = &data[..size];
25
26    if wasmparser::validate(wasm).is_ok() {
27        let mut wasm_mutate = wasm_mutate::WasmMutate::default();
28        wasm_mutate
29            .seed(seed.into())
30            .fuel(MUTATION_FUEL)
31            .reduce(max_size < size)
32            .raw_mutate_func(Some(Arc::new(move |data, max_size| {
33                let len = data.len();
34
35                // The given max could be very large, so clamp it to no more
36                // than `len * 2` in any single, given mutation. This way we
37                // don't over-allocate a bunch of space.
38                let max_size = std::cmp::min(max_size, len * 2);
39                // Also, the max must always be greater than zero (`libfuzzer`
40                // asserts this).
41                let max_size = std::cmp::max(max_size, 1);
42
43                // Make sure we have capacity in case `libfuzzer` decides to
44                // grow this data.
45                if max_size > len {
46                    data.resize(max_size, 0);
47                }
48
49                // Finally, have `libfuzzer` mutate the data!
50                let new_len = libfuzzer_mutate(data, len, max_size);
51
52                // Resize the data to the mutated size, releasing any extra
53                // capacity that we don't need anymore.
54                data.resize(new_len, 0);
55                data.shrink_to_fit();
56
57                Ok(())
58            })));
59
60        let wasm = wasm.to_vec();
61        let mutations = wasm_mutate.run(&wasm);
62        if let Ok(mutations) = mutations {
63            for mutation in mutations.take(MUTATION_ITERS) {
64                if let Ok(mutated_wasm) = mutation {
65                    if mutated_wasm.len() <= max_size {
66                        data[..mutated_wasm.len()].copy_from_slice(&mutated_wasm);
67                        return mutated_wasm.len();
68                    }
69                }
70            }
71        }
72    }
73
74    // If we can't mutate the input because it isn't valid Wasm or `wasm-mutate`
75    // otherwise fails, try to use `wasm-smith` to generate a new, arbitrary
76    // Wasm module that fits within the max-size limit.
77    let mut u = Unstructured::new(&data[..max_size]);
78    if let Ok(module) = wasm_smith::Module::arbitrary(&mut u) {
79        let wasm = module.to_bytes();
80        if wasm.len() <= max_size {
81            data[..wasm.len()].copy_from_slice(&wasm);
82            return wasm.len();
83        }
84    }
85
86    // Otherwise, try to return an empty Wasm module:
87    //
88    // ```
89    // (module)
90    // ```
91    static EMPTY_WASM: &[u8] = &[0x00, b'a', b's', b'm', 0x01, 0x00, 0x00, 0x00];
92    if EMPTY_WASM.len() <= max_size {
93        data[..EMPTY_WASM.len()].copy_from_slice(EMPTY_WASM);
94        return EMPTY_WASM.len();
95    }
96
97    // If the max size is even smaller than an empty Wasm module, then just let
98    // `libfuzzer` mutate the data.
99    libfuzzer_mutate(data, size, max_size)
100}
wasmtime_fuzzing/mutators.rs

wasmtime_fuzzing/
mutators.rs
