fuzz: allow generating shared memories (#4266)

`wasm-smith` v0.11 has support for generating shared memories when the
`threads_enabled` configuration flag is set. This change turns on that
flag occasionally. This also upgrades `wasm-smith` to v0.11.1 to always
generate shared memory with a known maximum.

Cargo.lock

@@ -3237,9 +3237,9 @@ dependencies = [
 
 [[package]]
 name = "wasm-smith"
-version = "0.11.0"
+version = "0.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4f0ed86e26ccdda3c178e557f8f2874e277c8d4f29c88e3c3320c58db12840ed"
+checksum = "c85cf25be85aac46356216b4662eb5768347046449a45c938ae1443b788665bb"
 dependencies = [
  "arbitrary",
  "flagset",

crates/fuzzing/Cargo.toml

@@ -20,7 +20,7 @@ wasmprinter = "0.2.36"
 wasmtime = { path = "../wasmtime" }
 wasmtime-wast = { path = "../wast" }
 wasm-encoder = "0.13.0"
-wasm-smith = "0.11.0"
+wasm-smith = "0.11.1"
 wasm-mutate = "0.2.4"
 wasm-spec-interpreter = { path = "./wasm-spec-interpreter", optional = true }
 wasmi = "0.7.0"

crates/fuzzing/src/generators.rs

@@ -163,6 +163,11 @@ impl<'a> Arbitrary<'a> for Config {
             ..
         } = &config.wasmtime.strategy
         {
+            // If the pooling allocator is used, do not allow shared memory to
+            // be created. FIXME: see
+            // https://github.com/bytecodealliance/wasmtime/issues/4244.
+            config.module_config.config.threads_enabled = false;
+
             // Force the use of a normal memory config when using the pooling allocator and
             // limit the static memory maximum to be the same as the pooling allocator's memory
             // page limit.
@@ -296,6 +301,7 @@ impl Config {
         config.reference_types_enabled = false;
         config.simd_enabled = false;
         config.memory64_enabled = false;
+        config.threads_enabled = false;
 
         // If using the pooling allocator, update the instance limits too
         if let InstanceAllocationStrategy::Pooling {
@@ -343,11 +349,11 @@ impl Config {
     pub fn set_spectest_compliant(&mut self) {
         let config = &mut self.module_config.config;
         config.memory64_enabled = false;
-        config.simd_enabled = false;
         config.bulk_memory_enabled = true;
         config.reference_types_enabled = true;
         config.multi_value_enabled = true;
         config.simd_enabled = true;
+        config.threads_enabled = false;
         config.max_memories = 1;
         config.max_tables = 5;
 
@@ -388,6 +394,7 @@ impl Config {
             .wasm_multi_memory(self.module_config.config.max_memories > 1)
             .wasm_simd(self.module_config.config.simd_enabled)
             .wasm_memory64(self.module_config.config.memory64_enabled)
+            .wasm_threads(self.module_config.config.threads_enabled)
             .wasm_backtrace(self.wasmtime.wasm_backtraces)
             .cranelift_nan_canonicalization(self.wasmtime.canonicalize_nans)
             .cranelift_opt_level(self.wasmtime.opt_level.to_wasmtime())
@@ -434,6 +441,17 @@ impl Config {
             }
         }
 
+        // Vary the memory configuration, but only if threads are not enabled.
+        // When the threads proposal is enabled we might generate shared memory,
+        // which is less amenable to different memory configurations:
+        // - shared memories are required to be "static" so fuzzing the various
+        //   memory configurations will mostly result in uninteresting errors.
+        //   The interesting part about shared memories is the runtime so we
+        //   don't fuzz non-default settings.
+        // - shared memories are required to be aligned which means that the
+        //   `CustomUnaligned` variant isn't actually safe to use with a shared
+        //   memory.
+        if !self.module_config.config.threads_enabled {
             match &self.wasmtime.memory_config {
                 MemoryConfig::Normal(memory_config) => {
                     cfg.static_memory_maximum_size(
@@ -451,6 +469,7 @@ impl Config {
                         .guard_before_linear_memory(false);
                 }
             }
+        }
 
         return cfg;
     }
@@ -634,6 +653,11 @@ impl<'a> Arbitrary<'a> for ModuleConfig {
         // `SwarmConfig::arbitrary`.
         config.memory64_enabled = u.arbitrary()?;
 
+        // Allow the threads proposal if memory64 is not already enabled. FIXME:
+        // to allow threads and memory64 to coexist, see
+        // https://github.com/bytecodealliance/wasmtime/issues/4267.
+        config.threads_enabled = !config.memory64_enabled && u.arbitrary()?;
+
         Ok(ModuleConfig { config })
     }
 }

fuzz/README.md

@@ -43,7 +43,7 @@ At the time of writing, we have the following fuzz targets:
   the same results as the `wasmi` interpreter.
 * `instantiate`: Generate a Wasm module and Wasmtime configuration and attempt
   to compile and instantiate with them.
-* `instantiate`: Generate many Wasm modules and attempt to compile and
+* `instantiate-many`: Generate many Wasm modules and attempt to compile and
   instantiate them concurrently.
 * `spectests`: Pick a random spec test and run it with a generated
   configuration.

fuzz/fuzz_targets/differential_v8.rs

@@ -17,9 +17,12 @@ fn run(data: &[u8]) -> Result<()> {
     config.set_differential_config();
 
     // Enable features that v8 has implemented
-    config.module_config.config.simd_enabled = true;
-    config.module_config.config.bulk_memory_enabled = true;
-    config.module_config.config.reference_types_enabled = true;
+    config.module_config.config.simd_enabled = u.arbitrary()?;
+    config.module_config.config.bulk_memory_enabled = u.arbitrary()?;
+    config.module_config.config.reference_types_enabled = u.arbitrary()?;
+    // FIXME: to enable fuzzing with the threads proposal, see
+    // https://github.com/bytecodealliance/wasmtime/issues/4268.
+    // config.module_config.config.threads_enabled = u.arbitrary()?;
 
     // Allow multiple tables, as set_differential_config() assumes reference
     // types are disabled and therefore sets max_tables to 1