feat(wasm-smith): add reserved_memory_size (#5)

crates/wasm-smith/src/config.rs

@@ -323,6 +323,11 @@ pub trait Config: 'static + std::fmt::Debug {
         false
     }
 
+    /// The size of reserved memory on the last memory page. Defaults to None.
+    fn reserved_memory_size(&self) -> Option<u64> {
+        None
+    }
+
     /// Determines whether the tail calls proposal is enabled for generating
     /// instructions.
     ///
@@ -560,6 +565,7 @@ pub struct SwarmConfig {
     pub min_uleb_size: u8,
     pub multi_value_enabled: bool,
     pub reference_types_enabled: bool,
+    pub reserved_memory_size: Option<u64>,
     pub tail_call_enabled: bool,
     pub relaxed_simd_enabled: bool,
     pub saturating_float_to_int_enabled: bool,
@@ -595,6 +601,7 @@ impl<'a> Arbitrary<'a> for SwarmConfig {
             max_tables,
             max_memory_pages: u.arbitrary()?,
             min_uleb_size: u.int_in_range(0..=5)?,
+            reserved_memory_size: None,
             bulk_memory_enabled: reference_types_enabled || u.arbitrary()?,
             reference_types_enabled,
             simd_enabled: u.arbitrary()?,
@@ -784,6 +791,10 @@ impl Config for SwarmConfig {
         self.reference_types_enabled
     }
 
+    fn reserved_memory_size(&self) -> Option<u64> {
+        self.reserved_memory_size
+    }
+
     fn tail_call_enabled(&self) -> bool {
         self.tail_call_enabled
     }

crates/wasm-smith/src/core/code_builder.rs

@@ -2,12 +2,11 @@ use super::{
     Elements, FuncType, GlobalInitExpr, Instruction, InstructionKind::*, InstructionKinds, Module,
     ValType,
 };
-use crate::unique_string;
 use arbitrary::{Result, Unstructured};
 use std::collections::{BTreeMap, BTreeSet};
 use std::convert::TryFrom;
 use std::rc::Rc;
-use wasm_encoder::{BlockType, ConstExpr, ExportKind, GlobalType, MemArg, RefType};
+use wasm_encoder::{BlockType, ConstExpr, GlobalType, MemArg, RefType};
 mod no_traps;
 
 macro_rules! instructions {
@@ -4626,6 +4625,13 @@ fn memory_offset(u: &mut Unstructured, module: &Module, memory_index: u32) -> Re
 
     let choice = u.int_in_range(0..=a + b + c - 1)?;
     if choice < a {
+        // Here we guarantee that writing to reserved memory will never happen.
+        // 16 is the number of bytes of the largest load type (V128).
+        let min = module
+            .config
+            .reserved_memory_size()
+            .map(|reserved| min.saturating_sub(reserved).saturating_sub(16))
+            .unwrap_or(min);
         u.int_in_range(0..=min)
     } else if choice < a + b {
         u.int_in_range(min..=max)