llvm-context: do not trap zero length OOB heap access (#389)

Fixes https://github.com/paritytech/contract-issues/issues/120

---------

Signed-off-by: xermicus <cyrill@parity.io>

CHANGELOG.md

@@ -6,6 +6,9 @@ This is a development pre-release.
 
 Supported `polkadot-sdk` rev: `2503.0.1`
 
+### Chnaged
+- Emulated EVM heap memory accesses of zero length are never out of bounds.
+
 ## v0.4.1
 
 This is a development pre-release.

Cargo.lock

@@ -8819,7 +8819,7 @@ dependencies = [
 
 [[package]]
 name = "revive-llvm-context"
-version = "0.4.1"
+version = "0.5.0"
 dependencies = [
  "anyhow",
  "hex",

Cargo.toml

@@ -24,7 +24,7 @@ revive-differential = { version = "0.2.0", path = "crates/differential" }
 revive-explorer = { version = "0.1.0", path = "crates/explore" }
 revive-integration = { version = "0.2.0", path = "crates/integration" }
 revive-linker = { version = "0.2.0", path = "crates/linker" }
-revive-llvm-context = { version = "0.4.1", path = "crates/llvm-context" }
+revive-llvm-context = { version = "0.5.0", path = "crates/llvm-context" }
 revive-runner = { version = "0.2.0", path = "crates/runner" }
 revive-runtime-api = { version = "0.3.0", path = "crates/runtime-api" }
 revive-solc-json-interface = { version = "0.4.0", path = "crates/solc-json-interface", default-features = false }

crates/integration/codesize.json

@@ -1,10 +1,10 @@
 {
-  "Baseline": 932,
+  "Baseline": 914,
-  "Computation": 2313,
+  "Computation": 2295,
-  "DivisionArithmetics": 9103,
+  "DivisionArithmetics": 9085,
-  "ERC20": 17479,
+  "ERC20": 17499,
-  "Events": 1692,
+  "Events": 1674,
-  "FibonacciIterative": 1508,
+  "FibonacciIterative": 1490,
-  "Flipper": 2098,
+  "Flipper": 2102,
-  "SHA1": 8176
+  "SHA1": 8158
 }

crates/integration/contracts/MemoryBounds.sol

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.24;
+
+/* runner.json
+{
+  "differential": true,
+  "actions": [
+    {
+      "Instantiate": {
+        "code": {
+          "Solidity": {
+            "contract": "MemoryBounds"
+          }
+        }
+      }
+    },
+    {
+      "Call": {
+        "dest": {
+          "Instantiated": 0
+        }
+      }
+    }
+  ]
+}
+*/
+
+contract MemoryBounds {
+    fallback() external {
+        assembly {
+            // Accessing OOB offsets should always work when the length is 0.
+            return(100000, 0)
+        }
+    }
+}

crates/integration/src/tests.rs

@@ -62,6 +62,7 @@ test_spec!(function_type, "FunctionType", "FunctionType.sol");
 test_spec!(layout_at, "LayoutAt", "LayoutAt.sol");
 test_spec!(shift_arithmetic_right, "SAR", "SAR.sol");
 test_spec!(add_mod_mul_mod, "AddModMulModTester", "AddModMulMod.sol");
+test_spec!(memory_bounds, "MemoryBounds", "MemoryBounds.sol");
 
 fn instantiate(path: &str, contract: &str) -> Vec<SpecsAction> {
     vec![Instantiate {

crates/llvm-context/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "revive-llvm-context"
-version = "0.4.1"
+version = "0.5.0"
 license.workspace = true
 edition.workspace = true
 repository.workspace = true

crates/llvm-context/src/polkavm/context/function/runtime/sbrk.rs

@@ -47,6 +47,17 @@ impl RuntimeFunction for Sbrk {
         let offset = Self::paramater(context, 0).into_int_value();
         let size = Self::paramater(context, 1).into_int_value();
 
+        let return_block = context.append_basic_block("return_pointer");
+        let body_block = context.append_basic_block("body");
+        let is_size_zero = context.builder().build_int_compare(
+            inkwell::IntPredicate::EQ,
+            size,
+            context.xlen_type().const_zero(),
+            "is_size_zero",
+        )?;
+        context.build_conditional_branch(is_size_zero, return_block, body_block)?;
+
+        context.set_basic_block(body_block);
         let trap_block = context.append_basic_block("trap");
         let offset_in_bounds_block = context.append_basic_block("offset_in_bounds");
         let is_offset_out_of_bounds = context.builder().build_int_compare(
@@ -91,7 +102,6 @@ impl RuntimeFunction for Sbrk {
         )?;
 
         context.set_basic_block(size_in_bounds_block);
-        let return_block = context.append_basic_block("return_pointer");
         let new_size_block = context.append_basic_block("new_size");
         let is_new_size = context.builder().build_int_compare(
             inkwell::IntPredicate::UGT,