deal with misaligned end of keccaks

contracts/MIPSMemory.sol

@@ -29,10 +29,10 @@ contract MIPSMemory {
     require(p.length == 0 || p.length == len, "length is somehow wrong");
     p.length = len;
     p.data[offset] = (1 << 32) |
-                     (uint32(uint8(anything[offset+0])) << 24) |
-                     (uint32(uint8(anything[offset+1])) << 16) |
-                     (uint32(uint8(anything[offset+2])) << 8) |
-                     (uint32(uint8(anything[offset+3])) << 0);
+                     ((len <= (offset+0) ? 0 : uint32(uint8(anything[offset+0]))) << 24) |
+                     ((len <= (offset+1) ? 0 : uint32(uint8(anything[offset+1]))) << 16) |
+                     ((len <= (offset+2) ? 0 : uint32(uint8(anything[offset+2]))) << 8) |
+                     ((len <= (offset+3) ? 0 : uint32(uint8(anything[offset+3]))) << 0);
   }
 
   // one per owner (at a time)
@@ -131,12 +131,16 @@ contract MIPSMemory {
     // MMIO preimage oracle
     if (addr >= 0x31000000 && addr < 0x32000000) {
       bytes32 pihash = ReadBytes32(stateHash, 0x30001000);
+      if (pihash == keccak256("")) {
+        // both the length and any data are 0
+        return 0;
+      }
       if (addr == 0x31000000) {
         return uint32(preimage[pihash].length);
       }
       uint offset = addr-0x31000004;
       uint64 data = preimage[pihash].data[offset];
-      require(data > 0, "offset must be loaded in");
+      require(data > 0, "offset not loaded");
       return uint32(data);
     }
 

package.json

@@ -3,6 +3,7 @@
     "@nomiclabs/hardhat-ethers": "^2.0.2",
     "@types/node": "^16.9.2",
     "chai": "^4.3.4",
+    "ethereum-waffle": "^3.4.0",
     "ethers": "^5.4.7",
     "hardhat": "^2.6.4",
     "hardhat-gas-reporter": "^1.0.4",

test/mips_test_oracle.js

 const { keccak256 } = require("@ethersproject/keccak256");
 const { expect } = require("chai");
+
+const chai = require("chai");
+const { solidity } = require("ethereum-waffle");
+chai.use(solidity);
+
 const { writeMemory } = require("../scripts/lib")
 
+async function loadPreimageAndSelect(mm, data, offset) {
+  // add in the preimage at offset 4
+  const hash = keccak256(data)
+  await mm.AddPreimage(data, offset)
+
+  // write the oracle selection address
+  let root = "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421"
+  root = await writeMemory(mm, root, 0x30001000, hash, true)
+  return root
+}
+
 describe("MIPSMemory oracle", function () {
   beforeEach(async function () {
     const MIPSMemory = await ethers.getContractFactory("MIPSMemory")
@@ -9,19 +25,21 @@ describe("MIPSMemory oracle", function () {
     await mm.AddTrieNode(new Uint8Array([0x80]))
   })
   it("simple oracle", async function() {
-    // add in the preimage at offset 4
-    const data = [0x11,0x22,0x33,0x44,0xaa,0xbb,0xcc,0xdd]
-    const hash = keccak256(data)
-    await mm.AddPreimage(data, 4)
-
-    // write the oracle selection address
-    let root = "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421"
-    root = await writeMemory(mm, root, 0x30001000, hash, true)
+    root = await loadPreimageAndSelect(mm, [0x11,0x22,0x33,0x44,0xaa,0xbb,0xcc,0xdd], 4)
 
     // length is 8
     expect(await mm.ReadMemory(root, 0x31000000)).to.equal(8)
 
     // offset 4 is 0xaabbccdd
     expect(await mm.ReadMemory(root, 0x31000008)).to.equal(0xaabbccdd)
+
+    // offset 0 isn't loaded
+    await expect(mm.ReadMemory(root, 0x31000004)).to.be.revertedWith("offset not loaded")
+  })
+
+  it("misaligned oracle", async function() {
+    root = await loadPreimageAndSelect(mm, [0x11,0x22,0x33,0x44,0xaa,0xbb,0xcc], 4)
+    expect(await mm.ReadMemory(root, 0x31000000)).to.equal(7)
+    expect(await mm.ReadMemory(root, 0x31000008)).to.equal(0xaabbcc00)
   })
 })
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