cannon: Fix oracle tests for mipsevm

cannon/mipsevm/open_mips_tests/test/oracle.asm

-    .section .test, "x"
+.section .test, "x"
     .balign 4
     .set    noreorder
     .global test
     .ent    test
 
 # load hash at 0x30001000
-# 0x47173285 a8d7341e 5e972fc6 77286384 f802f8ef 42a5ec5f 03bbfa25 4cb01fad = "hello world"
+# 0x47173285 a8d7341e 5e972fc6 77286384 f802f8ef 42a5ec5f 03bbfa25 4cb01fad = keccak("hello world")
+# 0x02173285 a8d7341e 5e972fc6 77286384 f802f8ef 42a5ec5f 03bbfa25 4cb01fad = keccak("hello world").key
 test:
   lui $s0, 0x3000
   ori $s0, 0x1000
 
-  lui $t0, 0x4717
+  lui $t0, 0x0217
   ori $t0, 0x3285
   sw $t0, 0($s0)
   lui $t0, 0xa8d7
@@ -35,20 +36,53 @@ test:
   ori $t0, 0x1fad
   sw $t0, 0x1c($s0)
 
-# syscall 4020 to trigger
-  li $v0, 4020
+# preimage request - write(fdPreimageWrite, preimageData, 32)
+  li $a0, 6
+  li $a1, 0x30001000
+  li $t0, 8
+  li $a2, 4
+$writeloop:
+  li $v0, 4004
   syscall
+  addiu $a1, $a1, 4
+  addiu $t0, $t0, -1
+  bnez $t0, $writeloop
+  nop
+
+# preimage response to 0x30002000 - read(fdPreimageRead, addr, count)
+# read preimage length
+  li $a0, 5
+  li $a1, 0x31000000
+  li $a2, 4
+  li $v0, 4003
+  syscall
+  li $a1, 0x31000004
+  li $v0, 4003
+  syscall
+# read the preimage data
+  li $a1, 0x31000008
+  li $t0, 3
+$readloop:
+  li $v0, 4003
+  syscall
+  addiu $a1, $a1, 4
+  addiu $t0, $t0, -1
+  bnez $t0, $readloop
+  nop
 
-# length at 0x31000000
+# length at 0x31000000. We also check that the lower 32 bits are zero
   lui $s1, 0x3100
   lw $t0, 0($s1)
-
+  sltiu $t6, $t0, 1
+  li $s1, 0x31000004
+  lw $t0, 0($s1)
 # should be len("hello world") == 11
   li $t4, 11
   subu $t5, $t0, $t4
-  sltiu $v0, $t5, 1 
+  sltiu $v0, $t5, 1
+  and $v0, $v0, $t6
 
-# data at 0x31000004
+# data at 0x31000008
   lw $t0, 4($s1)
   lui $t4, 0x6865
   ori $t4, 0x6c6c

cannon/mipsevm/state_test.go

@@ -31,8 +31,9 @@ func TestState(t *testing.T) {
 
 	for _, f := range testFiles {
 		t.Run(f.Name(), func(t *testing.T) {
+			var oracle PreimageOracle
 			if f.Name() == "oracle.bin" {
-				t.Skip("oracle test needs to be updated to use syscall pre-image oracle")
+				oracle = staticOracle(t, []byte("hello world"))
 			}
 			// TODO: currently tests are compiled as flat binary objects
 			// We can use more standard tooling to compile them to ELF files and get remove maketests.py
@@ -50,7 +51,7 @@ func TestState(t *testing.T) {
 			// set the return address ($ra) to jump into when test completes
 			state.Registers[31] = endAddr
 
-			us := NewInstrumentedState(state, nil, os.Stdout, os.Stderr)
+			us := NewInstrumentedState(state, oracle, os.Stdout, os.Stderr)
 
 			for i := 0; i < 1000; i++ {
 				if us.state.PC == endAddr {
@@ -196,3 +197,15 @@ func TestClaim(t *testing.T) {
 	require.Equal(t, expectedStdOut, stdOutBuf.String(), "stdout")
 	require.Equal(t, expectedStdErr, stdErrBuf.String(), "stderr")
 }
+
+func staticOracle(t *testing.T, preimageData []byte) *testOracle {
+	return &testOracle{
+		hint: func(v []byte) {},
+		getPreimage: func(k [32]byte) []byte {
+			if k != preimage.Keccak256Key(crypto.Keccak256Hash(preimageData)).PreimageKey() {
+				t.Fatalf("invalid preimage request for %x", k)
+			}
+			return preimageData
+		},
+	}
+}