cannon: Fix unaligned preimage read

cannon/mipsevm/open_mips_tests/test/oracle_unaligned_read.asm

+.section .test, "x"
+    .balign 4
+    .set    noreorder
+    .global test
+    .ent    test
+
+# load hash at 0x30001000
+# 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, 0x0217
+  ori $t0, 0x3285
+  sw $t0, 0($s0)
+  lui $t0, 0xa8d7
+  ori $t0, 0x341e
+  sw $t0, 4($s0)
+  lui $t0, 0x5e97
+  ori $t0, 0x2fc6
+  sw $t0, 8($s0)
+  lui $t0, 0x7728
+  ori $t0, 0x6384
+  sw $t0, 0xc($s0)
+  lui $t0, 0xf802
+  ori $t0, 0xf8ef
+  sw $t0, 0x10($s0)
+  lui $t0, 0x42a5
+  ori $t0, 0xec5f
+  sw $t0, 0x14($s0)
+  lui $t0, 0x03bb
+  ori $t0, 0xfa25
+  sw $t0, 0x18($s0)
+  lui $t0, 0x4cb0
+  ori $t0, 0x1fad
+  sw $t0, 0x1c($s0)
+
+# 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 to unaligned addr. This will read only up to the nearest aligned byte so we have to read again.
+  li $a0, 5
+  li $a1, 0x31000001
+  li $a2, 4
+  li $v0, 4003
+  syscall
+  li $a1, 0x31000004
+  li $v0, 4003
+  syscall
+  li $a1, 0x31000008
+  li $a2, 1
+  li $v0, 4003
+  syscall
+# read the preimage data
+  li $a1, 0x31000009
+  li $t0, 11
+$readloop:
+  li $v0, 4003
+  li $a2, 4
+  syscall
+  addu $a1, $a1, $v0
+  subu $t0, $t0, $v0
+  bnez $t0, $readloop
+  nop
+
+# length at 0x31000001. We also check that the lower 32 bits are zero
+  li $s1, 0x31000001
+  lb $t0, 0($s1)
+  lb $t2, 1($s1)
+  sll $t2, $t2, 8
+  or $t0, $t0, $t2
+  lb $t2, 2($s1)
+  sll $t2, $t2, 16
+  or $t0, $t0, $t2
+# assert len[0:3] == 0
+  sltiu $v0, $t0, 1
+
+# assert len[4:8] == 0
+  addiu $s1, $s1, 3
+  lw $t1, 0($s1)
+  sltiu $v1, $t1, 1
+  and $v0, $v0, $v1
+
+# assert len[8:9] == 11
+  addiu $s1, $s1, 4
+  lb $t2, 0($s1)
+  li $t4, 11
+  subu $t5, $t2, $t4
+  sltiu $v1, $t5, 1
+  and $v0, $v0, $v1
+
+# data at 0x31000009
+  addiu $s1, $s1, 1
+  lb $t0, 0($s1)
+  lb $t2, 1($s1)
+  sll $t0, $t0, 8
+  or $t0, $t0, $t2
+  lb $t2, 2($s1)
+  sll $t0, $t0, 8
+  or $t0, $t0, $t2
+  lb $t2, 3($s1)
+  sll $t0, $t0, 8
+  or $t0, $t0, $t2
+
+  #lw $t0, 0($s1)
+  lui $t4, 0x6865
+  ori $t4, 0x6c6c
+  subu $t5, $t0, $t4
+  sltiu $v1, $t5, 1
+
+  and $v0, $v0, $v1
+
+# save results
+  lui     $s0, 0xbfff         # Load the base address 0xbffffff0
+  ori     $s0, 0xfff0
+  ori     $s1, $0, 1          # Prepare the 'done' status
+
+  sw      $v0, 8($s0)         # Set the test result
+  sw      $s1, 4($s0)         # Set 'done'
+
+$done:
+  jr      $ra
+  nop
+
+  .end test

packages/contracts-bedrock/contracts/cannon/MIPS.sol

@@ -166,7 +166,7 @@ contract MIPS {
                     if lt(space, datLen) { datLen := space } // if less space than data, shorten data
                     if lt(a2, datLen) { datLen := a2 } // if requested to read less, read less
                     dat := shr(sub(256, mul(datLen, 8)), dat) // right-align data
-                    dat := shl(mul(add(sub(4, datLen), alignment), 8), dat) // position data to insert into memory word
+                    dat := shl(mul(sub(sub(4, datLen), alignment), 8), dat) // position data to insert into memory word
                     let mask := sub(shl(mul(sub(4, alignment), 8), 1), 1) // mask all bytes after start
                     let suffixMask := sub(shl(mul(sub(sub(4, alignment), datLen), 8), 1), 1) // mask of all bytes starting from end, maybe none
                     mask := and(mask, not(suffixMask)) // reduce mask to just cover the data we insert