fix modexp gasFee

nimbus/vm/interpreter/utils/utils_numeric.nim

@@ -68,16 +68,35 @@ proc rangeToPadded*[T: StUint](x: openarray[byte], first, last: int, toLen = 0):
 
   if toLen > hi-lo+1:
     var temp: array[N, byte]
-    temp[0..hi-lo] = x.toOpenArray(lo, hi)
+    temp[0..hi-lo] = x.toOpenArray(lo, hi)    
     result = T.fromBytesBE(
       temp,
-      allowPadding = true
+      allowPadding = false
     )
   else:
     result = T.fromBytesBE(
       x.toOpenArray(lo, hi),
       allowPadding = true
     )
+    
+proc rangeToPadded2*[T: StUint](x: openarray[byte], first, last: int, toLen = 0): T =
+  ## Convert take a slice of a sequence of bytes interpret it as the big endian
+  ## representation of an Uint256. Use padding for sequence shorter than 32 bytes
+  ## including 0-length sequences.
+  const N = T.bits div 8
+
+  let lo = max(0, first)
+  let hi = min(min(x.high, last), (lo+N)-1)
+
+  if not(lo <= hi):
+    return # 0
+
+  var temp: array[N, byte]
+  temp[0..hi-lo] = x.toOpenArray(lo, hi)    
+  result = T.fromBytesBE(
+    temp.toOpenArray(0, toLen-1),
+    allowPadding = true
+  )
 
 # calculates the memory size required for a step
 func calcMemSize*(offset, length: int): int {.inline.} =

nimbus/vm/interpreter_dispatch.nim

@@ -323,17 +323,19 @@ proc selectVM(computation: BaseComputation, fork: Fork) {.gcsafe.} =
     computation.constantinopleVM()
 
 proc executeOpcodes(computation: BaseComputation) =
-  try:
+  let fork = computation.getFork
-    let fork = computation.getFork
-    if computation.execPrecompiles(fork):
-      computation.nextProc()
-      return
-    computation.selectVM(fork)
-  except:
-    let msg = getCurrentExceptionMsg()
-    computation.setError(&"Opcode Dispatch Error msg={msg}, depth={computation.msg.depth}", true)
-  computation.nextProc()
 
+  block:
+    if computation.execPrecompiles(fork):
+      break
+
+    try:
+      computation.selectVM(fork)
+    except:
+      let msg = getCurrentExceptionMsg()
+      computation.setError(&"Opcode Dispatch Error msg={msg}, depth={computation.msg.depth}", true)
+
+  computation.nextProc()
   if computation.isError():
     if computation.tracingEnabled: computation.traceError()
     debug "executeOpcodes error", msg=computation.error.info

nimbus/vm/precompiles.nim

@@ -150,11 +150,11 @@ proc modExpInternal(computation: BaseComputation, base_len, exp_len, mod_len: in
 
     let adj_exp_len = block:
       # TODO deal with negative length
+      let first32 = rawMsg.rangeToPadded2[:Uint256](96 + base_len, 95 + base_len + exp_len, min(exp_len, 32))
       if exp_len <= 32:
-        if exp.isZero(): 0
+        if first32.isZero(): 0
-        else: log2(exp)    # highest-bit in exponent
+        else: log2(first32)    # highest-bit in exponent
       else:
-        let first32 = rawMsg.rangeToPadded[:Uint256](96 + base_len, 95 + base_len + exp_len)
         # TODO: `modexpRandomInput.json` require Uint256 arithmetic for this code below
         if not first32.isZero:
           8 * (exp_len - 32) + first32.log2