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Invalid masks cause panic

This minimal example for 10.0.0.0/40

package main

import (
	"github.com/malfunkt/iprange"
)

func main() {
	_, _ = iprange.ParseList("10.0.0.0/40")
}

results in a panic:

$ go run test1.go
panic: runtime error: index out of range

goroutine 1 [running]:
encoding/binary.binary.bigEndian.Uint32(...)
        Go/src/encoding/binary/binary.go:111
github.com/malfunkt/iprange.(*ipParserImpl).Parse(0xc04209e000, 0x500ba0, 0xc04209c050, 0x0)
        yaccpar:355 +0x2007
github.com/malfunkt/iprange.ipParse(0x500ba0, 0xc04209c050, 0xb)
        yaccpar:153 +0x5f
github.com/malfunkt/iprange.ParseList(0x4ec90f, 0xb, 0x0, 0x0, 0x4b2533, 0xc042085f78, 0xc042054058)
        ip.y:93 +0xbe
main.main()
        test1.go:8 +0x3d
exit status 2

With an invalid mask, the mask variable coming from net.CIDRMask at https://github.com/malfunkt/iprange/blob/master/y.go#L502 is nil:

case 5:
  ipDollar = ipS[ippt-3 : ippt+1]
  //line ip.y:54
  {
    mask := net.CIDRMask(int(ipDollar[3].num), 32) // mask == nil if original mask is invalid
    min := ipDollar[1].addrRange.Min.Mask(mask)   // min == nil in this case too
    maxInt := binary.BigEndian.Uint32([]byte(min)) +
      0xffffffff -
      binary.BigEndian.Uint32([]byte(mask)) // nil passed to Uint32 causes a panic 
    maxBytes := make([]byte, 4)

Passing a nil or short byte[] to `Uint32 results in a panic. The bounds check hint which causes the panic can be seen here:

func (bigEndian) Uint32(b []byte) uint32 {
  _ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
  return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
}

Solution

I am not very familiar with the codebase so these might be way off.

Quick: Check mask' and minvalues in [y.go#L502](https://github.com/malfunkt/iprange/blob/master/y.go#L502) before passing it toUint32`
Not quick but IMO better solution: Check input for validity before passing it down. For example check if mask value is valid (16-30 if I am not mistaken).

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