base-files: ipcalc.sh: Add netmask2prefix function

Also add is_contiguous to check if it's a valid netmask.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

package/base-files/files/bin/ipcalc.sh

@@ -64,7 +64,7 @@ fi
 hostmask=$((netmask ^ 0xffffffff))
 
 network=$((ipaddr & netmask))
-prefix=$((32 - $(bitcount $hostmask)))
+prefix=$(netmask2prefix "$netmask")
 broadcast=$((network | hostmask))
 
 count=$((hostmask + 1))

package/base-files/files/lib/functions/ipv4.sh

@@ -140,3 +140,24 @@ prefix2netmask() {
     echo "$(((~(uint_max >> n)) & uint_max))"
 }
 
+# check argument as being contiguous upper bits (and yes,
+# 0 doesn't have any discontiguous bits).
+is_contiguous() {
+    local x="$(check_uint32 "$1")"
+
+    local y=$((~x & uint_max))
+    local z=$(((y + 1) & uint_max))
+
+    [ $((z & y)) -eq 0 ]
+}
+
+# convert mask to prefix, validating that it's a conventional
+# (contiguous) netmask.
+netmask2prefix() {
+    local n="$(check_uint32 "$1")"
+
+    is_contiguous "$n" || __die "Not a contiguous netmask ($n)"
+
+    echo "$((32 - $(bitcount $n)))"
+}
+