fields: fix i2len of 3-byte fields returning 4 instead of 3

scapy/contrib/isis.py

@@ -350,7 +350,7 @@ class ISIS_UnreservedBandwidthSubTlv(ISIS_GenericSubTlv):
 class ISIS_TEDefaultMetricSubTlv(ISIS_GenericSubTlv):
     name = "TE Default Metric SubTLV"
     fields_desc = [ByteEnumField("type", 18, _isis_subtlv_names_1),
-                   FieldLenField("len", None, length_of="temetric", adjust=lambda pkt, x:x - 1, fmt="B"),  # noqa: E501
+                   FieldLenField("len", None, length_of="temetric", fmt="B"),
                    ThreeBytesField("temetric", 1000)]
 
 

scapy/fields.py

@@ -1072,6 +1072,7 @@ class ThreeBytesField(Field[int, int]):
     def __init__(self, name, default):
         # type: (str, int) -> None
         Field.__init__(self, name, default, "!I")
+        self.sz = 3  # emits/consumes 3 bytes; keep i2len consistent for FieldLenField
 
     def addfield(self, pkt, s, val):
         # type: (Packet, bytes, Optional[int]) -> bytes
@@ -1092,6 +1093,7 @@ class LEThreeBytesField(ByteField):
     def __init__(self, name, default):
         # type: (str, Optional[int]) -> None
         Field.__init__(self, name, default, "<I")
+        self.sz = 3  # emits/consumes 3 bytes; keep i2len consistent for FieldLenField
 
     def addfield(self, pkt, s, val):
         # type: (Packet, bytes, Optional[int]) -> bytes

test/fields.uts

@@ -174,6 +174,18 @@ print(p.sprintf('%test1% %test2% %test3% %test4%'))
 assert p.sprintf('%test1% %test2% %test3% %test4%') == '0x123456 123456 0xfedbca 567890'
 assert repr(p.test1) == '1193046'
 
+# i2len must match the 3 bytes actually emitted, so FieldLenField(length_of=)
+# on a 3-byte field computes the right length.
+for cls in [ThreeBytesField, X3BytesField, LEThreeBytesField, XLE3BytesField, OUIField]:
+    f = cls('x', 0)
+    assert f.i2len(None, 0x010203) == len(f.addfield(None, b'', 0x010203)) == 3, cls
+
+class ThreeBytesTLV(Packet):
+    fields_desc = [ FieldLenField('len', None, length_of='oui', fmt='B'),
+                    OUIField('oui', 0x00000c) ]
+
+assert ThreeBytesTLV(raw(ThreeBytesTLV())).len == 3
+
 
 = NBytesField
 ~ field nbytesfield