[pro-laxmi]

Description

When building IE_Indication, IE_MMContext_EPS, or PCO_S_Nssai with default parameters or without manually calculating and passing length, ConditionalField conditions evaluate pkt.length > ... when pkt.length is None. In Python 3, this raises TypeError: '>' not supported between instances of 'NoneType' and 'int':

from scapy.all import raw, load_contrib
load_contrib("gtp_v2")
from scapy.contrib.gtp_v2 import IE_Indication, IE_MMContext_EPS, PCO_S_Nssai

raw(IE_Indication())
# TypeError: While building field 'DAF': '>' not supported between instances of 'NoneType' and 'int'

raw(IE_MMContext_EPS())
# TypeError: While building field 'fields': '>' not supported between instances of 'NoneType' and 'int'

raw(PCO_S_Nssai())
# TypeError: While building field 'SST': '>' not supported between instances of 'NoneType' and 'int'

Fix

1. IE_Indication: Set default length to 3 (matching the 3 mandatory octets of basic flags defined in 3GPP TS 29.274 Table 8.12-1: Octets 5–7). Dissection and explicit lengths (length=8) remain unchanged.
2. IE_MMContext_EPS: In ConditionalField and StrLenField("fields", ...), guard pkt.length against None (pkt.length > 40 if pkt.length is not None else False) so the 41-octet base EPS context builds cleanly without an explicit length.
3. PCO_S_Nssai: In ConditionalField, guard against pkt.length is None, defaulting mandatory SST to True, allowing PCO_Protocol.post_build to automatically calculate the length.

Tests

Added test cases in test/contrib/gtp_v2.uts verifying that IE_Indication(PS=1), IE_MMContext_EPS(), and PCO_S_Nssai build and dissect cleanly without requiring an explicit length parameter. Full unit tests pass via test/run_tests -P "load_contrib('gtp_v2')" -t test/contrib/gtp_v2.uts.