J1939SoftSocket: expand edge-case unit tests, soft↔native interop coverage, and pgn filter

[Copilot]

Extends the J1939SoftSocket test suite with boundary/error edge cases and richer interoperability scenarios against NativeJ1939Socket. Also incorporates a pgn filter parameter inspired by BenGardiner's rx_pgn from PR #4961.

New feature: pgn filter parameter

Adds a pgn keyword argument to both J1939TPImplementation and J1939SoftSocket (default 0 = accept all PGNs). When set to a non-zero value, only messages whose PGN matches are delivered — covering single-frame, BAM, and RTS/CTS sessions. Inspired by BenGardiner's rx_pgn parameter.

Edge-case unit tests (TestSocket, no root)

• Single-frame boundary – 8-byte payload goes as one CAN frame; no TP.CM emitted
• TP boundary – 9-byte payload triggers BAM with exactly 2 DT frames; 14-byte payload fills both DT fields with no padding
• Session overwrite – new BAM from same peer resets an incomplete in-progress session
• Wrong-SA filtering – TP.DT frames from a different SA than the BAM initiator are dropped; correct-SA DT still accepted
• Priority preservation – J1939.priority (0/3/6/7) maps 1-to-1 to the CAN ID priority bits on the wire
• data_page=1 PGN – PGNs > 0xFFFF (e.g. 0x1FECA) encoded in BAM payload pgn field; TP.CM CAN ID keeps data_page=0 per spec
• Sequential messages – three back-to-back messages (mix of single-frame and BAM) delivered in order through one socket
• CTS hold – CTS(num_packets=0) pauses sender; no DT emitted until a releasing CTS(count>0) arrives
• TX timeout – after T3 (1.25 s) without CTS the TX state machine resets to IDLE and accepts new messages
• 255-byte BAM – 37 TP.DT frames encoded and reassembled end-to-end
• Large RTS/CTS (49 bytes) – full 7-DT unicast session between two soft sockets
• Loopback suppression – frames carrying own SA are silently discarded
• recv() after close – returns None, does not raise
• pgn filter – matching PGN delivered; non-matching single-frame and BAM silently dropped; pgn=0 accepts all PGNs

Interoperability tests (vcan, needs_root)

• PDU1 unicast short frame – soft→native and native→soft, both sides bound to specific SAs; dst field verified
• RTS/CTS unicast (20 bytes) soft→native – kernel J1939 stack completes the handshake and delivers one reassembled message
• Large BAM (100 bytes) in both directions – soft socket reassembles 15 DT frames from kernel; kernel reassembles 15 DT frames from soft socket
• CAN-level priority verification – two frames sent at priorities 3 and 6; confirmed in CAN ID via NativeCANSocket(basecls=J1939_CAN)
• Concurrent ping-pong – soft and native sockets exchange 3 messages simultaneously in both directions; payload and source address verified on both ends

[polybassa]

@copilot Analyze the J1939 SoftSocket implementation from BenGardiner:j1939_scanning https://github.com/secdev/scapy/pull/4961/changes#diff-5d6504c729d8bfec31ff30aec28e76f32ae70eba914b2338540fe2b13a221f34

Check for useful modifications

[Copilot]

@copilot Analyze the J1939 SoftSocket implementation from BenGardiner:j1939_scanning https://github.com/secdev/scapy/pull/4961/changes#diff-5d6504c729d8bfec31ff30aec28e76f32ae70eba914b2338540fe2b13a22...

Analyzed PR secdev#4961. Summary of what BenGardiner's implementation adds over ours, and what we incorporated:

Already in our implementation (confirmed equivalent):

• listen_only parameter – identical concept, fully implemented and tested
• Timeout extension factor (TP_DT_TIMEOUT_EXTENSION_FACTOR = 10) – we have _J1939_TP_DT_TIMEOUT_EXTENSION = 10 with the same total-elapsed-time logic; I clarified the comments to accurately describe the wall-clock ceiling approach (commit 63faffb)

Incorporated from BenGardiner's approach:

• pgn filter parameter (rx_pgn in his code) – added pgn_filter to J1939TPImplementation and pgn to J1939SoftSocket; pgn=0 accepts all PGNs (identical semantics to his rx_pgn=0); filters single-frame, BAM, and RTS/CTS sessions. Three new tests added (commit ea28d5a).

Not incorporated (out of scope / too complex for this PR):

• Configurable CMDT block size (bs parameter) – requires multi-block CTS state tracking
• J1939-81 address claiming (name parameter) – separate feature, significant complexity

Warning

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