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Zombie connection after lightningd ignores `connectd_peer_spoke`

Aperta
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Dal 2/8/2026.

  • #9370 di @morehouse — aperta

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
55/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Tranquilla
Stack tecnologico
c
Ambito
networking

Direzione di ricerca

Inizia dalla gestione collegata in connectd/multiplex.c e lightningd/peer_control.c, in particolare dai sei percorsi in cui handle_peer_spoke non risponde. Segui il messaggio connectd_peer_no_subd proposto attraverso entrambi i componenti; il lavoro è completo quando l'avvio o la ricerca falliti del subdaemon liberano il subd corrispondente e connectd riprende a leggere i messaggi del peer.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

When connectd reads a peer message whose channel_id has no attached subd, it creates a subd with conn == NULL and sends connectd_peer_spoke to lightningd, asking it to start up a subdaemon and respond with connectd_peer_connect_subd and the file descriptor that should be assigned to conn.

While connectd is waiting for lightningd's response, it stops reading all messages from the peer:

https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/connectd/multiplex.c#L1506-L1538

The io_wait at the end is only released when either lightningd responds, or when CLN needs to send a message to that peer (e.g., gossip flush). If neither happens, the connection becomes a zombie and all peer messages are ignored until eventually the ping timeout expires and the connection is dropped.

There are currently six situations where lightningd's handle_peer_spoke never responds:

  1. The peer sends an error for its first message (e.g., data loss recovery).
    https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/lightningd/peer_control.c#L2067-L2072

  2. channeld dies from a bug or protocol violation, and the peer sends another message for that channel before lightningd recognizes the channeld died. An alternative (benign) way to trigger this is when lightningd has already created the requested subdaemon but connectd hasn't processed it yet.
    https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/lightningd/peer_control.c#L2074-L2081

  3. The peer attempts a channel_reestablish while the node is shutting down.
    https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/lightningd/peer_control.c#L2083-L2097

  4. The peer sends a message after channeld was killed without a status message (e.g., OOM).
    https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/lightningd/peer_control.c#L2100-L2109

  5. openingd fails to spawn (e.g., fd limit reached).
    https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/lightningd/peer_control.c#L2143-L2145

  6. dualopend fails to spawn (e.g., fd limit reached).
    https://github.com/ElementsProject/lightning/blob/ae53e8775e57ddf8debfd6fc7b8e3cb5e2c93dc6/lightningd/peer_control.c#L2163-L2165

Suggested fix

Add a new message connectd_peer_no_subd that lightningd can respond with when handle_peer_spoke fails in the above situations. Then connectd knows to free the matching subd and continue reading from the connection.

Discovery

This bug was discovered while fuzzing CLN with smite. Smite would send a channel_ready message with an incorrect channel_id, causing channeld to exit. Then smite would send another channel message, which would trigger the Situation 2 race condition and zombify the connection.

Lingua principale
C
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Merge medio
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PR unite (30g)
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