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Merged
wpaulino merged 2 commits into from
May 19, 2026
Merged

fuzz: fix signature weight assumptions #4620

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f9269d1
Avoid grind signatures in fuzz builds
joostjager May 19, 2026
241ac47
Account for fuzz signature weight
joostjager May 19, 2026
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2 changes: 1 addition & 1 deletion fuzz/Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@ publish = false
edition = "2021"

[dependencies]
lightning = { path = "../lightning", features = ["regex", "_test_utils"] }
lightning = { path = "../lightning", default-features = false, features = ["std", "regex", "_test_utils"] }
lightning-invoice = { path = "../lightning-invoice" }
lightning-liquidity = { path = "../lightning-liquidity" }
lightning-rapid-gossip-sync = { path = "../lightning-rapid-gossip-sync" }
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18 changes: 9 additions & 9 deletions fuzz/src/full_stack.rs
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Expand Up @@ -1885,8 +1885,8 @@ fn splice_seed() -> Vec<u8> {
// CommitmentSigned message with proper signature (r=f7, s=01...) and funding_txid TLV
// signature r encodes sighash first byte f7, s follows the pattern from funding_created
// TLV type 1 (odd/optional) for funding_txid as per impl_writeable_msg!(CommitmentSigned, ...)
// Note: txid is encoded in reverse byte order (Bitcoin standard), so to get display 0000...0031, encode 3100...0000
ext_from_hex("0084 c000000000000000000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000000000000000f7 0100000000000000000000000000000000000000000000000000000000000000 0000 01 20 3100000000000000000000000000000000000000000000000000000000000000 03000000000000000000000000000000", &mut test);
// Note: txid is encoded in reverse byte order (Bitcoin standard), so to get display 0000...0032, encode 3200...0000
ext_from_hex("0084 c000000000000000000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000000000000000f7 0100000000000000000000000000000000000000000000000000000000000000 0000 01 20 3200000000000000000000000000000000000000000000000000000000000000 03000000000000000000000000000000", &mut test);

// After commitment_signed exchange, we need to exchange tx_signatures.
// Message type IDs: TxSignatures = 71 (0x0047)
Expand All @@ -1899,19 +1899,19 @@ fn splice_seed() -> Vec<u8> {
// inbound read from peer id 0 of len 150 (134 message + 16 MAC)
ext_from_hex("030096", &mut test);
// TxSignatures message with shared_input_signature TLV (type 0)
// txid must match the splice funding txid (0x31 in reverse byte order)
// txid must match the splice funding txid (0x32 in reverse byte order)
// shared_input_signature: 64-byte fuzz signature for the shared input
ext_from_hex("0047 c000000000000000000000000000000000000000000000000000000000000000 3100000000000000000000000000000000000000000000000000000000000000 0000 00 40 00000000000000000000000000000000000000000000000000000000000000dc 0100000000000000000000000000000000000000000000000000000000000000 03000000000000000000000000000000", &mut test);
ext_from_hex("0047 c000000000000000000000000000000000000000000000000000000000000000 3200000000000000000000000000000000000000000000000000000000000000 0000 00 40 00000000000000000000000000000000000000000000000000000000000000dc 0100000000000000000000000000000000000000000000000000000000000000 03000000000000000000000000000000", &mut test);

// Connect a block with the splice funding transaction to confirm it
// The splice funding tx: version(4) + input_count(1) + txid(32) + vout(4) + script_len(1) + sequence(4)
// + output_count(1) + value(8) + script_len(1) + script(34) + locktime(4) = 94 bytes = 0x5e
// Transaction structure from FundingTransactionReadyForSigning:
// - Input: spending c000...00:0 with sequence 0xfffffffd
// - Output: 115538 sats to OP_0 PUSH32 6e00...00
// - Output: 115537 sats to OP_0 PUSH32 6e00...00
// - Locktime: 13
ext_from_hex("0c005e", &mut test);
ext_from_hex("02000000 01 c000000000000000000000000000000000000000000000000000000000000000 00000000 00 fdffffff 01 52c3010000000000 22 00206e00000000000000000000000000000000000000000000000000000000000000 0d000000", &mut test);
ext_from_hex("02000000 01 c000000000000000000000000000000000000000000000000000000000000000 00000000 00 fdffffff 01 51c3010000000000 22 00206e00000000000000000000000000000000000000000000000000000000000000 0d000000", &mut test);

// Connect additional blocks to reach minimum_depth confirmations
for _ in 0..5 {
Expand All @@ -1928,8 +1928,8 @@ fn splice_seed() -> Vec<u8> {
// inbound read from peer id 0 of len 82 (66 message + 16 MAC)
ext_from_hex("030052", &mut test);
// SpliceLocked message (type 77 = 0x004d): channel_id + splice_txid + mac
// splice_txid must match the splice funding txid (0x31 in reverse byte order)
ext_from_hex("004d c000000000000000000000000000000000000000000000000000000000000000 3100000000000000000000000000000000000000000000000000000000000000 03000000000000000000000000000000", &mut test);
// splice_txid must match the splice funding txid (0x32 in reverse byte order)
ext_from_hex("004d c000000000000000000000000000000000000000000000000000000000000000 3200000000000000000000000000000000000000000000000000000000000000 03000000000000000000000000000000", &mut test);

test
}
Expand Down Expand Up @@ -2059,6 +2059,6 @@ mod tests {

// Splice locked
assert_eq!(log_entries.get(&("lightning::ln::peer_handler".to_string(), "Handling SendSpliceLocked event in peer_handler for node 030000000000000000000000000000000000000000000000000000000000000002 for channel c000000000000000000000000000000000000000000000000000000000000000".to_string())), Some(&1));
assert_eq!(log_entries.get(&("lightning::ln::channel".to_string(), "Promoting splice funding txid 0000000000000000000000000000000000000000000000000000000000000031".to_string())), Some(&1));
assert_eq!(log_entries.get(&("lightning::ln::channel".to_string(), "Promoting splice funding txid 0000000000000000000000000000000000000000000000000000000000000032".to_string())), Some(&1));
}
}
2 changes: 1 addition & 1 deletion lightning-persister/Cargo.toml
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Expand Up @@ -20,7 +20,7 @@ tokio = ["dep:tokio"]

[dependencies]
bitcoin = "0.32.2"
lightning = { version = "0.3.0", path = "../lightning" }
lightning = { version = "0.3.0", path = "../lightning", default-features = false, features = ["std"] }
tokio = { version = "1.35", optional = true, default-features = false, features = ["rt-multi-thread"] }

[target.'cfg(windows)'.dependencies]
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29 changes: 23 additions & 6 deletions lightning/src/events/bump_transaction/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -331,7 +331,13 @@ impl<B: BroadcasterInterface, C: CoinSelectionSource, SP: SignerProvider, L: Log
let anchor_input_witness_weight = if channel_type.supports_anchor_zero_fee_commitments() {
EMPTY_WITNESS_WEIGHT
} else {
ANCHOR_INPUT_WITNESS_WEIGHT
let weight = ANCHOR_INPUT_WITNESS_WEIGHT;
#[cfg(secp256k1_fuzz)]
let weight = {
// The secp256k1 fuzz signer does not low-S normalize dummy signatures.
weight + 1
};
weight
};

// First, check if the commitment transaction has sufficient fees on its own.
Expand Down Expand Up @@ -547,6 +553,18 @@ impl<B: BroadcasterInterface, C: CoinSelectionSource, SP: SignerProvider, L: Log
} else {
panic!("channel type should be either zero-fee HTLCs, or zero-fee commitments");
};
// The secp256k1 fuzz signer emits dummy signatures without low-S normalization, so
// DER+sighash can be one byte larger for each of the two HTLC signatures.
#[cfg(secp256k1_fuzz)]
let (htlc_success_witness_weight, htlc_timeout_witness_weight) =
(htlc_success_witness_weight + 2, htlc_timeout_witness_weight + 2);
Comment on lines +556 to +560
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@ldk-claude-review-bot ldk-claude-review-bot May 19, 2026

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Nit: The +2 correctly adjusts satisfaction_weight in the Input structs (used for the debug assertion at line 740), but the aggregated_htlc_success_input_output_pair_weight() / aggregated_htlc_timeout_input_output_pair_weight() functions called on lines 595/597 still use the unadjusted constants for batch sizing. Under secp256k1_fuzz, the per-HTLC weight is underestimated by 2 WU in those calls.

In practice this is safe — worst case is 483 HTLCs × 2 WU = 966 WU undercounting, which is absorbed by the 1000 WU USER_COINS_WEIGHT_BUDGET margin. But it's a conceptual inconsistency worth noting if this area evolves.

let (htlc_success_input_output_pair_weight, htlc_timeout_input_output_pair_weight) = (
chan_utils::aggregated_htlc_success_input_output_pair_weight(channel_type),
chan_utils::aggregated_htlc_timeout_input_output_pair_weight(channel_type),
);
#[cfg(secp256k1_fuzz)]
let (htlc_success_input_output_pair_weight, htlc_timeout_input_output_pair_weight) =
(htlc_success_input_output_pair_weight + 2, htlc_timeout_input_output_pair_weight + 2);

let max_tx_weight = if channel_type.supports_anchor_zero_fee_commitments() {
// Cap the size of transactions claiming `HolderHTLCOutput` in 0FC channels.
Expand Down Expand Up @@ -587,9 +605,9 @@ impl<B: BroadcasterInterface, C: CoinSelectionSource, SP: SignerProvider, L: Log
&htlc_descriptors[broadcasted_htlcs..broadcasted_htlcs + batch_size]
{
let input_output_weight = if htlc_descriptor.preimage.is_some() {
chan_utils::aggregated_htlc_success_input_output_pair_weight(channel_type)
htlc_success_input_output_pair_weight
} else {
chan_utils::aggregated_htlc_timeout_input_output_pair_weight(channel_type)
htlc_timeout_input_output_pair_weight
};
if htlc_weight_sum + input_output_weight >= max_tx_weight - USER_COINS_WEIGHT_BUDGET
{
Expand Down Expand Up @@ -649,9 +667,8 @@ impl<B: BroadcasterInterface, C: CoinSelectionSource, SP: SignerProvider, L: Log
{
Ok(selection) => selection,
Err(()) => {
let htlcs_to_remove = USER_COINS_WEIGHT_BUDGET.div_ceil(
chan_utils::aggregated_htlc_timeout_input_output_pair_weight(channel_type),
);
let htlcs_to_remove =
USER_COINS_WEIGHT_BUDGET.div_ceil(htlc_timeout_input_output_pair_weight);
batch_size = batch_size.checked_sub(htlcs_to_remove as usize).ok_or(())?;
if batch_size == 0 {
return Err(());
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3 changes: 3 additions & 0 deletions lightning/src/lib.rs
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Expand Up @@ -43,6 +43,9 @@
#[cfg(all(fuzzing, test))]
compile_error!("Tests will always fail with cfg=fuzzing");

#[cfg(all(fuzzing, feature = "grind_signatures"))]
compile_error!("Fuzz builds must not enable grind_signatures");

#[macro_use]
extern crate alloc;

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