cosmos-sdk/x/slashing/spec/04_begin_block.md

BeginBlock

Evidence Handling

Tendermint blocks can include Evidence, which indicates that a validator committed malicious behavior. The relevant information is forwarded to the application as ABCI Evidence in abci.RequestBeginBlock so that the validator an be accordingly punished.

For some Evidence submitted in block to be valid, it must satisfy:

Evidence.Timestamp >= block.Timestamp - MaxEvidenceAge

Where Evidence.Timestamp is the timestamp in the block at height Evidence.Height and block.Timestamp is the current block timestamp.

If valid evidence is included in a block, the validator's stake is reduced by some penalty (SlashFractionDoubleSign for equivocation) of what their stake was when the infraction occurred (rather than when the evidence was discovered). We want to "follow the stake", i.e. the stake which contributed to the infraction should be slashed, even if it has since been redelegated or started unbonding.

We first need to loop through the unbondings and redelegations from the slashed validator and track how much stake has since moved:

slashAmountUnbondings := 0
slashAmountRedelegations := 0

unbondings := getUnbondings(validator.Address)
for unbond in unbondings {

    if was not bonded before evidence.Height or started unbonding before unbonding period ago {
        continue
    }

    burn := unbond.InitialTokens * SLASH_PROPORTION
    slashAmountUnbondings += burn

    unbond.Tokens = max(0, unbond.Tokens - burn)
}

// only care if source gets slashed because we're already bonded to destination
// so if destination validator gets slashed our delegation just has same shares
// of smaller pool.
redels := getRedelegationsBySource(validator.Address)
for redel in redels {

    if was not bonded before evidence.Height or started redelegating before unbonding period ago {
        continue
    }

    burn := redel.InitialTokens * SLASH_PROPORTION
    slashAmountRedelegations += burn

    amount := unbondFromValidator(redel.Destination, burn)
    destroy(amount)
}

We then slash the validator and tombstone them:

curVal := validator
oldVal := loadValidator(evidence.Height, evidence.Address)

slashAmount := SLASH_PROPORTION * oldVal.Shares
slashAmount -= slashAmountUnbondings
slashAmount -= slashAmountRedelegations

curVal.Shares = max(0, curVal.Shares - slashAmount)

signInfo = SigningInfo.Get(val.Address)
signInfo.JailedUntil = MAX_TIME
signInfo.Tombstoned = true
SigningInfo.Set(val.Address, signInfo)

This ensures that offending validators are punished the same amount whether they act as a single validator with X stake or as N validators with collectively X stake. The amount slashed for all double signature infractions committed within a single slashing period is capped as described in overview.md under Tombstone Caps.

Liveness Tracking

At the beginning of each block, we update the ValidatorSigningInfo for each validator and check if they've crossed below the liveness threshold over a sliding window. This sliding window is defined by SignedBlocksWindow and the index in this window is determined by IndexOffset found in the validator's ValidatorSigningInfo. For each block processed, the IndexOffset is incrimented regardless if the validator signed or not. Once the index is determined, the MissedBlocksBitArray and MissedBlocksCounter are updated accordingly.

Finally, in order to determine if a validator crosses below the liveness threshold, we fetch the maximum number of blocks missed, maxMissed, which is SignedBlocksWindow - (MinSignedPerWindow * SignedBlocksWindow) and the minimum height at which we can determine liveness, minHeight. If the current block is greater than minHeight and the validator's MissedBlocksCounter is greater than maxMissed, they will be slashed by SlashFractionDowntime, will be jailed for DowntimeJailDuration, and have the following values reset: MissedBlocksBitArray, MissedBlocksCounter, and IndexOffset.

Note: Liveness slashes do NOT lead to a tombstombing.

height := block.Height

for vote in block.LastCommitInfo.Votes {
  signInfo := GetValidatorSigningInfo(vote.Validator.Address)

  // This is a relative index, so we counts blocks the validator SHOULD have
  // signed. We use the 0-value default signing info if not present, except for
  // start height.
  index := signInfo.IndexOffset % SignedBlocksWindow()
  signInfo.IndexOffset++

  // Update MissedBlocksBitArray and MissedBlocksCounter. The MissedBlocksCounter
  // just tracks the sum of MissedBlocksBitArray. That way we avoid needing to
  // read/write the whole array each time.
  missedPrevious := GetValidatorMissedBlockBitArray(vote.Validator.Address, index)
  missed := !signed

  switch {
  case !missedPrevious && missed:
    // array index has changed from not missed to missed, increment counter
    SetValidatorMissedBlockBitArray(vote.Validator.Address, index, true)
    signInfo.MissedBlocksCounter++

  case missedPrevious && !missed:
    // array index has changed from missed to not missed, decrement counter
    SetValidatorMissedBlockBitArray(vote.Validator.Address, index, false)
    signInfo.MissedBlocksCounter--

  default:
    // array index at this index has not changed; no need to update counter
  }

  if missed {
    // emit events...
  }

  minHeight := signInfo.StartHeight + SignedBlocksWindow()
  maxMissed := SignedBlocksWindow() - MinSignedPerWindow()

  // If we are past the minimum height and the validator has missed too many
  // jail and slash them.
  if height > minHeight && signInfo.MissedBlocksCounter > maxMissed {
    validator := ValidatorByConsAddr(vote.Validator.Address)

    // emit events...

    // We need to retrieve the stake distribution which signed the block, so we
    // subtract ValidatorUpdateDelay from the block height, and subtract an
    // additional 1 since this is the LastCommit.
    //
    // Note, that this CAN result in a negative "distributionHeight" up to
    // -ValidatorUpdateDelay-1, i.e. at the end of the pre-genesis block (none) = at the beginning of the genesis block.
    // That's fine since this is just used to filter unbonding delegations & redelegations.
    distributionHeight := height - sdk.ValidatorUpdateDelay - 1

    Slash(vote.Validator.Address, distributionHeight, vote.Validator.Power, SlashFractionDowntime())
    Jail(vote.Validator.Address)

    signInfo.JailedUntil = block.Time.Add(DowntimeJailDuration())

    // We need to reset the counter & array so that the validator won't be
    // immediately slashed for downtime upon rebonding.
    signInfo.MissedBlocksCounter = 0
    signInfo.IndexOffset = 0
    ClearValidatorMissedBlockBitArray(vote.Validator.Address)
  }

  SetValidatorSigningInfo(vote.Validator.Address, signInfo)
}