feat: transient per-block limit for zero gas tx per block

[matthiasmatt]

What

Adds a per-block quota for zero-gas EVM transactions to limit free blockspace consumption.

Why

Zero-gas EVM calls bypass fee deduction and mempool min-gas-price checks. A deterministic per-block cap provides a protocol-level
backstop against spam/DoS targeting ZeroGasActors.always_zero_gas_contracts.

How

• Introduces a transient (per-block) counter EvmState.BlockZeroGasTxCount stored under a new transient namespace. (x/evm/const.go,
  x/evm/evmstate/evm_state.go)
• Adds a new EVM ante step AnteStepZeroGasBlockQuota that runs immediately after AnteStepDetectZeroGas and:
  • applies DeliverTx-only
  • increments the counter when evm.IsZeroGasEthTx(ctx)
  • rejects once the block quota is exceeded with evm.ErrZeroGasBlockQuotaExceeded. (x/evm/evmante/evmante_zero_gas_quota.go, x/
    evm/errors.go, x/evm/evmante/all_evmante.go)
• Quota is currently a consensus-critical constant maxZeroGasTxsPerBlock (set to 0 to disable). (x/evm/evmante/
  evmante_zero_gas_quota.go)

Testing

• Added TestAnteStepZeroGasBlockQuota_PerBlockTxCount verifying the first maxZeroGasTxsPerBlock zero-gas txs pass and the next one
  fails in the same block. (x/evm/evmante/evmante_zero_gas_quota_test.go)
• go test ./x/evm/...

Follow-ups

• Consider making the quota governance-controlled (params/state) instead of a hardcoded constant.
• If we want mempool-level protection too, add a separate CheckTx/local policy (this change intentionally enforces DeliverTx
  only).

[gemini-code-assist]

Code Review

This pull request introduces a per-block quota for zero-gas EVM transactions to mitigate spam and DoS risks. The implementation is solid, adding new ante handler steps for detection and quota enforcement, and correctly modifying existing ante handlers to bypass fee-related logic for these transactions. The changes are well-tested, covering various scenarios including normal transactions, zero-gas transactions, and edge cases like new accounts and reverted executions. My main concern is the inclusion of hardcoded testnet private keys and mnemonic phrases in the repository, which poses a significant security risk and should be addressed immediately.

I am having trouble creating individual review comments. Click here to see my feedback.

evm-e2e/.passkey-testnet2-privkey.txt (1)

Committing a private key to the repository, even for a testnet, is a significant security risk. This key should be removed from the repository's history. It's recommended to use environment variables or a secure secret management system to handle such credentials, or generate them on the fly for tests.

evm-e2e/.passkey-testnet2-wallet.json (1)

Committing a wallet mnemonic phrase to the repository is a critical security vulnerability. This allows anyone with access to the repository to control the associated funds. This file should be removed from the repository and its history. For testing, consider generating wallets dynamically or using environment variables to supply credentials securely.

x/evm/evmante/evmante_gas_consume.go (119-121)

The removal of debugging fmt.Printf and log.Printf statements is a good cleanup. However, there are more instances of such logging in the all_evmante.go file that should also be removed before merging to keep the codebase clean.

x/evm/evmstate/msg_server.go (76)

This log.Printf statement appears to be for debugging and should be removed before merging to avoid polluting logs in production environments.

[codecov]

Codecov Report

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.07%. Comparing base (f89059d) to head (eb0387e).

Files with missing lines	Patch %	Lines
x/evm/evmante/evmante_zero_gas_quota.go	73.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           ud/zero-gas-evm    #2519      +/-   ##
===================================================
+ Coverage            58.98%   59.07%   +0.09%     
===================================================
  Files                  361      362       +1     
  Lines                24415    24436      +21     
===================================================
+ Hits                 14401    14436      +35     
+ Misses                8777     8758      -19     
- Partials              1237     1242       +5     

Files with missing lines	Coverage Δ
x/evm/const.go	43.39% <ø> (ø)
x/evm/errors.go	0.00% <ø> (ø)
x/evm/evmante/all_evmante.go	64.86% <100.00%> (+0.31%)	⬆️
x/evm/evmstate/evm_state.go	92.94% <100.00%> (+0.44%)	⬆️
x/evm/evmante/evmante_zero_gas_quota.go	73.33% <73.33%> (ø)

... and 1 file with indirect coverage changes

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