Aggkit Optimistic Mode Bug: Commit_imported_bridge_exits Mismatch

by Alex Johnson 66 views

Introduction

In the realm of blockchain technology, optimistic execution is a strategy employed to enhance transaction processing speed and efficiency. However, like any intricate system, it's susceptible to bugs and discrepancies. This article delves into a specific bug encountered within Aggkit's optimistic mode, focusing on a mismatch in commit_imported_bridge_exits. Understanding this issue is crucial for developers and users alike, as it sheds light on the challenges inherent in optimistic systems and the importance of rigorous testing and debugging.

Bug Report Overview

This report addresses a bug encountered in the optimistic mode of Aggkit, specifically within the Fusaka upgrade environment (spec-7-op). The core issue revolves around a discrepancy in the signed data between the aggsend and aggkit-prover components. The aggkit-prover rejects optimistic requests due to an InvalidSigner error. This error indicates a mismatch between the declared and recovered addresses, suggesting a problem with the signature verification process. To provide a detailed understanding, let's dissect the various facets of this bug.

Environment Details:

  • Aggkit Version: v1.7.1
  • Aggkit-prover version: 1.5.4
  • op-succinct-proposer: v3.3.3-agglayer

The heart of the matter lies in the differing signed data between aggsend and the prover. A deeper dive into the specifics reveals that the commit_imported_bridge_exits value is the point of divergence.

Technical Discrepancies

Let's analyze the technical data to pinpoint the exact nature of the discrepancy. On the aggsender side, the signed data includes:

OptimisticSignatureCalculatorImpl.Sign agg:AggregationProofPublicValues{l1Head: 0xec7e91ce940c42823297880603db77ceaa7c8002f5a42322dcf1772b3616cbc9, l2PreRoot: 0x8937a54ddde7452b76f52455dc4e69c8d10f925677b67c588ff35134eaa40a27, claimRoot: 0x3218654aa9e8957b4937538def2c92991ec01bf64f91c96d04d92c573f0b69c0, l2BlockNumber: 398822, rollupConfigHash: 0xbc48466a9be068b2e5b855867987b06751a4ad75eea12b717b1b0a80d7d0af9b, multiBlockVKey: 0x6f311cdf654e648a010a9b5831e334ee112a8f183718cd1c631610a93b7fa02b, proverAddress: 0x8087BF04FEdE30c6cfF965330AD658bB75Add9eC}

OptimisticSignatureCalculatorImpl.Sign OptimisticSignatureData{AggregationProofPublicValuesHash: 0xe6feab15ae70b7b82a1a4afb3beab2e43b02cdd2ad53d08da80936f7a715273e, NewLocalExitRoot: 0x625e889ced3c31277c6653229096374d396a2fd3564a8894aaad2ff935d2fc8c, CommitImportedBridgeExits: 0xf4dd04c8fc169fa04b248be710b7f8681fd8b45cc3ffcb2c47cbe9a27ed0536f}

On the other hand, the aggkit-prover presents a different view:

stderr: fep public values: AggregationProofPublicValues { l1_head: 0xec7e91ce940c42823297880603db77ceaa7c8002f5a42322dcf1772b3616cbc9, l2_pre_root: 0x8937a54ddde7452b76f52455dc4e69c8d10f925677b67c588ff35134eaa40a27, l2_post_root: 0x3218654aa9e8957b4937538def2c92991ec01bf64f91c96d04d92c573f0b69c0, l2_block_number: 398822, rollup_config_hash: 0xbc48466a9be068b2e5b855867987b06751a4ad75eea12b717b1b0a80d7d0af9b, multi_block_vkey: 0x6f311cdf654e648a010a9b5831e334ee112a8f183718cd1c631610a93b7fa02b, prover_address: 0x8087bf04fede30c6cff965330ad658bb75add9ec }

stderr:                      625e889ced3c31277c6653229096374d396a2fd3564a8894aaad2ff935d2fc8c || commit_imported_bridge_exits: a13425f7de8162c05dd397c4b6ebcc8fa09993a73166564a2062395f94fe6d05)

The critical difference lies in the commit_imported_bridge_exits value. Aggsender reports 0xf4dd04c8fc169fa04b248be710b7f8681fd8b45cc3ffcb2c47cbe9a27ed0536f, while aggkit-prover presents 0xa13425f7de8162c05dd397c4b6ebcc8fa09993a73166564a2062395f94fe6d05. This discrepancy is the crux of the issue.

Impact of the Discrepancy

The mismatch in commit_imported_bridge_exits leads to the InvalidSigner error, as the signature verification fails due to the differing data. This, in turn, causes the rejection of optimistic requests, hindering the smooth operation of the system. To fully grasp the implications, it's essential to consider the role of commit_imported_bridge_exits within the broader context of Aggkit's optimistic mode.

Understanding Optimistic Mode and commit_imported_bridge_exits

Optimistic mode in blockchain systems is designed to expedite transaction processing by assuming that most transactions are valid. Instead of immediately verifying each transaction, the system optimistically executes them and posts the results. A challenge period follows, during which validators can challenge the results if they detect any discrepancies. This approach significantly reduces latency and improves throughput.

The commit_imported_bridge_exits value plays a vital role in this process. It represents a commitment to the state of imported bridge exits, which are crucial for cross-chain interoperability. Any discrepancy in this value can lead to inconsistencies in the system's state, potentially jeopardizing the integrity of the blockchain.

Historical Context: PR 1146

It's worth noting that a previous issue with optimistic mode was addressed in PR 1146, which was incorporated into Aggkit since version v0.7.1-rc2. This historical context underscores the ongoing efforts to refine and stabilize Aggkit's optimistic mode. The current bug, however, indicates that further investigation and resolution are necessary.

Reproducing the Behavior

To effectively address this bug, reproducing it is paramount. While the steps to reproduce were not explicitly detailed in the original report (indicated by "Go to '...'", "Click on '....'", etc.), the technical details provided offer a starting point for developers to reconstruct the scenario. This typically involves setting up a similar environment (Fusaka upgrade -spec-7-op), using the specified versions of Aggkit, Aggkit-prover, and op-succinct-proposer, and then triggering optimistic requests that involve bridge exits. Detailed logging and debugging tools can then be employed to capture the data flow and pinpoint the source of the discrepancy.

Expected Behavior

The expected behavior is straightforward: the aggsend and aggkit-prover should agree on the commit_imported_bridge_exits value. This alignment is crucial for the correct functioning of the optimistic mode. When the values match, the signature verification process should succeed, and optimistic requests should be processed without errors.

Environment Details

  • OS: The operating system was not specified in the original report.
  • Browser: The browser is not relevant to this particular bug, as it pertains to backend components.
  • Version: As mentioned earlier:
    • Aggkit Version: v1.7.1
    • Aggkit-prover version: 1.5.4
    • op-succinct-proposer: v3.3.3-agglayer

Additional Context

The original report references a Slack conversation (https://0xpolygon.slack.com/archives/C07J2T5QBM4/p1762538780107989), which likely contains further insights and discussions related to this bug. Accessing this conversation could provide valuable context for developers working on a solution.

Conclusion

The optimistic mode bug in Aggkit, specifically the discrepancy in commit_imported_bridge_exits, presents a significant challenge to the system's reliability and efficiency. Understanding the technical details, the role of optimistic mode, and the historical context is crucial for effective debugging and resolution. By meticulously reproducing the bug, analyzing the data flow, and leveraging available resources like the Slack conversation, developers can work towards a robust solution that ensures the integrity of Aggkit's optimistic execution.

For further reading on optimistic rollups and their security considerations, you can explore resources like the Offchain Labs documentation.