EVM Puzzle 7 & 8: Advanced Memory Operations

Introduction

In this post, we'll tackle Puzzles 7 and 8, which introduce more advanced EVM concepts including memory operations and complex data manipulation. These puzzles will test your understanding of how the EVM handles data storage and retrieval.

Puzzle 7

Copy and Paste → 36600080373660006000F03B600114601357FD5B00 into the EVM Playground. Then click RUN.

# Puzzle 7
# Goal:
# Make it to the JUMPDEST at position 13
#
# The contract's code is:
#
# [00]    CALLDATASIZE    # Get size of calldata
# [01]    PUSH1 00        # Push 0 onto stack
# [03]    DUP1            # Duplicate 0
# [04]    CALLDATACOPY    # Copy calldata to memory
# [05]    CALLDATASIZE    # Get size of calldata again
# [06]    PUSH1 00        # Push 0 onto stack
# [08]    PUSH1 00        # Push 0 onto stack
# [0A]    CREATE          # Create new contract
# [0B]    EXTCODESIZE     # Get size of created contract
# [0C]    PUSH1 01        # Push 1 onto stack
# [0E]    EQ              # Check if sizes are equal
# [0F]    PUSH1 13        # Push 19 (0x13) onto stack
# [11]    JUMPI           # Conditional jump
# [12]    REVERT          # Revert if jump fails
# [13]    JUMPDEST        # Success destination
# [14]    STOP            # Halt execution
#
# Bytecode: 0x36600080373660006000F03B600114601357FD5B00

Understanding the Puzzle

This puzzle introduces contract creation and code size comparison. Let's break down what's happening:

1. The code first gets the size of our calldata
2. It copies our calldata to memory starting at position 0
3. It then tries to create a new contract using that calldata as the contract's code
4. Finally, it checks if the new contract's code size equals 1

Puzzle 8:

CREATE and CALL

Puzzle 8 combines contract creation with contract interaction, requiring us to create a contract and then call it successfully.

Copy and Paste → 36600080373660006000F0600080808080945AF1600014601B57FD5B00 into the EVM Playground. Then click RUN.

# Puzzle 8
# Goal:
# Create a contract and call it successfully

00      36      CALLDATASIZE
01      60      00      PUSH1 00
03      80      DUP1
04      37      CALLDATACOPY
05      36      CALLDATASIZE
06      60      00      PUSH1 00
08      60      00      PUSH1 00
0A      F0      CREATE
0B      60      00      PUSH1 00
0D      80      DUP1
0E      80      DUP1
0F      80      DUP1
10      80      DUP1
11      86      SWAP5
12      5A      GAS
13      F1      CALL
14      60      00      PUSH1 00
16      14      EQ
17      60      1B      PUSH1 1B
19      57      JUMPI
1A      FD      REVERT
1B      5B      JUMPDEST
1C      00      STOP

# Bytecode: 36600080373660006000F0600080808080945AF1600014601B57FD5B00

Understanding the Puzzle

This puzzle introduces two new opcodes: CREATE and CALL. Let's break down what each does:

Key Takeaways

• Contract creation can be done dynamically using calldata
• The CREATE opcode uses memory contents as the new contract's code
• Contract calls can be used to test contract behavior
• Return values from contract calls can be checked

Common Pitfalls

• Forgetting that created contract code must be valid EVM bytecode
• Not accounting for gas costs in contract creation and calls
• Creating contracts that revert instead of returning successfully
• Misunderstanding how the CALL opcode affects the stack

Practice Exercises

1. Can you create a contract that returns a specific non-zero value?
2. Try creating a contract that performs a calculation before returning
3. Experiment with different ways to make contract calls fail or succeed