[General] <BootLoader smart contract>

[sumit03guha]

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Environment

Mainnet

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Provide a brief description of the functionality you're trying to implement and the issue you are running into.

In the system-contracts constants.sol, the BOOTLOADER_FORMAL_ADDRESS is 0x0000000000000000000000000000000000008001
But on the zksync explorer, it says EmptyContract

Can anyone help me understand this?

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[sarahschwartz]

Hi @sumit03guha, great question! I would refer you to this section in our docs:

The bootloader's code is not stored on Layer 2 (L2) but its hash is stored on Layer 1 (L1) and can only be modified through a system upgrade. This setup ensures that the bootloader functions as a kind of "formal" address that provides context and identity to this, msg.sender, and similar references during transaction processing. If someone interacts with this address, for instance, to handle transaction fees, it triggers the EmptyContract’s code.

[sumit03guha]

Hey @sarahschwartz, thanks for the reference to the docs. I have gone through it but it is not fully clear to me as I am still unable to connect the dots.

I found the code for the BootLoader here.

In the readme, it says that

Bootloader For greater extensibility and to lower the overhead, some parts of the protocol (e.g. account abstraction rules) were moved to an ephemeral contract called a bootloader.

We call it ephemeral because it is not physically deployed and cannot be called, but it has a formal address that is used on msg.sender, when it calls other contracts.

My question is, if the code is not physically deployed, then how is it being executed?

[sumit03guha]

To be more specific, let's say I have the following line of code in my smart contract on Zksync
if (msg.sender != BOOTLOADER_FORMAL_ADDRESS)

According to my understanding, the msg.sender is going to be the EmptyContract, right?

[sarahschwartz]

My question is, if the code is not physically deployed, then how is it being executed?

It's basically a part of the execution environment, rather than a contract.

To be more specific, let's say I have the following line of code in my smart contract on Zksync
if (msg.sender != BOOTLOADER_FORMAL_ADDRESS)

Afaik the msg.sender would never be the bootloader address in a normal smart contract. The only cases this would apply to are smart account contracts and paymaster contracts, which have functions that need to only be able to be called by the execution environment.

You can see an example of what that looks like in this tutorial. The smart account uses a onlyBootloader modifier to make sure certain functions can't be called externally.

modifier onlyBootloader() {
      require(
          msg.sender == BOOTLOADER_FORMAL_ADDRESS,
          "Only bootloader can call this function"
      );
      // Continue execution if called from the bootloader.
      _;
  }