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Update EIP-7069: Padding oob behavior of RETURNDATA* #8617

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Update EIP-7069: Padding oob behavior of RETURNDATA* #8617

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Update EIP-7069: Padding oob behavior of RETURNDATA*
pdobacz Jun 3, 2024
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Update rationale section
pdobacz Jun 3, 2024
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24 changes: 20 additions & 4 deletions EIPS/eip-7069.md
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type: Standards Track
category: Core
created: 2023-05-05
requires: 150, 211, 214, 2929
requires: 150, 211, 214, 2929, 3540
---

## Abstract

Introduce three new call instructions, `EXTCALL`, `EXTDELEGATECALL` and `EXTSTATICCALL`, with simplified semantics. Introduce another instruction, `RETURNDATALOAD` for loading a word from return data into stack. The existing call instructions remain unchanged.
Introduce three new call instructions, `EXTCALL`, `EXTDELEGATECALL` and `EXTSTATICCALL`, with simplified semantics. Introduce another instruction, `RETURNDATALOAD` for loading a word from return data into stack. Modify the behavior of `RETURNDATACOPY` instruction executed within EOF formatted code (as defined by [EIP-3540](./eip-3540.md)). The existing `*CALL` instructions remain unchanged.

The new instructions do not allow specifying a gas limit, but rather rely on the "63/64th rule" ([EIP-150](./eip-150.md)) to limit gas. An important improvement is the rules around the "stipend" are simplified, and callers do not need to perform special calculation whether the value is sent or not.

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1. Charge `G_verylow` (3) gas
2. Pop 1 item from the stack, to be referred to as `offset`
3. If `offset + 32 > len(returndata buffer)`, halt with exceptional failure.
4. Push 1 item onto the stack, the 32-byte word read from the returndata buffer starting at `offset`.
3. Push 1 item onto the stack, the 32-byte word read from the returndata buffer starting at `offset`.
4. If `offset + 32 > len(returndata buffer)`, the result is zero-padded.

In case this EIP is included as part of the greater EOF upgrade, execution semantics of `RETURNDATACOPY` in EOF formatted code ([EIP-3540](./eip-3540.md)) is modified as follows:

1. Assume the 3 arguments popped from stack are `destOffset`, `offset` and `size`.
2. If `offset + size > len(returndata buffer)` **do not** halt with exceptional failure, but instead set the `offset + size - len(returndata buffer)` memory bytes after the copied ones to zero.
3. Gas charged for memory copying remains `3 * num_words(size)`, regardless of the number of bytes actually copied or set to zero.

Execution of `RETURNDATACOPY` which is not in EOF formatted code (i.e. is in legacy code) is not changed.

<!-- *TODO:* Clarify which side (caller/callee) is gas deducted from and where an error originates from. -->

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<!-- *TODO:* Mention gas refunds? -->

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There is an alternative scenario where, in case this EIP is included as part of the greater EOF upgrade, the four new instructions are **additionally** available in legacy EVM. There is, however, a preference to limit changes to legacy EVM in the fork where EOF is included as well as in subsequent ones.

### `RETURNDATALOAD` and `RETURNDATACOPY` padding behavior

This EIP initially proposed keeping the halt-on-OOB behavior of legacy `RETURNDATACOPY`. This makes compilers optimizations harder, because unnecessary `RETURNDATA*` instructions cannot be optimized out without change to code semantics.

It could be that only `RETURNDATALOAD` is given the padding behavior, but that would make it confusingly inconsistent with the closely related `RETURNDATACOPY` instruction.

There also was the alternative to have `RETURNDATACOPY2` introduced with the padding behavior, available in EOF only, at the same time banning `RETURNDATACOPY` in EOF. This has been rejected in order to avoid multiplying opcodes, and also as suboptimal from the point of view of compiler implementation.

## Backwards Compatibility

No existing instructions are changed and so we do not think any backwards compatibility issues can occur.
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