Add EIP: NONREENTRANT and REENTRANT opcodes

EIPS/eip-7705.md

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+---
+eip: 7705
+title: NONREENTRANT and REENTRANT opcodes
+description: Opcodes to mark a contract as nonreentrant
+author: Charles Cooper (@charles-cooper)
+discussions-to: https://ethereum-magicians.org/t/eip-7705-nonreentrant-opcodes/19957
+status: Draft
+type: Standards Track
+category: Core
+created: 2024-05-09
+---
+
+## Abstract
+
+Add two opcodes, `NONREENTRANT` and `REENTRANT`, which set and clear a contract's reentrancy status. After invoking `NONREENTRANT`, a contract cannot be `CALL`ed (or `STATICCALL`ed, or `DELEGATECALL`ed) until `REENTRANT` is invoked.
+
+## Motivation
+
+Reentrancy attacks account for a substantial portion of user funds stolen on EVM chains, including the famous "DAO hack". However, due to the cost of preventing against reentrancy attacks in application code, developers often opt-out of reentrancy protection. This cost has come down with the advent of transient storage ([EIP-1153](./eip-1153.md)), but it is still not cheap enough where it is a "no-brainer" to use it by default. This EIP proposes opcodes which make it cheaper to protect against reentrancy in application code.
+
+## Specification
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.
+
+Two new opcodes are introduced, `NONREENTRANT` (0xF6) and `REENTRANT` (0xF7), which set and clear a contract's nonreentrancy flag. The effect of invoking `NONREENTRANT` is that a contract can no longer have execution context transferred to it (via any of the `*CALL` opcodes), until `REENTRANT` is invoked. `CALL`ing a contract which has the nonreentrancy flag set is equivalent to executing a single `REVERT` opcode.
+
+Both `NONREENTRANT` and `REENTRANT` are idempotent; that is, invoking `NONREENTRANT` when a contract already has nonreentrant status is a no-op, and likewise for `REENTRANT`.
+
+The scope of the nonreentrant flag is limited to the current transaction. That is, nonreentrant flags are cleared at the end of every transaction. In the presence of reversion (`REVERT` or exceptional halt), the contract's nonreentrancy flag reverts to whatever its value was before the call.
+
+The cost of `NONREENTRANT` and `REENTRANT` are both set at 5 (`G_mid`).
+
+## Rationale
+
+The computational cost of pushing the current value to the call stack (for handling reverts) is accounted for in the overhead cost of the `*CALL` opcodes.
+
+An alternative design could be considered which only introduces one opcode. This opcode, `NONREENTRANT`, would take a single stack item and set the nonreentrancy flag based on its value. This alternative design can be considered based on feedback.
+
+## Backwards Compatibility
+
+No backward compatibility issues found.
+
+## Test Cases
+
+## Reference Implementation
+
+## Security Considerations
+
+TBD
+
+## Copyright
+
+Copyright and related rights waived via [CC0](../LICENSE.md).