Exception return state lock parameter

A method comprising:

• storing exception return state information in exception return state register storage;
• and using processing circuitry, performing processing operations in response to instructions;
• in response to a guarded control stack (GCS) exception return state push instruction, obtaining the exception return state information from the exception return state register storage and pushing the exception return state information to a GCS data structure;
• in response to a GCS exception return state pop instruction, obtaining GCS-protected exception return state information from the GCS data structure;
• rejecting a write request specifying a target address in response to determining that a memory region corresponding to the target address is specified as a GCS region for storing the GCS data structure and the write request is a request other than a GCS memory access request triggered by one of a restricted subset of GCS-accessing types of instruction, the restricted subset of GCS-accessing types of instruction including at least the GCS exception return state push instruction;
• and
• in at least one operating state of the processing circuitry, detecting, in response to an attempt to modify the exception return state information stored in the exception return state register storage, whether an exception return state lock parameter is in a locked state or an unlocked state, and signalling a fault when the exception return state lock parameter is detected to be in the locked state.

A non-transitory, computer-readable storage medium storing a computer program comprising instructions which, when executed by a host data processing apparatus, control the host data processing apparatus to provide an instruction execution environment for executing target code, the computer program comprising:

• register emulating program logic to control the host data processing apparatus to simulate exception return state register storage for storing exception return state information;
• and
• processing program logic to control the host data processing apparatus to simulate execution of instructions of the target code;
• in which:
• in response to a guarded control stack (GCS) exception return state push instruction of the target code, the processing program logic is configured to control the host data processing apparatus to obtain the exception return state information from the simulated exception return state register storage and push the exception return state information to a GCS data structure;
• in response to a GCS exception return state pop instruction of the target code, the processing program logic is configured to obtain GCS-protected exception return state information obtained from the GCS data structure;
• the computer program comprises memory management program logic to reject a write request specifying a target address in response to determining that a memory region corresponding to the target address is specified as a GCS region for storing the GCS data structure and the write request is a request other than a GCS memory access request triggered by one of a restricted subset of GCS-accessing types of instruction, the restricted subset of GCS-accessing types of instruction including at least the GCS exception return state push instruction;
• and
• in at least one simulated operating state of the processing program logic, the processing program logic is configured to control the host data processing apparatus to detect, in response to an attempt to modify the exception return state information stored in the simulated exception return state register storage, whether an exception return state lock parameter is in a locked state or an unlocked state, and to signal a fault when the exception return state lock parameter is detected to be in the locked state.