Handling and Returning from ARM Cortex-M4 MemManageFault Exceptions

MemManageFault Triggered on Write to Read-Only Memory Region

The MemManageFault is a critical exception in ARM Cortex-M4 processors that occurs when a memory access violation is detected. In this scenario, the fault is triggered by attempting to write to a read-only memory region configured via the Memory Protection Unit (MPU). When the fault occurs, the MemManage Fault Address Register (MMFAR) is updated with the address that caused the violation, and the MemManage Fault Status Register (MMFSR) within the Configurable Fault Status Register (CFSR) is set to 0x82, indicating a data access violation.

The primary challenge arises when attempting to return from the MemManageFault exception handler. After clearing the fault flags in the CFSR and MMFAR, the processor re-executes the same instruction that caused the fault, leading to an infinite loop of exceptions. This behavior is expected because the processor resumes execution from the point where the fault occurred, and if the underlying issue (e.g., writing to a read-only region) is not resolved, the fault will reoccur.

The ARMv7-M Architecture Reference Manual specifies that writing a ‘1’ to the corresponding bit in the CFSR clears the fault. However, simply clearing the fault flags does not address the root cause of the violation. As a result, the processor repeatedly triggers the MemManageFault exception, making it impossible to continue normal execution without intervention.

Fault Recurrence Due to Unresolved Memory Access Violation

The recurrence of the MemManageFault is primarily due to the unresolved memory access violation. When the exception handler clears the fault flags but does not modify the instruction causing the fault, the processor re-executes the same instruction upon returning from the exception. This creates a loop where the fault is repeatedly triggered.

The fault recurrence can be attributed to several factors:

1. Unmodified Faulting Instruction: The instruction that caused the fault remains unchanged in the program memory. Upon returning from the exception, the processor fetches and executes the same instruction, leading to the same memory access violation.

2. Incomplete Fault Handling: Clearing the fault flags in the CFSR and MMFAR is necessary but insufficient. The handler must also ensure that the faulting instruction is either skipped or modified to prevent re-execution.

3. Stack Frame Configuration: The exception handler must correctly manipulate the stack frame to adjust the return address. This ensures that the processor resumes execution from a point after the faulting instruction, effectively skipping it.

4. Instruction Size Consideration: ARM Cortex-M4 processors support both 16-bit (Thumb) and 32-bit instructions. The handler must determine the size of the faulting instruction to correctly adjust the return address.

Implementing Instruction Skipping and Exception Return

To resolve the MemManageFault recurrence, the exception handler must implement a mechanism to skip the faulting instruction. This involves manipulating the stack frame to adjust the return address, ensuring that the processor resumes execution from the next instruction. The following steps outline the process:

1. Clear Fault Flags: Write ‘1’ to the appropriate bits in the CFSR to clear the MemManageFault status. This step is necessary to allow the processor to exit the exception handler.

2. Determine Stack Pointer: Identify the stack pointer (SP) used during the exception entry. The Cortex-M4 uses either the Main Stack Pointer (MSP) or the Process Stack Pointer (PSP) depending on the context.

3. Locate Return Address: The return address is stored in the stack frame created during exception entry. This address points to the instruction that caused the fault.

4. Identify Instruction Size: Determine whether the faulting instruction is 16-bit or 32-bit. This can be inferred from the Thumb instruction set encoding or by examining the program memory.

5. Adjust Return Address: Increment the return address by 2 for a 16-bit instruction or by 4 for a 32-bit instruction. This adjustment ensures that the processor skips the faulting instruction upon exception return.

6. Restore Context and Return: Restore the processor context from the stack frame and execute the exception return instruction. The processor will resume execution from the adjusted return address, effectively skipping the faulting instruction.

The following table summarizes the key steps and their corresponding actions:

Detailed Implementation

1. Clear Fault Flags:

   • Access the CFSR register and write ‘1’ to the MemManageFault bits (MMFSR). This clears the fault status and allows the processor to exit the exception handler.
   • Example: CFSR |= 0x00000084; (Clears both MMARVALID and DACCVIOL flags).

2. Determine Stack Pointer:

   • Check the CONTROL register to determine whether the MSP or PSP is active.
   • Example: uint32_t sp = (CONTROL & 0x2) ? PSP : MSP;

3. Locate Return Address:

   • The stack frame created during exception entry contains the return address at a specific offset.
   • Example: uint32_t return_address = *(uint32_t*)(sp + 0x18);

4. Identify Instruction Size:

   • Decode the faulting instruction to determine its size. For Thumb instructions, the first 16 bits can indicate whether it is a 16-bit or 32-bit instruction.
   • Example: uint16_t instruction = *(uint16_t*)return_address;

5. Adjust Return Address:

   • Increment the return address based on the instruction size.
   • Example: return_address += (instruction & 0xF800) == 0xF000 ? 4 : 2;

6. Restore Context and Return:

   • Update the return address in the stack frame and execute the exception return instruction.
   • Example: *(uint32_t*)(sp + 0x18) = return_address; __asm volatile("bx lr");

By following these steps, the MemManageFault handler can effectively skip the faulting instruction and allow the processor to continue normal execution. This approach avoids the need for a system reset and ensures that the application can recover from memory access violations gracefully.