ARM Cortex-R52 Exception Triggering Encoding for A32 and T32 Instruction Sets

ARM Cortex-R52 Exception Triggering via UDF Instruction in A32 and T32

The ARM Cortex-R52 processor, designed for real-time and safety-critical applications, supports both the A32 (ARM) and T32 (Thumb) instruction sets. A common requirement in embedded systems is to ensure that any unintended execution of code from unused memory regions triggers an exception. This is particularly important in safety-critical systems where unintended code execution can lead to catastrophic failures. The Undefined Instruction (UDF) opcode is a powerful tool for this purpose, as it forces the processor to raise an exception when executed. However, the encoding of the UDF instruction differs between the A32 and T32 instruction sets, making it challenging to create a single encoding that triggers an exception in both instruction sets.

The A32 instruction set uses a 32-bit encoding for the UDF instruction, while the T32 instruction set uses a 16-bit encoding. The challenge lies in creating a pattern that, when executed in either A32 or T32 mode, will be interpreted as a UDF instruction and trigger an exception. This requires a deep understanding of the instruction encoding formats for both A32 and T32, as well as the behavior of the Cortex-R52 processor when encountering these encodings.

Memory Layout and Instruction Encoding Overlap

The primary challenge in creating a single encoding that triggers an exception in both A32 and T32 modes is the difference in instruction length and encoding formats. In A32 mode, all instructions are 32 bits long, while in T32 mode, instructions can be either 16 or 32 bits long. The UDF instruction in A32 mode is encoded as a 32-bit value, while in T32 mode, it is encoded as a 16-bit value. This difference in encoding means that a single pattern cannot directly serve as a UDF instruction in both modes.

However, it is possible to create a pattern that, when interpreted in A32 mode, corresponds to a UDF instruction, and when interpreted in T32 mode, also corresponds to a UDF instruction. This requires careful alignment of the instruction encodings so that the 16-bit T32 UDF encoding overlaps with a portion of the 32-bit A32 UDF encoding. The key is to ensure that the overlapping bits are interpreted correctly in both modes.

For example, the A32 UDF instruction is typically encoded as 0xE7FxxxFx, where the xxx bits can be used to specify a custom undefined instruction number. The T32 UDF instruction, on the other hand, is encoded as 0xDE00, where the lower 8 bits can be used to specify a custom undefined instruction number. By carefully selecting the values of the xxx bits in the A32 encoding and the lower 8 bits in the T32 encoding, it is possible to create a pattern that, when placed in memory, will trigger an exception in both modes.

Implementing Exception-Triggering Encoding in Unused Memory

To implement an exception-triggering encoding in unused memory, the following steps should be taken:

1. Identify Unused Memory Regions: The first step is to identify the memory regions that are unused and should be filled with the exception-triggering encoding. This typically includes regions of flash or RAM that are not allocated to any specific function or data structure.

2. Determine the Alignment Requirements: Since the Cortex-R52 processor can execute instructions in both A32 and T32 modes, it is important to ensure that the exception-triggering encoding is aligned correctly in memory. In A32 mode, instructions must be aligned to 4-byte boundaries, while in T32 mode, instructions can be aligned to 2-byte boundaries. Therefore, the exception-triggering encoding should be placed at addresses that are aligned to 4-byte boundaries to ensure correct interpretation in both modes.

3. Create the Exception-Triggering Encoding: The next step is to create the actual encoding that will trigger an exception in both A32 and T32 modes. As discussed earlier, this involves selecting values for the xxx bits in the A32 UDF encoding and the lower 8 bits in the T32 UDF encoding such that the resulting pattern is interpreted as a UDF instruction in both modes. For example, the following encoding could be used:

   • A32 UDF Encoding: 0xE7F000F0
   • T32 UDF Encoding: 0xDE00

   When placed in memory, the 32-bit value 0xE7F000F0 will be interpreted as a UDF instruction in A32 mode, while the 16-bit value 0xDE00 will be interpreted as a UDF instruction in T32 mode. By placing these values at aligned memory addresses, the processor will trigger an exception if it attempts to execute code from these regions.

4. Fill Unused Memory with the Encoding: Once the exception-triggering encoding has been created, it should be used to fill the identified unused memory regions. This can be done during the initialization phase of the firmware, or it can be done by the linker script if the memory regions are known at compile time.

5. Verify the Exception Handling: After filling the unused memory regions with the exception-triggering encoding, it is important to verify that the processor correctly triggers an exception when attempting to execute code from these regions. This can be done by writing a test case that intentionally jumps to an address within the unused memory region and verifying that the exception handler is invoked.

6. Handle the Exception: Finally, the exception handler should be implemented to handle the undefined instruction exception. This typically involves logging the error, resetting the system, or taking other appropriate action depending on the requirements of the application.

By following these steps, it is possible to create a robust mechanism for triggering exceptions in unused memory regions, ensuring that any unintended code execution is caught and handled appropriately. This is particularly important in safety-critical systems where unintended code execution can lead to catastrophic failures.

Detailed Analysis of UDF Instruction Encoding

To fully understand how to create an exception-triggering encoding that works in both A32 and T32 modes, it is necessary to delve deeper into the encoding formats of the UDF instruction in both instruction sets.

A32 UDF Instruction Encoding

In the A32 instruction set, the UDF instruction is encoded as a 32-bit value with the following format:

31 28 27 24 23 20 19 16 15 12 11 8 7 4 3 0
+-------+-------+-------+-------+-------+-------+-------+-------+
| 1 1 1 0 | 0 1 1 1 | 1 1 1 1 | x x x x | x x x x | x x x x | 1 1 1 1 |
+-------+-------+-------+-------+-------+-------+-------+-------+

The x bits represent the custom undefined instruction number, which can be used to differentiate between different undefined instructions. The value 0xE7FxxxFx is typically used to encode the UDF instruction, where xxx can be any value between 0x000 and 0xFFF.

T32 UDF Instruction Encoding

In the T32 instruction set, the UDF instruction is encoded as a 16-bit value with the following format:

15 12 11 8 7 4 3 0
+-------+-------+-------+-------+
| 1 1 0 1 | 1 1 1 0 | 0 0 0 0 | x x x x |
+-------+-------+-------+-------+

The x bits represent the custom undefined instruction number, which can be used to differentiate between different undefined instructions. The value 0xDE00 is typically used to encode the UDF instruction, where the lower 8 bits can be any value between 0x00 and 0xFF.

Overlapping Encoding Strategy

To create a single encoding that triggers an exception in both A32 and T32 modes, the 16-bit T32 UDF encoding must overlap with a portion of the 32-bit A32 UDF encoding. Specifically, the 16-bit T32 UDF encoding must align with the lower 16 bits of the 32-bit A32 UDF encoding. This can be achieved by carefully selecting the values of the xxx bits in the A32 encoding and the lower 8 bits in the T32 encoding.

For example, consider the following encoding:

• A32 UDF Encoding: 0xE7F000F0
• T32 UDF Encoding: 0xDE00

When placed in memory, the 32-bit value 0xE7F000F0 will be interpreted as a UDF instruction in A32 mode, while the 16-bit value 0xDE00 will be interpreted as a UDF instruction in T32 mode. The key is to ensure that the 16-bit T32 UDF encoding (0xDE00) aligns with the lower 16 bits of the 32-bit A32 UDF encoding (0xE7F000F0). In this case, the lower 16 bits of 0xE7F000F0 are 0x00F0, which does not match 0xDE00. Therefore, this encoding will not work as intended.

To create a valid overlapping encoding, the lower 16 bits of the A32 UDF encoding must match the T32 UDF encoding. For example, consider the following encoding:

• A32 UDF Encoding: 0xE7F0DE00
• T32 UDF Encoding: 0xDE00

In this case, the lower 16 bits of the A32 UDF encoding (0xDE00) match the T32 UDF encoding (0xDE00). When placed in memory, the 32-bit value 0xE7F0DE00 will be interpreted as a UDF instruction in A32 mode, while the 16-bit value 0xDE00 will be interpreted as a UDF instruction in T32 mode. This encoding will trigger an exception in both modes.

Practical Implementation

To implement this strategy in practice, the following steps should be taken:

1. Define the Exception-Triggering Encoding: Define the 32-bit A32 UDF encoding and the 16-bit T32 UDF encoding such that the lower 16 bits of the A32 encoding match the T32 encoding. For example:

   #define A32_UDF_ENCODING 0xE7F0DE00
   #define T32_UDF_ENCODING 0xDE00
   

2. Fill Unused Memory with the Encoding: Fill the unused memory regions with the 32-bit A32 UDF encoding. This can be done using a loop or by specifying the encoding in the linker script. For example:

   uint32_t *unused_memory = (uint32_t *)UNUSED_MEMORY_START;
   for (uint32_t i = 0; i < UNUSED_MEMORY_SIZE / 4; i++) {
       unused_memory[i] = A32_UDF_ENCODING;
   }
   

3. Verify the Exception Handling: Write a test case that intentionally jumps to an address within the unused memory region and verify that the exception handler is invoked. For example:

   void test_exception_triggering() {
       void (*unused_memory_function)() = (void (*)())UNUSED_MEMORY_START;
       unused_memory_function();  // This should trigger an exception
   }
   

4. Handle the Exception: Implement the exception handler to handle the undefined instruction exception. For example:

   void undefined_instruction_handler() {
       // Log the error, reset the system, or take other appropriate action
   }
   

By following these steps, it is possible to create a robust mechanism for triggering exceptions in unused memory regions, ensuring that any unintended code execution is caught and handled appropriately.

Conclusion

Creating an exception-triggering encoding that works in both A32 and T32 modes on the ARM Cortex-R52 processor requires a deep understanding of the instruction encoding formats for both instruction sets. By carefully aligning the 16-bit T32 UDF encoding with the lower 16 bits of the 32-bit A32 UDF encoding, it is possible to create a single pattern that triggers an exception in both modes. This strategy can be used to fill unused memory regions with an exception-triggering encoding, ensuring that any unintended code execution is caught and handled appropriately. This is particularly important in safety-critical systems where unintended code execution can lead to catastrophic failures.