[SOLVED] FreeRTOS MPU: privileged access to bss?

FreeRTOS MPU: privileged access to bss?

I am using FreeRTOS 10.5.1 with MPU support on a Cortex M4 microcontroller (STM32F4).

I created some tasks (all privileged -> portPRIVILEGE_BIT set) and my system is running fine.

Now I tested again with one task not having the portPRIVILEGE_BIT set (= User Mode).

When entering the task right up in the first function (HAL_TIM_Base_Start()) I am getting a memory management exception. The mem manage address register shows an address in the bss (where the timer is located).

Why does the MPU trigger when accessing the bss? (I checked the __privileged_data_end__ ends before) How does a User Mode task behave when accessing an area (like bss) that is not explicitly configured as e.g. portMPU_REGION_READ_WRITE in xRegions[]?

Access to bss with portPRIVILEGE_BIT set -> no problem.
Access to bss with portPRIVILEGE_BIT reset -> mem management fault.

configured addresses:

0x20000000   privileged_data_start 
0x20006000   privileged_data_end 

0x2000AF65   Mem manage address register <- fault location

Edit: I did another test and created a MPU region definition for 0x2000A000 to 0x2000F000 to be portMPU_REGION_READ_WRITE.

Now the program is running fine for function no.1 in my task, but it's failing in function no.2 -> fault location is now 0x20006498.

Is it really neccessary for an User mode task to explicitly define all areas that it is allowed to access? Is leaving a memory area undefined not sufficient?

Solution

To everybody with the same issue, I just received the answer from FreeRTOS support:

Yes, it is necessary to grant an unprivileged task all the accesses explicitly. By default, an unprivileged task has only access to its stack. The application writer is expected to grant access to required memory regions explicitly.