tests/kernel/mem_protect/stackprot failure in frdm_k64f due to limited privilege stack size

[ioannisg]

Describe the bug
stackprot test does not run properly on frdm_k64f, triggering a BusFault

To Reproduce
Steps to reproduce the behavior:

1. mkdir build; cd build
2. cmake -DBOARD=board\frdm_k64f ..
3. make
4. See error

Expected behavior
Test should pass.

Logs

***** Booting Zephyr OS v1.14.0-rc1-563-g51c8c550d8 *****
***** BUS FAULT *****
  Stacking error
  Imprecise data bus error
  NXP MPU error, port 3
    Mode: Supervisor, Data Address: 0x20002e08
    Type: Write, Master: 0, Regions: 0x8000
***** HARD FAULT *****
  Fault escalation (see below)
***** BUS FAULT *****
  Precise data bus error
  BFAR Address: 0x20002e08
  NXP MPU error, port 3
    Mode: Supervisor, Data Address: 0x20002e08
    Type: Read, Master: 0, Regions: 0x8000
***** Hardware exception *****
Current thread ID = 0x20000080
Faulting instruction address = 0x31b6
Fatal fault in ISR! Spinning...

Additional context
The test started failing after (45631c3). This commit forced the test to run on user mode. Alone, this is not a reason for the test to fail, but:

• Running the test on user mode implies re-programming the MPU using (on ARM) the Privileged Stack, which has a limited default size (512 bytes)
• Unfortunately, due to the re-work of MPU in Arch arm mpu re work #12817, the MPU re-programming uses, roughly, 250 bytes of stack memory
• This test sets CONFIG_STACK_CANARIES=y, which, forces the compiler to embed more stuff into the stack frame(s).

Therefore, we are running out of privileged stack space, when re-programming the MPU, which causes a BusFault on accessing the Guard area.

---

Workaround:
Setting CONFIG_PRIVILEGED_STACK_SIZE=1024 makes the test pass.

While we do need to decrease the stack usage of MPU re-programming in ARM builds with User-mode, I believe, it is ok to have an increased size for privileged stack areas, when compiling with support for CANARIES, since, itself, that feature adds to the stack usage. That is, I propose:

config PRIVILEGED_STACK_SIZE
	int "Size of privileged stack"
	default 512 if MPU_STACK_GUARD || BUILTIN_STACK_GUARD
	default 512 if COVERAGE_GCOV
	default 384 if ARC
  >>>   default 1024 if ARM && STACK_CANARIES && (MPU_STACK_GUARD || BUILTIN_STACK_GUARD)
	default 256
	depends on ARCH_HAS_USERSPACE

[andrewboie]

This default is getting too complicated.
How was the original value of 256 arrived at?

[andrewboie]

Based on discussion this morning we could perhaps just do:

config PRIVILEGED_STACK_SIZE
	int "Size of privileged stack"
	default 1024
	depends on ARCH_HAS_USERSPACE

[ioannisg]

#14090 fixes this one.