MPU fault during application startup

[nordic-krch]

Describe the bug
Application can hit mpu fault before starting main thread. This is caused by stack misuse. In function z_arch_switch_to_main_thread thread entry is provided as forth argument (_main). Then within the function stack pointer is changed:

zephyr/arch/arm/include/kernel_arch_func.h

Line 106 in 269935d

__set_PSP((u32_t)start_of_main_stack);

After stack is changed, call to thread entry occurs which is using _main as the argument. If variable is stored on stack, it will be fetched from stack but at that point this is already different stack and value is corrupted:

zephyr/arch/arm/include/kernel_arch_func.h

Line 115 in 269935d

z_thread_entry(_main, 0, 0, 0);

So _main variable will be corrupted.

To Reproduce
Modify hello_world example by using following prj.conf:

CONFIG_MPU_STACK_GUARD=y
CONFIG_NO_OPTIMIZATIONS=y
CONFIG_LOG=y

Steps to reproduce the behavior:

1. mkdir build; cd build
2. cmake -GNinja -DBOARD=nrf52840_pca10056 ..
3. ninja flash
4. See error

Fatal fault in essential thread! Spinning...
***** MPU FAULT *****
  Instruction Access Violation
***** Hardware exception *****
Current thread ID = 0x200005d8
Faulting instruction address = 0x20001b48
Fatal fault in essential thread! Spinning...

Expected behavior
Sample should start.

Impact
Suddenly system crashes during initialization.

[ioannisg]

Going to look into this.

[ioannisg]

Yes, this looks like a bug. It was, probably, inserted when we refactored this file to look like a C file, instead of assembly code.