Potential thread stack overflow in AP_MSP::loop

[mirusu400]

Bug report

Issue details

While some detailed build configuration, There are potential stack overflow in thread function named AP_MSP::loop.

ardupilot/libraries/AP_MSP/AP_MSP.cpp

Lines 114 to 121 in 92693e0

	if (backends_using_msp_thread > 0) {
	// we've found at least 1 msp backend, start protocol handler
	if (!hal.scheduler->thread_create(FUNCTOR_BIND_MEMBER(&AP_MSP::loop, void),
	"MSP",
	1024, AP_HAL::Scheduler::PRIORITY_IO, 1)) {
	return;
	}
	}

In this line, AP_MSP::loop Allows 1024 stack size.

However, after manually checking stack, it might have 1096 Bytes in with some detailed configurations.

Steps to reproduce
Step to reproduce is quite same as #28374 and #27937

1. In boards.py, Add this line:

# In 224 line..
        env.CFLAGS += [
++          '-fstack-usage',
            '-ffunction-sections',
            '-fdata-sections',
            '-fsigned-char',
...
# In 343 line...
        env.CXXFLAGS += [
++          '-fstack-usage',
            '-std=gnu++11',

            '-fdata-sections',

This will help you automatically calculating stack size of each function.

2. Build ardupilot normally, using this configurations:

./waf configure --board r9pilot -g --check-verbose --disable-Werror --toolchain=arm-none-eabi --notests --enable-scripting --enable-opendroneid --enable-check-firmware --enable-custom-controller --enable-gps-logging --enable-header-checks --enable-stats 
./waf -v copter

Now we can get stack usage file (*.su) for each source file, So we can manually check stack size of each function.

In case of AP_MSP::loop:

There are large call stack with this flow:

AP_MSP::loop() => 32 byte
AP_MSP_Telem_Backend::hide_osd_items() => 64 byte
AP_MSP_Telem_Backend::get_rssi => 8 byte
AP_RSSI::read_receiver_rssi => 8 byte
AP_RSSI::read_pwm_pin_rssi => 8 byte
AP_HAL::PWMSource::set_pin => 40 byte
GCS::send_text => 0 byte
GCS::send_textv => 160 byte
GCS::service_statustext => 24 byte
GCS_MAVLINK::service_statustext => 120 byte
_mav_finalize_message_chan_send => 88 byte
mavlink_sign_packet => 136 byte
mavlink_sha256_update => 416 byte

=> SUM: 1096

So, there are potentially occur stack overflow in AP_SmartAudio::loop Thread function.

I only check with R9Pilot with copter, but it seems that there are lots of any other configurations that potentially make thread stack overflow.

Version

Ardupilot latest version (commit 92693e0)

Platform
[ ] All
[ ] AntennaTracker
[ x ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Hardware type
R9Pilot