sched_register_task() may register too many tasks

[sjorsdewit]

sched_register_task() checks if NUM_TASKS is reached. It seems there is an off-by-one in this check:

The bug

If NG(num_registered_tasks) == NUM_TASKS, no further tasks should be register-able, as all available slots are already claimed. The current implementation returns SUCCESS and results in the situation NG(num_registered_tasks) == (NUM_TASKS+1). This also means that the arrays NG(m_index) and NG(m_info) are accessed out-of-bounds (indexed one past the end).

How to reproduce

• Register exactly NUM_TASKS + 1 tasks and observe that no assertion is triggered (though the program may crash due to undefined behaviour).
• Register at least NUM_TASKS + 2 tasks and observe that the assertion triggers correctly

Impact

This only impacts firmwares that try to register exactly one task too many, which is probably fairly rare (but can be hard to debug).

[LOorts-Aloxy]

Thank you for the pull request! It seems this is indeed a problem in the code.

[LOorts-Aloxy]

Thank you for your pull request @sjorsdewit, this can prevent a lot of weird behavior! :+1: