Linux kernel MicroPython snippet "disabling" /dev/urandom for specific commands.

disable_urandom.py

from kernel_ffi import callback, current, str as s

task_struct = partial_struct("task_struct")
file_operations = partial_struct("file_operations")

real_urandom_read = urandom_read
no_random_progs = ["ssh-keygen"]

def my_urandom_read(filp, buf, count, ppos):
    # technically should take task lock for 'comm' but meh.
    if s(int(task_struct(current()).comm)) in no_random_progs:
        print("no random for you!")
        # lousy copy_to_user, should be improved if you use with untrusted programs
        memcpy(buf, b"a" * count, count)
        return count

    return real_urandom_read(filp, buf, count, ppos)

cb = callback(my_urandom_read)
file_operations(urandom_fops).read = cb.ptr()

# to stop
file_operations(urandom_fops).read = int(real_urandom_read)