Bringing food on board a flight is allowed, especially if it’s a 6 or so hour flight and the complimentary snacks don’t quite fill you up. If you choose to bring food with you, consider packing something that isn’t too pungent to not bother those around you. As someone with food allergies, I’ve always packed my own meals and snacks for flights and I don’t see it as rude at all, just necessary.
Most airlines on a flight that long will offer food for purchase onboard, and even if not, they could have brought something or picked something up in the airport to bring along if they’d wanted to have a meal on the plane. It’s important to remember that you’re taking care of yourself and that it would be rude to eat a full meal in front of others who may not have brought any food with them. However, it’s not rude to bring food on board a plane – just be thoughtful about smells.
My suggestion is to pack an insulated bento box with some hearty snacks like onigiri, grapes, cut up veggies and dip, crackers and string cheese. This way, you’re getting the nourishment you need without being too intrusive to those around you.