When i eat salt in solid form like on french fries, it makes sores on the roof of my mouth. foods high in sodium dry out my mouth. is this an allergy?

No: Reaction to salt in the mouth is an irritant and i suspect that you are prone to have canker sores in the first place too much salt causes one to feel thirsty since the body needs more water to establish a new balance. As anyone knows, thirst occurs after one swallows a mouthful of seawater even when the body is adequately hydrated. Do note that too much salt intake is not good for your heal.