I noticed water on my chimney (inside the garage), and after going into the attic, it was obvious there has been a leak problem for a long time around the chimney. I have seen various rubberized patching products that can be trowled on to seal this common leak area. Does anyone advise against using this product? Basically, you apply one coat, set the fabric mesh in place, and recoat. The main downside I could think of is that it would be a pain in the butt to remove when it was time to reflash, reroof, etc..
Would really help if we knew what kind of roof you have. Pictures inside and above would help a lot.
Just based on your initial post, can't give you very much guidance. I can tell you definitely "NO", do not use mastic to repair it. Odds are you won't fix your problem but you will create a bigger one and make it harder and more expensive to repair correctly
"Rubberized patching products" for something like this are just that; patches, at best. Doing it right is more than a 5 or 6 on a scale of 1 to 10, but manageable as long as you have the determination to do so and are comfortable working up there. Simultaneously, I would look at the crown on the chimney as well, as that can also be a path for water to migrate down the chimney. If you want to take it on yourself, post some pictures, and there are a number of knowledgable folks here who can assist you. Otherwise, I would get a reputable local roofer out, and would skip any thoughts of bandaids. Just my 2 cents worth.