Originally Posted by fatbat
don't sew through the tarp! your bag will leak like crazy. get a curved needle and sew onto just the the canvas layer of the bag.
It'll leak a bit more, but I'm not so sure about "like crazy." You might notice that area of the bag gets a little moist in the rain, but you probably won't have a river running though it into a lake at the bottom of your bag or anything. Also, you could treat it with some sealant.
Either way, if you can avoid it, don't sew through the liner.