comment-search-backward is a compiled Lisp function in `newcomment.el'.

(comment-search-backward &optional LIMIT NOERROR)

Find a comment start between LIMIT and point.
Moves point to inside the comment and returns the position of the
comment-starter. If no comment is found, moves point to LIMIT
and raises an error or returns nil if NOERROR is non-nil.

Ensure that `comment-normalize-vars' has been called before you use this.