Although a ba'al mum (a kohen/priest with a blemish or defect) can't approach to bring near to offer food for his G-d (Vayikra/Leviticus 21:17), can't approach to bring near the offerings made by Fire for HaShem (21:21). Which tasks in his role as kohen may he fulfill?
The gemara in Yoma 23b (ArtScroll 23b2 notes 12 and 17) in discussing the removal of the ashes, states that the restrictions on a baal mum applies only to those duties which are considered avodah. The gemara states that if removing the ash is considered an avodah (tana kamah), then a kohen baal mum may not do so. Rabbi Eliezer says that it is not an avodah and a kohen baal mum may do so. This would also apply to duties such as guarding the doors, and checking the wood for worms.
Such a kohen must also avoid tum'ah and follow the marriage laws as well as being allowed to eat from the korbanos with the rest of the kohanim. Emor 21:22
His God's food from the most holy and from the holy ones, he may eat.
from the most holy: [This phrase] refers to [those sacrifices] with a higher degree of holiness;
and from the holy ones, he may eat: These are the sacrifices with a lesser degree of holiness. Now, if sacrifices with a higher degree of holiness are mentioned [that a kohen with a defect may eat of them], why is it necessary to state [the same of] sacrifices with a lesser degree of holiness? [The answer is that] had they not been stated, I would have concluded [the following]: Indeed [a kohen] with a defect may eat of the sacrifices with a higher degree of holiness, since we find that these were allowed to [be eaten even by] a non- kohen , since Moses [who was not a kohen ,] ate of the flesh of the sacrifices of the investitures. Consequently, perhaps the most holy sacrifices carry with them this leniency]. The breast and thigh of sacrifices with a lesser degree of holiness, however, shall not be eaten [by a kohen with a defect], for we do not find an instance where a non- kohen takes a share of these!“ Therefore, Scripture states here ”or from the holy" [thereby permitting a kohen with a defect to eat from the sacrifices with a lesser degree of holiness as well]. In this way the matter is explained in [Tractate] Zevachim (101b).