himself because on 76a s.v. u'parchinan he says goy v'eved haba al bas yisrael havlad kasher, ela shifcha vladah eved, which implies the child with a Jewish mother is Jewish, and does not need geirus. Rashi also seems to contradict himself on 70 s.v. kol that Herod probably did not marry Jewish wives. That implies had he married Jewish wives, his children (he was an eved) would be full Jews. But Rashi holds goy v'eved haba al bas yisrael havlad kosher but needs geirus! But the answer is a non-Jewish father is worse than an eved father. See Shaar Hamelech, Issurei Biah 15:3, and Rav Moshe Soloveichik zt"l in Chiddushei Hagram Halevi Issurei Biah 15:3. A non-Jewish father
passes on his psul of non-Jew to the child. But an eved has no yichus (pedigree) so he has no psul to pass on. So When Rashi says goy v'eved haba al bas yisrael havald kasher but needs geirus, he only means goy haba. But eved sheba is completely Jewish and needs no geirus. This shita is also brought in Piskei Tosfos, Asara Yuchsin, 142.
Rav Ahron Soloveichik zt"l explained that Rashi doesn't mean - in the case of goy haba - the child is a goy. The child is Jewish, but he has a psul kahal (he's not allowed to enter the community), but that psul is removed with tevila. It is similar to a ger who had kabalas mitzvos, milah and tevila but didn't bring his korban (in the time of the mikdash) that he is Jewish but may not marry a Jewish woman until he brings his korban, according to Shita Mekubetzes, Kerisus 9a. (See Perach Mateh Aharon, Milah 1:7. The application to this Rashi is not in that piece, but he said it in shiur on Bava Basra.)
An almost identical explanation like Rav Ahron was offered by Rav Itzele Ponovicher in Zecher Yitzchak, siman 4. His thesis is that all aspects of gerus - milah, tevila and korban - are to purify the ger from his previous state of non-Jewishness. He then says with this we can explain [Rashi's shita] that the child is Jewish but needs tevila because he has the psul of goy, that the psul is the psul of both of the parents, but the yichus is after the mother... He is a full Yisrael but needs tevila to marry a bas yisrael....