Open Source Development Labs, the organization for which Torvalds currently works, will henceforth require contributors to Linux to certify their contributions. The certification is rendered below. Option (b) seems a bit confusing; I suppose that certifications under (b) rest on other open source material--but does it leave open the possibility of multiple, possibly inconsistent, open source licenses being stacked on top of each other?
Developer's Certificate of Origin 1.0
By making a contribution to this project, I certify that:
(a) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or
(b) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or
(c) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.