Well, the reason Linus could confidently choose to license the kernel as GPLv2 is exactly because it inherited no incompatibly-licensed code. USL v. BSDi (also

Well, the reason Linus could confidently choose to license the kernel as GPLv2 is exactly because it inherited no incompatibly-licensed code. USL v. BSDi (also 1992 — what a year!) is evidence of how non-trivial this actually is: https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc._v._Berkeley_Software_Design,_Inc https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc.....