This is a regression of bug #170544. Trying to build Usermode Linux (kernel sources 2.6.23) errors out because of a missing "asm/user.h" Reproducible: Always Steps to Reproduce:
It's not a regression, it's never been fixed anywhere but in sys-kernel/usermode-sources-2.6.18-r2., as noted on the other bug. *** This bug has been marked as a duplicate of bug 170544 ***