It appears that a file created by gentoo kernel patches fails to compile:
drivers/char/kcopy/kcopy.c: In function 'kcopy_validate_task':
drivers/char/kcopy/kcopy.c:257:31: error: invalid operands to binary == (have 'kuid_t' and 'kuid_t')
drivers/char/kcopy/kcopy.c:257:67: error: invalid operands to binary == (have 'kuid_t' and 'kuid_t')
drivers/char/kcopy/kcopy.c:258:1: warning: control reaches end of non-void function [-Wreturn-type]
I was using gcc-4.6.3 to build the kernel on a non-hardened x86_64 machine.
I have sys-kernel/linux-headers-3.7 installed in case it matters. I don't think the rest of "emerge --info" is relevant, so I won't bother pasting it.
*** This bug has been marked as a duplicate of bug 458426 ***