Created attachment 365270 [details] build log sys-kernel/gentoo-sources-3.12.5 fails during install phase.
Created attachment 365272 [details] emerge --info
Funnily enough, you are hitting bug #488492 which only affects 3.12.{0,1) and will be fixed by this upgrade to 3.12.5. The issue only happens intermittently, I would try the emerge again. Then if it installs ok, build it and get booted to is ASAP to get this fix. Nothing really to be done for this specific package, though. *** This bug has been marked as a duplicate of bug 488492 ***
It's going to be an ironic answer, but normally you would want to upgrade to another kernel to fix this; as 3.12.0 and 3.12.1 affected by this sandbox bug. So, unless you have another kernel handy to switch to, you'll need to download the patch http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/patch/?id=ede4cebce16f5643c61aedd6d88d9070a1d23a68 and apply it to your current kernel and boot into that. You can apply a patch by doing `patch -p1 < ABC` in /usr/src/linux/ (replace ABC by the path to the patch); after you have done that, you can rebuild that kernel and boot into it.
*** This bug has been marked as a duplicate of bug 488492 ***