~ $ ls -la /usr/src/

total 20
drwxr-xr-x  5 root root 4096 Oct 20 20:22 .
drwxr-xr-x 17 root root 4096 Aug  4 22:01 ..
-rw-r--r--  1 root root    0 Jun  6 00:52 .keep
lrwxrwxrwx  1 root root   19 Oct 20 15:52 linux -> linux-3.10.7-gentoo
drwxr-xr-x 25 root root 4096 Oct 20 22:12 linux-3.10.7-gentoo
drwxr-xr-x 24 root root 4096 Aug 25 13:03 linux-3.8.13-gentoo
drwxr-xr-x  2 root root 4096 Oct 20 21:39 nvidia-325.15
lrwxrwxrwx  1 root root   28 Sep 11 21:36 vboxhost-4.2.18 -> /opt/VirtualBox/src/vboxhost

~ $ ls -la /usr/src/linux/

total 81960
drwxr-xr-x  25 root root     4096 Oct 20 22:12 .
drwxr-xr-x   5 root root     4096 Oct 20 20:22 ..
-rw-r-----   1 root root   105863 Oct 20 20:54 .config
-rw-r--r--   1 root root   105852 Oct 20 13:07 .config.old
-rw-r--r--   1 root root     1091 Jun 30 19:13 .gitignore
-rw-r--r--   1 root root     4465 Jun 30 19:13 .mailmap
-rw-r--r--   1 root root      668 Oct 20 21:09 .missing-syscalls.d
-rw-r--r--   1 root root  2990201 Oct 20 21:30 .tmp_System.map
-rw-r-----   1 root root  1625680 Oct 20 21:30 .tmp_kallsyms1.o
-rw-r-----   1 root root  1625680 Oct 20 21:30 .tmp_kallsyms2.o
drwxr-xr-x   2 root root    24576 Oct 20 21:30 .tmp_versions
-rwxr-x---   1 root root 20102957 Oct 20 21:30 .tmp_vmlinux1
-rwxr-x---   1 root root 22200109 Oct 20 21:30 .tmp_vmlinux2
-rw-r-----   1 root root        2 Oct 20 21:30 .version
-rw-r--r--   1 root root       75 Oct 20 21:30 .vmlinux.cmd
-rw-r--r--   1 root root    18693 Jun 30 19:13 COPYING
-rw-r--r--   1 root root    95317 Jun 30 19:13 CREDITS
drwxr-xr-x 100 root root    12288 Aug 22 21:53 Documentation
-rw-r--r--   1 root root     2536 Jun 30 19:13 Kbuild
-rw-r--r--   1 root root      277 Aug 22 21:53 Kconfig
-rw-r--r--   1 root root   255053 Aug 22 21:53 MAINTAINERS
-rw-r--r--   1 root root    48424 Aug 22 21:53 Makefile
-rw-r--r--   1 root root   548540 Oct 20 21:31 Module.symvers
-rw-r--r--   1 root root    18736 Jun 30 19:13 README
-rw-r--r--   1 root root     7485 Jun 30 19:13 REPORTING-BUGS
-rw-r--r--   1 root root  2990201 Oct 20 21:30 System.map
drwxr-xr-x  33 root root     4096 Aug 24 21:21 arch
drwxr-xr-x   3 root root     4096 Oct 20 21:16 block
drwxr-xr-x   4 root root    20480 Oct 20 21:31 crypto
drwxr-xr-x   2 root root     4096 Aug 22 21:54 distro
drwxr-xr-x 112 root root     4096 Oct 20 21:24 drivers
drwxr-xr-x  36 root root     4096 Oct 20 21:25 firmware
drwxr-xr-x  73 root root    12288 Oct 20 21:16 fs
drwxr-xr-x  28 root root     4096 Aug 24 21:00 include
drwxr-xr-x   2 root root     4096 Oct 20 21:30 init
drwxr-xr-x   2 root root     4096 Oct 20 21:16 ipc
drwxr-xr-x  11 root root    12288 Oct 20 21:12 kernel
drwxr-xr-x   9 root root    20480 Oct 20 21:32 lib
drwxr-xr-x   2 root root    12288 Oct 20 21:12 mm
-rw-r--r--   1 root root     8920 Oct 20 21:30 modules.builtin
-rw-r--r--   1 root root    24416 Oct 20 21:30 modules.order
drwxr-xr-x  55 root root     4096 Oct 20 21:30 net
drwxr-xr-x  12 root root     4096 Aug 22 21:54 samples
drwxr-xr-x  13 root root     4096 Oct 20 21:09 scripts
drwxr-xr-x   9 root root     4096 Oct 20 21:16 security
drwxr-xr-x  22 root root     4096 Oct 20 21:25 sound
drwxr-xr-x  17 root root     4096 Aug 22 21:54 tools
drwxr-xr-x   2 root root     4096 Oct 20 21:09 usr
drwxr-xr-x   3 root root     4096 Aug 22 21:54 virt
-rwxr-x---   1 root root 22200109 Oct 20 21:30 vmlinux
-rw-r-----   1 root root 25068651 Oct 20 21:30 vmlinux.o

-------------------- bbswitch build log

The complete build log is located at '/var/log/portage/elog/sys-power:bbswitch-0.7:20131022-223026.log'

~ # cat /var/log/portage/elog/sys-power:bbswitch-0.7:20131022-223026.log

 * Package:    sys-power/bbswitch-0.7
 * Repository: gentoo
 * Maintainer: pacho@gentoo.org mva@mva.name,rei4dan@gmail.com,proxy-maint@gentoo.org
 * USE:        amd64 elibc_glibc kernel_linux userland_GNU
 * FEATURES:   ccache preserve-libs sandbox userpriv usersandbox
 * Determining the location of the kernel source code
 * Found kernel source directory:
 *     /usr/src/linux
 * Found kernel object directory:
 *     /lib/modules/3.10.7-gentoo/build
 * Found sources for kernel version:
 *     3.10.7-gentoo
>>> Unpacking source...
>>> Unpacking bbswitch-0.7.tar.gz to /var/tmp/portage/sys-power/bbswitch-0.7/work
>>> Source unpacked in /var/tmp/portage/sys-power/bbswitch-0.7/work
>>> Preparing source in /var/tmp/portage/sys-power/bbswitch-0.7/work/bbswitch-0.7 ...
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/sys-power/bbswitch-0.7/work/bbswitch-0.7 ...
>>> Source configured.
>>> Compiling source in /var/tmp/portage/sys-power/bbswitch-0.7/work/bbswitch-0.7 ...
ln: failed to create symbolic link ‘Module.symvers’: File exists
 * Preparing bbswitch module
make -j5 HOSTCC=x86_64-pc-linux-gnu-gcc CROSS_COMPILE=x86_64-pc-linux-gnu- 'LDFLAGS=-m elf_x86_64' KVERSION=3.10.7-gentoo default 
make -C /lib/modules/3.10.7-gentoo/build M="$(pwd)" modules
make: *** /lib/modules/3.10.7-gentoo/build: Permission denied.  Stop.
make: *** [default] Error 2
 * ERROR: sys-power/bbswitch-0.7::gentoo failed (compile phase):
 *   emake failed
 * 
 * If you need support, post the output of `emerge --info '=sys-power/bbswitch-0.7::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=sys-power/bbswitch-0.7::gentoo'`.
 * The complete build log is located at '/var/log/portage/elog/sys-power:bbswitch-0.7:20131022-223026.log'.
 * The ebuild environment file is located at '/var/tmp/portage/sys-power/bbswitch-0.7/temp/environment'.
 * Working directory: '/var/tmp/portage/sys-power/bbswitch-0.7/work/bbswitch-0.7'
 * S: '/var/tmp/portage/sys-power/bbswitch-0.7/work/bbswitch-0.7'