Difference between pages "Linux Containers" and "Genkernel Quick Start Tutorial/pt-br"

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Linux Containers, or LXC, is a Linux feature that allows Linux to run one or more isolated virtual systems (with their own network interfaces, process namespace, user namespace, and power state) using a single Linux kernel on a single server.
== Conceitos ==


== Status ==
TBC


As of Linux kernel 3.1.5, LXC is usable for isolating your own private workloads from one another. It is not yet ready to isolate potentially malicious users from one another or the host system. For a more mature containers solution that is appropriate for hosting environments, see [[OpenVZ]].
== Recompilando os códigos fonte do kernel Gentoo a partir do SystemRescue CD chroot ==


LXC containers don't yet have their own system uptime, and they see everything that's in the host's {{c|dmesg}} output, among other things. But in general, the technology works.
Um segundo caso que muitos usuários de Funtoo users encararão é recompilar seu próprio kernel quando instalar um um Funtoo novinho a partir de um stage 3 archive (o cenário mais comum é inicializar a maquina com o SystemRescue CD).


== Basic Info ==
{{Fancynote| Se quiser utilizar o System rescue CD sources fornecido pelo Funtoo {{Package|sys-kernel/sysrescue-std-sources}}, a filosofia permanece exatamente a mesma.}}


== First step emerging the required packages ==


* Linux Containers are based on:
The first step is to emerge:
** Kernel namespaces for resource isolation
** CGroups for resource limitation and accounting


{{Package|app-emulation/lxc}} is the userspace tool for Linux containers
# The Gentoo kernel sources: {{Package|sys-kernel/gentoo-sources}}
# Genkernel itself: {{Package|sys-kernel/genkernel}}


== Control groups ==
This is achieved by running the following:
<console>
###i## emerge sys-kernel/gentoo-sources sys-kernel/genkernel
</console>


* Control groups (cgroups) in kernel since 2.6.24
Once the Gentoo kernel sources are deployed, you should find a directory named '''linux-''version''-gentoo''' (e.g. linux-2.6.39-gentoo) under ''<code>/usr/src</code>''. Update the ''<code>linux</code>'' symlink to point to this directory:
** Allows aggregation of tasks and their children
<console>
** Subsystems (cpuset, memory, blkio,...)
###i## cd /usr/src
** accounting - to measure how much resources certain systems use
###i## rm linux
** resource limiting - groups can be set to not exceed a set memory limit
###i## ln -s linux-2.6.39-gentoo linux
** prioritization - some groups may get a larger share of CPU
</console>
** control - freezing/unfreezing of cgroups, checkpointing and restarting
== Second step: Grabbing and tweaking a configuration file ==
** No disk quota limitation ( -> image file, LVM, XFS, directory tree quota,...)


== Subsystems ==
How to start your kernel configuration? Simply by using the same configuration template the running System Rescue CD kernel had been built with! Before chrooting in your Funtoo instance, you did something like:
<br>
{{console|body=
###i## cat /proc/cgroups
subsys_name hierarchy num_cgroups enabled
cpuset
cpu
cpuacct
memory
devices
freezer
blkio
perf_event
hugetlb
}}


#cpuset    -> limits tasks to specific CPU/CPUs
<console>
#cpu        -> CPU shares
###i## mount -o bind /proc /mnt/gentoo/proc
#cpuacct    -> CPU accounting
</console>
#memory    -> memory and swap limitation and accounting
Or:
#devices    -> device allow deny list
<console>
#freezer    -> suspend/resume tasks
###i## mount -t proc none /mnt/gentoo/proc
#blkio      -> I/O priorization (weight, throttle, ...)
</console>
#perf_event -> support for per-cpu per-cgroup monitoring [http://lwn.net/Articles/421574/ perf_events]
#hugetlb    -> cgroup resource controller for HugeTLB pages  [http://lwn.net/Articles/499255/ hugetlb]


== Configuring the Funtoo Host System ==
In your chroot environment (or from a System Rescue CD virtual terminal) if you look what ''<code>/proc</code>'' contains you will notice a file named ''<code>config.gz</code>'':


=== Install LXC kernel ===
<console>
Any kernel beyond 3.1.5 will probably work. Personally I prefer {{Package|sys-kernel/gentoo-sources}} as these have support for all the namespaces without sacrificing the xfs, FUSE or NFS support for example. These checks were introduced later starting from kernel 3.5, this could also mean that the user namespace is not working optimally.
###i## ls /proc


* User namespace (EXPERIMENTAL) depends on EXPERIMENTAL and on UIDGID_CONVERTED
...
** config UIDGID_CONVERTED
dr-xr-xr-x  7 root      root                    0 May 23 03:13 952
*** True if all of the selected software components are known to have uid_t and gid_t converted to kuid_t and kgid_t where appropriate and are otherwise safe to use with the user namespace.
dr-xr-xr-x  7 root      root                    0 May 23 03:13 953
**** Networking - depends on NET_9P = n
dr-xr-xr-x  7 root      root                    0 May 23 18:42 9834
**** Filesystems - 9P_FS = n, AFS_FS = n, AUTOFS4_FS = n, CEPH_FS = n, CIFS = n, CODA_FS = n, FUSE_FS = n, GFS2_FS = n, NCP_FS = n, NFSD = n, NFS_FS = n, OCFS2_FS = n, XFS_FS = n
...
**** Security options - Grsecurity - GRKERNSEC = n (if applicable)
-r--r--r--  1 root      root                16024 May 23 22:27 config.gz
-r--r--r--  1 root      root                    0 May 23 22:27 consoles
-r--r--r-- 1 root      root                    0 May 23 22:27 cpuinfo
...
</console>


** As of 3.10.xx kernel, all of the above options are safe to use with User namespaces, except for XFS_FS, therefore with kernel >=3.10.xx, you should answer XFS_FS = n, if you want User namespaces support.
''<code>config.gz</code>'' holds the running kernel (System Rescue CD) configuration, just copy the unziped content into the Gentoo sources directory:
** in your kernel source directory, you should check init/Kconfig and find out what UIDGID_CONVERTED depends on
<console>
###i## cd /usr/src/linux
###i## zcat /proc/config.gz > .config
</console>


==== Kernel configuration ====
Next, run ''<code>make oldconfig</code>'' to set all newly added options:
These options should be enable in your kernel to be able to take full advantage of LXC.
<console>
###i## make oldconfig
</console>


* General setup
Next, tweak the kernel configuration in the way you prefer (manually edition of the .config file, make nconfig, make menuconfig....) if you wish. You are not ready yet! A final step is required: '''you ''must'' either set CONFIG_INITRAMFS_SOURCE to a blank value (CONFIG_INITRAMFS_SOURCE="") either delete the statement in the .config file'''. Forgotting to do that will make Genkernel abort the compilation process with a message like:
** CONFIG_NAMESPACES
<console>
*** CONFIG_UTS_NS
/usr/src/linux-2.6.39-gentoo/scripts/gen_initramfs_list.sh: Cannot open '/var/tmp/genkernel/initramfs-2.6.32.14-std155-i386.cpio.gz'
*** CONFIG_IPC_NS
make[1]: *** [usr/initramfs_data.cpio.lzma] Error 1
*** CONFIG_PID_NS
</console>
*** CONFIG_NET_NS
== Third step: Building and installing the kernel ==
*** CONFIG_USER_NS
** CONFIG_CGROUPS
*** CONFIG_CGROUP_DEVICE
*** CONFIG_CGROUP_SCHED
*** CONFIG_CGROUP_CPUACCT
*** CONFIG_CGROUP_MEM_RES_CTLR (in 3.6+ kernels it's called CONFIG_MEMCG)
*** CONFIG_CGROUP_MEM_RES_CTLR_SWAP (in 3.6+ kernels it's called CONFIG_MEMCG_SWAP)
*** CONFIG_CPUSETS (on multiprocessor hosts)
* Networking support
** Networking options
*** CONFIG_VLAN_8021Q
* Device Drivers
** Character devices
*** Unix98 PTY support
**** CONFIG_DEVPTS_MULTIPLE_INSTANCES
** Network device support
*** Network core driver support
**** CONFIG_VETH
**** CONFIG_MACVLAN


Once you have lxc installed, you can then check your kernel config with:
This is simply achieved by:
{{console|body=
<console>
# ##i##CONFIG=/path/to/config /usr/sbin/lxc-checkconfig
###i## genkernel --no-mrproper all
}}
</console>


=== Emerge lxc ===
The same remarks written in the [[Genkernel_Quick_Start_Tutorial#Third_step:_Building_and_installing_the_kernel|third paragraph]] of the first use case are still valid here.
{{console|body=
# ##i##emerge app-emulation/lxc
}}


=== Configure Networking For Container ===
[[Category:Kernel]]
 
Typically, one uses a bridge to allow containers to connect to the network. This is how to do it under Funtoo Linux:
 
# create a bridge using the Funtoo network configuration scripts. Name the bridge something like {{c|brwan}} (using {{c|/etc/init.d/netif.brwan}}). Configure your bridge to have an IP address.
# Make your physical interface, such as {{c|eth0}}, an interface with no IP address (use the Funtoo {{c|interface-noip}} template.)
# Make {{c|netif.eth0}} a slave of {{c|netif.brwan}} in {{c|/etc/conf.d/netif.brwan}}.
# Enable your new bridged network and make sure it is functioning properly on the host.
 
You will now be able to configure LXC to automatically add your container's virtual ethernet interface to the bridge when it starts, which will connect it to your network.
 
== Setting up a Funtoo Linux LXC Container ==
 
Here are the steps required to get Funtoo Linux running <i>inside</i> a container. The steps below show you how to set up a container using an existing Funtoo Linux OpenVZ template. It is now also possible to use [[Metro]] to build an lxc container tarball directly, which will save you manual configuration steps and will provide an {{c|/etc/fstab.lxc}} file that you can use for your host container config. See [[Metro Recipes]] for info on how to use Metro to generate an lxc container.
 
=== Create and Configure Container Filesystem ===
 
# Start with a Funtoo LXC template, and unpack it to a directory such as {{c|/lxc/funtoo0/rootfs/}}
# Create an empty {{c|/lxc/funtoo0/fstab}} file
# Ensure {{c|c1}} line is uncommented (enabled) and {{c|c2}} through {{c|c6}} lines are disabled in {{c|/lxc/funtoo0/rootfs/etc/inittab}}
 
That's almost all you need to get the container filesystem ready to start.
 
=== Create Container Configuration Files ===
 
Create the following files:
 
==== {{c|/lxc/funtoo0/config}} ====
 
 
and also create symlink from
==== {{c|/lxc/funtoo0/config to /etc/lxc/funtoo0.conf }} ====
{{console|body=
###i## install -d /etc/lxc/funtoo0
###i## ln -s /lxc/funtoo0/config /etc/lxc/funtoo0/config
}}
 
{{note| Daniel Robbins needs to update this config to be more in line with http://wiki.progress-linux.org/software/lxc/ -- this config appears to have nice, refined device node permissions and other goodies. // note by Havis to Daniel, this config is already superior.}}
 
 
Read "man 5 lxc.conf" , to get more information about linux container configuration file.
<pre>
## Container
lxc.utsname                            = funtoo0
lxc.rootfs                              = /lxc/funtoo0/rootfs/
lxc.arch                                = x86_64
#lxc.console                            = /var/log/lxc/funtoo0.console  # uncomment if you want to log containers console
lxc.tty                                = 6  # if you plan to use container with physical terminals (eg F1..F6)
#lxc.tty                                = 0  # set to 0 if you dont plan to use the container with physical terminal, also comment out in your containers /etc/inittab  c1 to c6 respawns (e.g. c1:12345:respawn:/sbin/agetty 38400 tty1 linux)
lxc.pts                                = 1024
 
 
## Capabilities
lxc.cap.drop                            = audit_control
lxc.cap.drop                            = audit_write
lxc.cap.drop                            = mac_admin
lxc.cap.drop                            = mac_override
lxc.cap.drop                            = mknod
lxc.cap.drop                            = setfcap
lxc.cap.drop                            = setpcap
lxc.cap.drop                            = sys_admin
#lxc.cap.drop                            = sys_boot # capability to reboot the container
#lxc.cap.drop                            = sys_chroot # required by SSH
lxc.cap.drop                            = sys_module
#lxc.cap.drop                            = sys_nice
lxc.cap.drop                            = sys_pacct
lxc.cap.drop                            = sys_rawio
lxc.cap.drop                            = sys_resource
lxc.cap.drop                            = sys_time
#lxc.cap.drop                            = sys_tty_config # required by getty
 
## Devices
#lxc.cgroup.devices.allow              = a # Allow access to all devices
lxc.cgroup.devices.deny                = a # Deny access to all devices
 
# Allow to mknod all devices (but not using them)
lxc.cgroup.devices.allow                = c *:* m
lxc.cgroup.devices.allow                = b *:* m
 
lxc.cgroup.devices.allow                = c 1:3 rwm # /dev/null
lxc.cgroup.devices.allow                = c 1:5 rwm # /dev/zero
lxc.cgroup.devices.allow                = c 1:7 rwm # /dev/full
lxc.cgroup.devices.allow                = c 1:8 rwm # /dev/random
lxc.cgroup.devices.allow                = c 1:9 rwm # /dev/urandom
#lxc.cgroup.devices.allow                = c 4:0 rwm # /dev/tty0 ttys not required if you have lxc.tty = 0
#lxc.cgroup.devices.allow                = c 4:1 rwm # /dev/tty1 devices with major number 4 are "real" tty devices
#lxc.cgroup.devices.allow                = c 4:2 rwm # /dev/tty2
#lxc.cgroup.devices.allow                = c 4:3 rwm # /dev/tty3
lxc.cgroup.devices.allow                = c 5:0 rwm # /dev/tty
lxc.cgroup.devices.allow                = c 5:1 rwm # /dev/console
lxc.cgroup.devices.allow                = c 5:2 rwm # /dev/ptmx
lxc.cgroup.devices.allow                = c 10:229 rwm # /dev/fuse
lxc.cgroup.devices.allow                = c 136:* rwm # /dev/pts/* devices with major number 136 are pts
lxc.cgroup.devices.allow                = c 254:0 rwm # /dev/rtc0
 
## Limits#
lxc.cgroup.cpu.shares                  = 1024
lxc.cgroup.cpuset.cpus                = 0        # limits container to CPU0
lxc.cgroup.memory.limit_in_bytes      = 512M
lxc.cgroup.memory.memsw.limit_in_bytes = 1G
#lxc.cgroup.blkio.weight                = 500      # requires cfq block scheduler
 
## Filesystem
#containers fstab should be outside it's rootfs dir (e.g. /lxc/funtoo0/fstab is ok, but /lxc/funtoo0/rootfs/etc/fstab is wrong!!!)
#lxc.mount                              = /lxc/funtoo0/fstab     
 
#lxc.mount.entry is prefered, because it supports relative paths
lxc.mount.entry                        = proc proc proc nosuid,nodev,noexec  0 0
lxc.mount.entry                        = sysfs sys sysfs nosuid,nodev,noexec,ro 0 0
lxc.mount.entry                        = devpts dev/pts devpts nosuid,noexec,mode=0620,ptmxmode=000,newinstance 0 0
lxc.mount.entry                        = tmpfs dev/shm tmpfs nosuid,nodev,mode=1777 0 0
lxc.mount.entry                        = tmpfs run tmpfs nosuid,nodev,noexec,mode=0755,size=128m 0 0
lxc.mount.entry                        = tmpfs tmp tmpfs nosuid,nodev,noexec,mode=1777,size=1g 0 0
 
##Example of having /var/tmp/portage as tmpfs in container
#lxc.mount.entry                        = tmpfs var/tmp/portage tmpfs defaults,size=8g,uid=250,gid=250,mode=0775 0 0
##Example of bind mount
#lxc.mount.entry                        = /srv/funtoo0 /lxc/funtoo0/rootfs/srv/funtoo0 none defaults,bind 0 0
 
## Network
lxc.network.type                        = veth
lxc.network.flags                      = up
lxc.network.hwaddr                      = #put your MAC address here, otherwise you will get a random one
lxc.network.link                        = br0
lxc.network.name                        = eth0
#lxc.network.veth.pair                  = veth-example
</pre>
 
Read "man 7 capabilities" to get more information aboout Linux capabilities.
 
Above, use the following command to generate a random MAC for {{c|lxc.network.hwaddr}}:
 
{{console|body=
###i## openssl rand -hex 6 | sed 's/\(..\)/\1:/g; s/.$//'
}}
 
It is a very good idea to assign a static MAC address to your container using {{c|lxc.network.hwaddr}}. If you don't, LXC will auto-generate a new random MAC every time your container starts, which may confuse network equipment that expects MAC addresses to remain constant.
 
It might happen from case to case that you aren't able to start your LXC Container with the above generated MAC address so for all these who run into that problem here is a little script that connects your IP for the container with the MAC address. Just save the following code as {{c|/etc/lxc/hwaddr.sh}}, make it executable and run it like {{c|/etc/lxc/hwaddr.sh xxx.xxx.xxx.xxx}} where xxx.xxx.xxx.xxx represents your Container IP. <br>{{c|/etc/lxc/hwaddr.sh}}:
 
<pre>
#!/bin/sh
IP=$*
HA=`printf "02:00:%x:%x:%x:%x" ${IP//./ }`
echo $HA
</pre>
 
==== {{c|/lxc/funtoo0/fstab}} ====
{{fancynote| It is now preferable to have mount entries directly in config file instead of separate fstab:}}
Edit the file {{c|/lxc/funtoo0/fstab}}:
<pre>
none /lxc/funtoo0/dev/pts devpts defaults 0 0
none /lxc/funtoo0/proc proc defaults 0 0
none /lxc/funtoo0/sys sysfs defaults 0 0
none /lxc/funtoo0/dev/shm tmpfs nodev,nosuid,noexec,mode=1777,rw 0 0
</pre>
 
== LXC Networking ==
*veth - Virtual Ethernet (bridge)
*vlan - vlan interface (requires device able to do vlan tagging)
*macvlan (mac-address based virtual lan tagging) has 3 modes:
**private
**vepa (Virtual Ethernet Port Aggregator)
**bridge
*phys - dedicated host NIC
[https://blog.flameeyes.eu/2010/09/linux-containers-and-networking Linux Containers and Networking]
 
Enable routing on the host:
By default Linux workstations and servers have IPv4 forwarding disabled.
{{console|body=
###i## echo "1" > /proc/sys/net/ipv4/ip_forward
###i## cat /proc/sys/net/ipv4/ip_forward
# 1
}}
 
== Initializing and Starting the Container ==
 
You will probably need to set the root password for the container before you can log in. You can use chroot to do this quickly:
 
{{console|body=
###i## chroot /lxc/funtoo0/rootfs
(chroot) ###i## passwd
New password: XXXXXXXX
Retype new password: XXXXXXXX
passwd: password updated successfully
(chroot) ###i## exit
}}
 
Now that the root password is set, run:
 
{{console|body=
###i## lxc-start -n funtoo0 -d
}}
 
The {{c|-d}} option will cause it to run in the background.
 
To attach to the console:
 
{{console|body=
###i## lxc-console -n funtoo0
}}
 
You should now be able to log in and use the container. In addition, the container should now be accessible on the network.
 
To directly attach to container:
 
{{console|body=
###i## lxc-attach -n funtoo0
}}
 
To stop the container:
 
{{console|body=
###i## lxc-stop -n funtoo0
}}
 
Ensure that networking is working from within the container while it is running, and you're good to go!
 
== Starting LXC container during host boot ==
 
# You need to create symlink in {{c|/etc/init.d/}} to {{c|/etc/init.d/lxc}} so that it reflects your container.
# {{c|ln -s /etc/init.d/lxc /etc/init.d/lxc.funtoo0}}
# now you can add {{c|lxc.funtoo0}} to default runlevel
# {{c|rc-update add lxc.funtoo0 default}}
{{console|body=
###i## rc
* Starting funtoo0 ...                  [ ok ]
}}
 
== LXC Bugs/Missing Features ==
 
This section is devoted to documenting issues with the current implementation of LXC and its associated tools. We will be gradually expanding this section with detailed descriptions of problems, their status, and proposed solutions.
 
=== reboot ===
 
* By default, lxc does not support rebooting a container from within. It will simply stop and the host will not know to start it.
* If you want your container to reboot gracefully, you need sys_boot capability (comment out lxc.cap.drop = sys_boot in your container config)
 
=== PID namespaces ===
 
Process ID namespaces are functional, but the container can still see the CPU utilization of the host via the system load (ie. in {{c|top}}).
 
=== /dev/pts newinstance ===
 
* Some changes may be required to the host to properly implement "newinstance" {{c|/dev/pts}}. See [https://bugzilla.redhat.com/show_bug.cgi?id=501718 This Red Hat bug].
 
=== lxc-create and lxc-destroy ===
 
* LXC's shell scripts are badly designed and are sure way to destruction, avoid using lxc-create and lxc-destroy.
 
=== network initialization and cleanup ===
 
* If used network.type = phys after lxc-stop the interface will be renamed to value from lxc.network.link. It supposed to be fixed in 0.7.4, happens still on 0.7.5 - http://www.mail-archive.com/lxc-users@lists.sourceforge.net/msg01760.html
 
* Re-starting a container can result in a failure as network resource are tied up from the already-defunct instance: [http://www.mail-archive.com/lxc-devel@lists.sourceforge.net/msg00824.html]
 
=== graceful shutdown ===
 
* To gracefully shutdown a container, it's init system needs to properly handle kill -PWR signal
* For funtoo/gentoo make sure that you have:
** pf:12345:powerwait:/sbin/halt
** in your containers /etc/inittab
* For debian/ubuntu make sure that you have:
** pf::powerwait:/sbin/shutdown -t1 -a -h now
** in your container /etc/inittab
** and also comment out other line starting with pf:powerfail (such as pf::powerwait:/etc/init.d/powerfail start) <- these are used if you have UPS monitoring daemon installed!
* /etc/init.d/lxc seems to have broken support for graceful shutdown (it sends proper signal, but then also tries to kill the init with lxc-stop)
 
=== funtoo ===
 
* Our udev should be updated to contain {{c|-lxc}} in scripts. (This has been done as of 02-Nov-2011, so should be resolved. But not fixed in our openvz templates, so need to regen them in a few days.)
* Our openrc should be patched to handle the case where it cannot mount tmpfs, and gracefully handle this situation somehow. (Work-around in our docs above, which is to mount tmpfs to {{c|/libexec/rc/init.d}} using the container-specific {{c|fstab}} file (on the host.)
* Emerging udev within a container can/will fail when realdev is run, if a device node cannot be created (such as /dev/console) if there are no mknod capabilities within the container. This should be fixed.
 
== References ==
 
* {{c|man 7 capabilities}}
* {{c|man 5 lxc.conf}}
 
== Links ==
 
* There are a number of additional lxc features that can be enabled via patches: [http://lxc.sourceforge.net/patches/linux/3.0.0/3.0.0-lxc1/]
* [https://wiki.ubuntu.com/UserNamespace Ubuntu User Namespaces page]
* lxc-gentoo setup script [https://github.com/globalcitizen/lxc-gentoo on GitHub]
 
* '''IBM developerWorks'''
** [http://www.ibm.com/developerworks/linux/library/l-lxc-containers/index.html LXC: Linux Container Tools]
** [http://www.ibm.com/developerworks/linux/library/l-lxc-security/ Secure Linux Containers Cookbook]
 
* '''Linux Weekly News'''
** [http://lwn.net/Articles/244531/ Smack for simplified access control]
 
[[Category:Labs]]
[[Category:HOWTO]]
[[Category:Virtualization]]

Revision as of 01:51, April 3, 2015

Conceitos

TBC

Recompilando os códigos fonte do kernel Gentoo a partir do SystemRescue CD chroot

Um segundo caso que muitos usuários de Funtoo users encararão é recompilar seu próprio kernel quando instalar um um Funtoo novinho a partir de um stage 3 archive (o cenário mais comum é inicializar a maquina com o SystemRescue CD).

   Note
Se quiser utilizar o System rescue CD sources fornecido pelo Funtoo No results, a filosofia permanece exatamente a mesma.

First step emerging the required packages

The first step is to emerge:

  1. The Gentoo kernel sources: No results
  2. Genkernel itself: No results

This is achieved by running the following:

root # emerge sys-kernel/gentoo-sources sys-kernel/genkernel

Once the Gentoo kernel sources are deployed, you should find a directory named linux-version-gentoo (e.g. linux-2.6.39-gentoo) under /usr/src. Update the linux symlink to point to this directory:

root # cd /usr/src
root # rm linux
root # ln -s linux-2.6.39-gentoo linux

Second step: Grabbing and tweaking a configuration file

How to start your kernel configuration? Simply by using the same configuration template the running System Rescue CD kernel had been built with! Before chrooting in your Funtoo instance, you did something like:

root # mount -o bind /proc /mnt/gentoo/proc

Or:

root # mount -t proc none /mnt/gentoo/proc

In your chroot environment (or from a System Rescue CD virtual terminal) if you look what /proc contains you will notice a file named config.gz:

root # ls /proc

...
dr-xr-xr-x  7 root       root                     0 May 23 03:13 952
dr-xr-xr-x  7 root       root                     0 May 23 03:13 953
dr-xr-xr-x  7 root       root                     0 May 23 18:42 9834
...
-r--r--r--  1 root       root                 16024 May 23 22:27 config.gz
-r--r--r--  1 root       root                     0 May 23 22:27 consoles
-r--r--r--  1 root       root                     0 May 23 22:27 cpuinfo
...

config.gz holds the running kernel (System Rescue CD) configuration, just copy the unziped content into the Gentoo sources directory:

root # cd /usr/src/linux
root # zcat /proc/config.gz > .config

Next, run make oldconfig to set all newly added options:

root # make oldconfig

Next, tweak the kernel configuration in the way you prefer (manually edition of the .config file, make nconfig, make menuconfig....) if you wish. You are not ready yet! A final step is required: you must either set CONFIG_INITRAMFS_SOURCE to a blank value (CONFIG_INITRAMFS_SOURCE="") either delete the statement in the .config file. Forgotting to do that will make Genkernel abort the compilation process with a message like:

/usr/src/linux-2.6.39-gentoo/scripts/gen_initramfs_list.sh: Cannot open '/var/tmp/genkernel/initramfs-2.6.32.14-std155-i386.cpio.gz'
make[1]: *** [usr/initramfs_data.cpio.lzma] Error 1

Third step: Building and installing the kernel

This is simply achieved by:

root # genkernel --no-mrproper all

The same remarks written in the third paragraph of the first use case are still valid here.