Articles by adrian

  1. CRIU and SELinux

    When I started to include container migration into Podman using CRIU over a year ago I did not really think about SELinux. I was able to checkpoint a running container and I was also able to restore it later https://podman.io/blogs/2018/10/10/checkpoint-restore.html. I never looked at the process labels of the restored containers. But I really should have.

    After my initial implementation of container checkpoint and restore for Podman I started to work on live container migration for Podman in October last year (2018). I opened the corresponding pull request end of January 2019. I immediately started to get SELinux related failures from the CI.

    Amongst other SELinux denials the main SELinux related problem was a blocked connectto.

    avc: denied { connectto } for pid=23569 comm="top" path=002F6372746F6F6C732D70722D3233363139 scontext=system_u:system_r:container_t:s0:c245,c463 tcontext=unconfined_u:system_r:container_runtime_t:s0-s0:c0.c1023 tclass=unix_stream_socket permissive=0

    This is actually a really interesting denial, because it gives away details about how CRIU works. This denial was caused by a container running top (podman run -d alpine top) which I tried to checkpoint.

    To understand why a denial like this is reported by top it helps to understand how CRIU works. To be able to access all resources of the process CRIU tries to checkpoint (or dump), CRIU injects parasite code into the process. The parasite code allows CRIU to act from within the process's address space. Once the parasite is injected and running it connects to the main CRIU process and is ready to receive commands.

    The parasite's attempt to connect to the main CRIU process is exactly the step SELinux is blocking. Looking at the denial it seems that a process top running as system_u:system_r:container_t:s0:c245,c463 is trying to connectto a socket labeled as unconfined_u:system_r:container_runtime_t:s0-s0:c0.c1023, which is indeed suspicious: something running in a container tries to connect to something running on the outside of the container. Knowing that this is CRIU and knowing how CRIU works it is, however, required that the parasite code connects to the main process using connectto.

    Fortunately SELinux has the necessary interface to solve this: setsockcreatecon(3). Using setsockcreatecon(3) it is possible to specify the context of newly created sockets. So all we have to do is get the context of the process to checkpoint and tell SELinux to label newly created sockets accordingly (8eb4309). Once understood that was easy. Unfortunately this is also where the whole thing got really complicated.

    The CRIU RPM package in Fedora is built without SELinux support, because CRIU's SELinux support until now was limited and not tested. CRIU's SELinux support used to be: If the process context does not start with unconfined_ CRIU just refuses to dump the process and exits. Being unaware of SELinux a process restored with CRIU was no longer running with the context it was started but with the context of CRIU during the restore. So if a container was running with a context like system_u:system_r:container_t:s0:c248,c716 during checkpointing it was running with the wrong context after restore: unconfined_u:system_r:container_runtime_t:s0, which is the context of the container runtime and not of the actual container process.

    So first I had to fix CRIU's SELinux handling to be able to use setsockcreatecon(3). Fortunately, once I understood the problem, it was pretty easy to fix CRIU's SELinux process labeling. Most of the LSM code in CRIU was written by Tycho in 2015 with focus on AppArmor which luckily uses the same interfaces as SELinux. So all I had to do is remove the restrictions on which SELinux context CRIU is willing to operate on and make sure that CRIU stores the information about the process context in its image files 796da06.

    Once the next CRIU release with these patches included is available I have to add BuildRequires: libselinux-devel to the RPM to build Fedora's CRIU package with SELinux support. This, however, means that CRIU users on Fedora might see SELinux errors they have not seen before. CRIU now needs SELinux policies which allow CRIU to change the SELinux context of a running process. For the Podman use case which started all of this there has been the corresponding change in container-selinux to allow container_runtime_t to dyntransition to container domains.

    For CRIU use cases outside of containers additional policies have been created which are also used by the new CRIU ZDTM test case selinux00. A new boolean exists which allows CRIU to use "setcon to dyntrans to any process type which is part of domain attribute". So with setsebool -P unconfined_dyntrans_all 1 it should be possible to use CRIU on Fedora just like before.

    After I included all those patches and policies into Podman's CI almost all checkpoint/restore related tests were successful. Except one test which was testing if it is possible to checkpoint and restore a container with established TCP connections. In this test case a container with Redis is started, a connection to Redis is opened and the container is checkpointed and restored. This was still failing in CI which was interesting as this seemed unrelated to SELinux.

    Trying to reproduce the test case locally I actually saw the following SELinux errors during restore:

    audit: SELINUX_ERR op=security_bounded_transition seresult=denied oldcontext=unconfined_u:system_r:container_runtime_t:s0 newcontext=system_u:system_r:container_t:s0:c218,c449

    This was unusual as it did not look like something that could be fixed with a policy.

    The reason my test case for checkpointing and restoring containers with established TCP connections failed was not the fact that it is testing established TCP connections, but the fact that it is a multithreaded process. Looking at the SELinux kernel code I found following comment in security/selinux/hooks.c:

    /* Only allow single threaded processes to change context */

    This line is unchanged since 2008 so it seemed unlikely that it would be possible to change SELinux in such a way that it would be possible to label each thread separately. My first attempt to solve this was to change the process label with setcon(3) before CRIU forks the first time. This kind of worked but at the same time created lots of SELinux denials (over 50), because during restore CRIU changes itself and the forks it creates into the process it wants to restore. So instead of changing the process label just before forking the first time I switched to setting the process label just before CRIU creates all threads (e86c2e9).

    Setting the context just before creating the threads resulted in only two SELinux denials. The first is about CRIU accessing the log file during restore which is not critical and the other denial happens when CRIU tries to influence the PID of the threads it wants to create via /proc/sys/kernel/ns_last_pid. As CRIU is now running in the SELinux context of the to be restored container and to avoid allowing the container to access all files which are labeled as sysctl_kernel_t, Fedora's selinux-policy contains a patch to label /proc/sys/kernel/ns_last_pid as sysctl_kernel_ns_last_pid_t.

    So with the latest CRIU and selinux-policy installed and the following addition to my local SELinux policy (kernel_rw_kernel_ns_lastpid_sysctl(container_domain)) I can now checkpoint and restore a Podman container (even multi-threaded) with the correct SELinux process context after a restore and no further SELinux denials blocking the checkpointing or restoring of the container. There are a few SELinux denials which are mainly related to not being able to write to the log files. Those denials, however, do not interfere with the checkpoint and restoring.

    For some time (two or three years) I was aware that CRIU was never verified to work correctly with SELinux but I always ignored it and I should have just fixed it a long time ago. Without the CRIU integration into Podman, however, I would have not been able to test my changes as I was able to do.

    I would like to thank Radostin for his feedback and ideas when I was stuck and his overview of the necessary CRIU changes, Dan for his help in adapting the container-selinux package to CRIU's needs and Lukas for the necessary changes to Fedora's selinux-policy package to make CRIU work with SELinux on Fedora. All these combined efforts made it possible to have the necessary policies and code changes ready to support container migration with Podman.

    Tagged as : criu podman selinux fedora
  2. Animated Download Maps

    Our mirror server has been generating download maps for almost 10 years (since August 2009). This is done by going through all our download log files (HTTP, FTP, RSYNC) and using GeoIP and the Matplotlib Basemap Toolkit to draw maps from where our mirror server is being accessed.

    I have taken the output from almost ten years and created the following animations. The first animation shows clients accessing all mirrored content:

    As the mirror server is running Fedora it is updated once a year which might result in an updated version of Basemap once a year. The update usually happens in December or January which sometimes can be seen in the animation when the output changes. Updating to Fedora 27 (December 2017) resulted in a Basemap version which started to draw different results and the last update to Fedora 29 (December 2018) can also be seen as switching to Python 3 removed most of the clients from the map (only visible in the last second of the animation). It seems some of the calculations are giving different results in Python 3.

    In addition to the map showing the accesses for all mirrored data, there is also an animation for clients accessing files from our Fedora mirror:

    The interesting thing about only looking at clients accessing Fedora files is that it can be seen that most accesses are actually from Europe. This seems to indicate that Fedora's mirroring system partially succeeds in directing clients to close by mirrors. Looking at the location of clients accessing our EPEL mirror it seems to work even better. This is probably related to the much larger number of existing EPEL mirrors:

    Another interesting effect of upgrading once a year can be seen around 6:42 in the EPEL animation. After upgrading to Fedora 25 the generated maps where upside down for a few days until I was able to fix it.

    Tagged as : fedora traffic
  3. CRIU configuration files

    One of the CRIU uses cases is container checkpointing and restoring, which also can be used to migrate containers. Therefore container runtimes are using CRIU to checkpoint all the processes in a container as well as to restore the processes in that container. Many container runtimes are layered, which means that the user facing layer (Podman, Docker, LXD) calls another layer to checkpoint (or restore) the container (runc, LXC) and this layer then calls CRIU.

    This leads to the problem that if CRIU introduces a new feature or option, all involved layers need code changes. Or if one of those layers made assumption about how to use CRIU, the user must live with that assumption, which may be wrong for the user's use case.

    To offer the possibility to change CRIU's behaviour through all these layers, be it that the container runtime has not implemented a certain CRIU feature or that the user needs a different CRIU behaviour, we started to discuss configuration files in 2016.

    Configuration files should be evaluated by CRIU and offer a third way to influence CRIU's behaviour. Setting options via CLI and RPC are the other two ways.

    At the Linux Plumbers Conference in 2016 during the Checkpoint/Restore micro-conference I gave a short introduction talk about how configuration files could look and everyone was nodding their head.

    In early 2017 Veronika Kabatova provided patches which were merged in CRIU's development branch criu-dev. At that point the development stalled a bit and only in early 2018 the discussion was picked up again. To have a feature merged into the master branch, which means it will be part of the next release, requires complete documentation (man-pages and wiki) and feature parity for CRIU's CLI and RPC mode. At this point it was documented but not supported in RPC mode.

    Adding configuration file support to CRIU's RPC mode was not a technical challenge, but if any recruiter ever asks me which project was the most difficult, I will talk about this. We were exchanging mails and patches for about half a year and it seems everybody had different expectations how everything should behave. I think at the end they pitied me and just merged my patches...

    CRIU 3.11 which was released on 2018-11-06 is the first release which includes support for configuration files and now (finally) I want to write about how it could be used.

    I am using the Simple_TCP_pair example from CRIU's wiki. First start the server:

    #️  ./tcp-howto 10000
    

    Then I am starting the client:

    # ./tcp-howto 127.0.0.1 10000
    Connecting to 127.0.0.1:10000
    PP 1 -> 1
    PP 2 -> 2
    PP 3 -> 3
    PP 4 -> 4
    

    Once client and server are running, let's try to checkpoint the client:

    # rm -f /etc/criu/default.conf
    # criu dump -t `pgrep -f 'tcp-howto 127.0.0.1 10000'`
    Error (criu/sk-inet.c:188): inet: Connected TCP socket, consider using --tcp-established option.
    

    CRIU tells us that it needs a special option to checkpoint processes with established TCP connections. No problem, but instead of changing the command-line, let's add it to the configuration file:

    # echo tcp-established > /etc/criu/default.conf
    # criu dump -t `pgrep -f 'tcp-howto 127.0.0.1 10000'`
    Error (criu/tty.c:1861): tty: Found dangling tty with sid 16693 pgid 16711 (pts) on peer fd 0.
    Task attached to shell terminal. Consider using --shell-job option. More details on http://criu.org/Simple_loop
    

    Alright, let's also add shell-job to the configuration file:

    # echo shell-job >> /etc/criu/default.conf
    # criu dump -t `pgrep -f 'tcp-howto 127.0.0.1 10000'` && echo OK
    OK
    

    That worked. Cool. Finally! Most CLI options can be used in the configuration file(s) and more detailed documentation can be found in the CRIU wiki.

    I want to thank Veronika for her initial implementation and everyone else helping, discussing and reviewing emails and patches to get this ready for release.

    Tagged as : criu podman
  4. Nextcloud in a Container

    After using Podman a lot during the last weeks while adding checkpoint/restore support to Podman I was finally ready to use containers in production on our mirror server. We were still running the ownCloud version that came via RPMs in Fedora 27 and it seems like many people have moved on to Nextcloud from tarballs.

    One of the main reason to finally use containers is Podman's daemonless approach.

    The first challenge while moving from ownCloud 9.1.5 to Nextcloud 14 is the actual upgrade. To make sure it works I first made a copy of all the uploaded files and of the database and did a test upgrade yesterday using a CentOS 7 VM. With PHP 7 from Software Collections it was not a real problem. It took some time, but it worked. I used the included upgrade utility to upgrade from ownCloud 9 to Nextcloud 10, to Nextcloud 11, to Nextcloud 12, to Nextcloud 13, to Nextcloud 14. Lots of upgrades. Once I verified that everything was still functional I did it once more, but this time I used the real data and disabled access to our ownCloud instance.

    The next step was to start the container. I decided to use the nextcloud:fpm container as I was planning to use the existing web server to proxy the requests. The one thing which makes using containers on our mirror server a bit difficult, is that it is not possible to use any iptables NAT rules. At some point there are just too many network connections in the NAT table from all the clients connecting to our mirror server that it used to drop network connections. This is a problem which is probably fixed since a long time, but it used to be a problem and I try to avoid it. That is why my Nextcloud container is using the host network namespace:

    podman run --name nextcloud-fpm -d --net host \
      -v /home/containers/nextcloud/html:/var/www/html \
      -v /home/containers/nextcloud/apps:/var/www/html/custom_apps \
      -v /home/containers/nextcloud/config:/var/www/html/config \
      -v /home/containers/nextcloud/data:/var/www/html/data \
      nextcloud:fpm
    

    I was reusing my existing config.php in which the connection to PostgreSQL on 127.0.0.1 was still configured.

    Once the container was running I just had to add the proxy rules to the Apache HTTP Server and it should have been ready. Unfortunately this was not as easy as I hoped it to be. All the documentation I found is about using the Nextcloud FPM container with NGINX. I found nothing about Apache's HTTPD. The following lines required most of the time of the whole upgrade to Nextcloud project:

    <FilesMatch \.php.*>
       SetHandler proxy:fcgi://127.0.0.1:9000/
       ProxyFCGISetEnvIf "reqenv('REQUEST_URI') =~ m|(/owncloud/)(.*)$|" SCRIPT_FILENAME "/var/www/html/$2"
       ProxyFCGISetEnvIf "reqenv('REQUEST_URI') =~ m|^(.+\.php)(.*)$|" PATH_INFO "$2"
    </FilesMatch>
    

    I hope these lines are actually correct, but so far all clients connecting to it seem to be happy. To have the Nextcloud container automatically start on system startup I based my systemd podman service file on the one from the Intro to Podman article.

    [Unit]
    Description=Custom Nextcloud Podman Container
    After=network.target
    
    [Service]
    Type=simple
    TimeoutStartSec=5m
    ExecStartPre=-/usr/bin/podman rm nextcloud-fpm
    
    ExecStart=/usr/bin/podman run --name nextcloud-fpm --net host \
       -v /home/containers/nextcloud/html:/var/www/html \
       -v /home/containers/nextcloud/apps:/var/www/html/custom_apps \
       -v /home/containers/nextcloud/config:/var/www/html/config \
       -v /home/containers/nextcloud/data:/var/www/html/data \
       nextcloud:fpm
    
    ExecReload=/usr/bin/podman stop nextcloud-fpm
    ExecReload=/usr/bin/podman rm nextcloud-fpm
    ExecStop=/usr/bin/podman stop nextcloud-fpm
    Restart=always
    RestartSec=30
    
    [Install]
    WantedBy=multi-user.target
    
    Tagged as : fedora nextcloud podman
  5. Antimatter Factory

    On October 19th, 2018, I was giving a talk about OpenHPC at the CentOS Dojo at CERN.

    I really liked the whole event and my talk was also recorded. Thanks for everyone involved for organizing it. The day before FOSDEM 2019 there will be another CentOS Dojo in Brussels. I hope I have the chance to also attend it.

    The most interesting thing during my two days in Geneva was, however, the visit of the Antimatter Factory:

    Antimatter Factory

    Assuming I actually understood anything we were told about it, it is exactly that: an antimatter factory.

    Tagged as : fedora centos openhpc
  6. S3 sleep with ThinkPad X1 Carbon 6th Generation

    Since a few weeks I have the new ThinkPad X1 Carbon 6th Generation and as many people I really like it.

    The biggest problem is that suspend does not work as expected.

    The issue seems to be that the X1 is using a new suspend technology called "Windows Modern Standby," or S0i3, and has removed classic S3 sleep.[1]

    Following the instructions in Alexander's article it was possible to get S3 suspend to work as expected and everything was perfect.

    With the latest Firmware update to 0.1.28 (using sudo fwupdmgr update (thanks a lot to Linux Vendor Firmware Service (LVFS) that this works!!!)) I checked if the patch mentioned in Alexander's article still applies and it did not.

    So I modified the patch to apply again and made it available here: https://lisas.de/~adrian/X1C6_S3_DSDT_0_1_28.patch

    Talking with Christian about it he mentioned an easier way to include the changed ACPI table into grub. For my Fedora system this looks like this:

    • cp dsdt.aml /boot/efi/EFI/fedora/
    • echo 'acpi $prefix/dsdt.aml' > /boot/efi/EFI/fedora/custom.cfg

    Thanks to Alexander and Christian I can correctly suspend my X1 again.

    Update 2018-09-09: Lenovo fixed the BIOS and everything described above is no longer necessary with version 0.1.30. Also see https://brauner.github.io/2018/09/08/thinkpad-6en-s3.html

    Tagged as : fedora X1
  7. Latest CRIU for CentOS COPR

    The version of CRIU which is included with CentOS is updated with every minor CentOS release (at least at the time of writing this) since 7.2, but once the minor CentOS release is available CRIU is not updated anymore until the next minor release. To make it easier to use the latest version of CRIU on CentOS I am now also rebuilding the latest version in COPR for CentOS: https://copr.fedorainfracloud.org/coprs/adrian/criu-el7/.

    To enable my CRIU COPR on CentOS following steps are necessary:

    • yum install yum-plugin-copr
    • yum copr enable adrian/criu-el7

    And then the latest version of CRIU can be installed using yum install criu.

    Tagged as : CentOS criu migration
  8. archive.rpmfusion.org

    After many years the whole RPM Fusion repository has grown to over 320GB. There have been occasional requests to move the unsupported releases to an archive, just like Fedora handles its mirror setup, but until last week this did not happen.

    As of now we have moved all unsupported releases (EL-5, Fedora 8 - 25) to our archive (http://archive.rpmfusion.org/) and clients are now being redirected to the new archive system. The archive consists of 260GB which means we can reduce the size mirrors need to carry by more than 75%.

    From a first look at the archive logs the amount of data requested by all clients for the archived releases is only about 30GB per day. Those 30GB are downloaded by over 350000 HTTP requests and over 98% of those requests are downloading the repository metdata only (repomd.xml, *filelist*, *primary*, *comps*).

  9. OpenHPC: Building Blocks

    I will be giving two talks about OpenHPC in the next weeks. The first talk will be at DevConf.cz 2018: OpenHPC Introduction

    The other talk will be at the CentOS Dojo in Brussels.

    I hope I will be able to demonstrate my two node HPC system based on Raspberry Pis and it definitely will be about OpenHPC's building blocks:

    OpenHPC Building
Blocks

    And the results:

    OpenHPC Building BlocksOpenHPC Building
BlocksOpenHPC Building
BlocksOpenHPC Building
Blocks

    Come to one of my talks and you will able to build your OpenHPC engineer from the available building blocks.

    Tagged as : CentOS OpenHPC
  10. Optimizing live container migration in LXD

    After having worked on optimizing live container migration based on runc (pre-copy migration and post-copy migration) I tried to optimize container migration in LXD.

    After a few initial discussions with Christian I started with pre-copy migration. Container migration in LXD is based on CRIU, just as in runc and CRIU's pre-copy migration support is based on dirty page tracking support of Linux: SOFT-DIRTY PTEs.

    As LXD uses LXC for the actual container checkpointing and restoring I was curious if there was already pre-copy migration support in LXC. After figuring out the right command-line parameters it almost worked thanks to the great checkpoint and restore support implemented by Tycho some time ago.

    Now that I knew that it works in LXC I focused on getting pre-copy migration support into LXD. LXD supports container live migration using the move command: lxc move <container> <remote>:<container>
    This move command, however, did not use any optimization yet. It basically did:

    1. Initial sync of the filesystem
    2. Checkpoint container using CRIU
    3. Transfer container checkpoint
    4. Final sync of the filesystem
    5. Restart container on the remote system

    The downtime for the container in this scenario is between step 2 and step
    5 and depends on the used memory of the processes inside the container. The goal of pre-copy migration is to dump the memory of the container and transfer it to the remote destination while the container keeps on running and doing a final dump with only the memory pages that changed since the last pre-dump (more about process migration optimization theories).

    Back to LXD: At the end of the day I had a very rough (and very hardcoded) first pre-copy migration implementation ready and I kept working on it until it was ready to be submitted upstream. The pull request has already been merged upstream and now LXD supports pre-copy migration.

    As not all architecture/kernel/criu combinations support pre-copy migration it has to be turned on manually right now, but we already discussed adding pre-copy support detection to LXC. To tell LXD to use pre-copy migration, the parameter 'migration.incremental.memory' needs to be set to 'true'. Once that is done and if LXD is instructed to migrate a container the following will happen:

    • Initial sync of the filesystem
    • Start pre-copy checkpointing loop using CRIU
      • Check if maximum number pre-copy iterations has been reached
      • Check if threshold of unchanged memory pages has been reached
      • Transfer container checkpoint
      • Continue pre-copy checkpointing loop if neither of those conditions is true
    • Final container delta checkpoint using CRIU
    • Transfer final delta checkpoint
    • Final sync of the filesystem
    • Restart container on the remote system

    So instead of doing a single checkpoint and transferring it, there are now multiple pre-copy checkpoints and the container keeps on running during those transfers. The container is only suspended during the last delta checkpoint and the transfer of the last delta checkpoint. In many cases this reduces the container downtime during migration, but there is the possibility that pre-copy migration also increases the container downtime during migration. This depends (as always) on the workload.

    To control how many pre-copy iterations LXD does there are two additional variables:

    1. migration.incremental.memory.iterations (defaults to 10)
    2. migration.incremental.memory.goal (defaults to 70%)

    The first variable (iterations) is used to tell LXD how many pre-copy iterations it should do before doing the final dump and the second variable (goal) is used to tell LXD the percentage of pre-copied memory pages that should not change between pre-copy iterations before doing the final dump.

    So LXD, in the default configuration, does either 10 pre-copy iterations before doing the final migration or the final migration is triggered when at least 70% of the memory pages have been transferred by the last pre-copy iteration.

    Now that this pull request is merged and if pre-copy migration is enabled a lxc move <container> <remote>:<container> should live migrate the container with a reduced downtime.

    I want to thank Christian for the collaboration on getting CRIU's pre-copy support into LXD, Tycho for his work preparing LXC and LXD to support migration so nicely and the developers of p.haul for the ideas how to implement pre-copy container migration. Next step: lazy migration.

    Tagged as : criu migration pre-copy

Page 1 / 5