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Tuesday, November 15, 2011

pcscd auto start using systemd

In pcsc-lite version 1.6.0 (5 May 2010) I introduced a mechanism to start the pcscd daemon only when an application called SCardstablishContext() to get a PC/SC context. Read "Configuring your system for pcscd auto-start" for more information.

Old way: start by the libpcsclite library

One of the problem is that pcscd is then started as the user running the application. The pcscd process has to be setgid to get special privileges to get access to the USB devices (the smart card readers). Some people (and I understand) do not like files with special access right (like setgid).

New way: systemd

Around the same time (23 Aug 2010) the first article about systemd was posted by systemd author: Lennart Poettering

Socket activation

I will not describe systemd in details. But one of the services of systemd is to start a process when an application tries to communicate to a socket. The good news is that libpcsclite and pcscd are communicating through a UNIX (local) socket. So systemd provides a way to start pcscd when libpcsclite starts the communication.

The good news is that the changes are really minimal. libpcsclite has not changed and only the initialisation of pcscd has been updated a bit to use systemd services.

systemd configuration

pcsc-lite now use 2 systemd configuration files: pcscd.service and pcscd.socket

File pcscd.service:
[Unit]
Description=PC/SC Smart Card Daemon
Requires=pcscd.socket

[Service]
ExecStart=/usr/sbin/pcscd --foreground --auto-exit
ExecReload=/usr/sbin/pcscd --hotplug
StandardOutput=syslog

[Install]
Also=pcscd.socket

File pcscd.socket:
[Unit]
Description=PC/SC Smart Card Daemon Activation Socket

[Socket]
ListenStream=/var/run/pcscd/pcscd.comm

[Install]
WantedBy=sockets.target

These two files are installed in /lib/systemd/system/ on my Debian (testing) system.

systemd main comands


Status

$ systemctl status pcscd.service
pcscd.service - PC/SC Smart Card Daemon
   Loaded: loaded (/lib/systemd/system/pcscd.service)
   Active: inactive (dead) since Mon, 14 Nov 2011 16:33:40 +0100; 17h ago
  Process: 15489 ExecStart=/usr/sbin/pcscd --foreground --auto-exit (code=exited, status=0/SUCCESS)
   CGroup: name=systemd:/system/pcscd.service

$ systemctl status pcscd.socket
pcscd.socket - PC/SC Smart Card Daemon Activation Socket
   Loaded: loaded (/lib/systemd/system/pcscd.socket)
   Active: active (listening) since Mon, 14 Nov 2011 14:49:45 +0100; 19h ago
   CGroup: name=systemd:/system/pcscd.socket

The pcscd socket is configured and active but the pcscd process is not running. After starting a PC/SC application we get:

$ systemctl status pcscd.service
pcscd.service - PC/SC Smart Card Daemon
   Loaded: loaded (/lib/systemd/system/pcscd.service)
   Active: active (running) since Tue, 15 Nov 2011 10:27:39 +0100; 9s ago
 Main PID: 26929 (pcscd)
   CGroup: name=systemd:/system/pcscd.service
    └ 26929 /usr/sbin/pcscd --foreground --auto-exit

Stop

It is possible to stop the running pcscd process.

$ sudo systemctl stop pcscd.service
$ systemctl status pcscd.service
pcscd.service - PC/SC Smart Card Daemon
   Loaded: loaded (/lib/systemd/system/pcscd.service)
   Active: failed since Tue, 15 Nov 2011 10:30:13 +0100; 1s ago
  Process: 26965 ExecStart=/usr/sbin/pcscd --foreground --auto-exit (code=exited, status=1/FAILURE)
   CGroup: name=systemd:/system/pcscd.service

But in general I just kill(1) the pcscd process.

Start

If you start pcscd by hand, for example in debug and foreground mode, the daemon will remove the socket /var/run/pcscd/pcscd.comm on exit. This socket is not recreated automatically by systemd. You need to stop and start the pcscd.socket, using just start is not enough.

$ ls /var/run/pcscd/pcscd.comm 
ls: cannot access /var/run/pcscd/pcscd.comm: No such file or directory
$ sudo systemctl stop pcscd.socket
$ sudo systemctl start pcscd.socket
$ ls /var/run/pcscd/pcscd.comm 
/var/run/pcscd/pcscd.comm

The pcscd process is not started but the socket is now listening.

Migration

I removed the old autostart code in revision 6105. If you have systemd installed on your system I recommend using it to start pcscd. If you do not have systemd installed (maybe you do not use a Linux kernel) then you have to start pcscd at boot (as before version 1.6.0).

systemd availability

systemd is only available with a Linux kernel. systemd is now installed by default in Fedora 14. systemd is provided by Debian but Debian is not just limited to a Linux kernel. Debian also provides Hurd and FreeBSD kernel based Debian systems and systemd is not (yet) available for these kernels.

Major GNU/Linux distributions should provide systemd now. If your distribution do not have systemd you can still use the old way of starting pcscd at boot.

For non-Linux systems I have no auto-start solution. Just start pcscd at boot.

Conclusion

systemd is a nice new system to replace init and a lot more. It is a good piece of code to implement auto start for pcsc-lite.

Thanks to Kalev Lember for pushing the systemd patches.
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