Hi Lennart, On Mon, Dec 4, 2017 at 9:11 AM, Pekka Paalanen <ppaala...@gmail.com> wrote: > On Fri, 1 Dec 2017 18:25:35 +0100 > Lennart Poettering <lenn...@poettering.net> wrote: > >> On Fr, 01.12.17 13:42, Pekka Paalanen (ppaala...@gmail.com) wrote: >> >> > > > > This is racy, as the session ID is not really reliably predictable, >> > > > > and is synthesized in different contexts in different ways, for >> > > > > example depnding on whether audit is enabled in the kernel it might >> > > > > be >> > > > > session-1.scope rather than session-c1.scope. >> > >> > > > If we could determine the bug doesn't exist anymore, that would be >> > > > awesome and I could just drop this. >> > >> > Hi Lennart, >> > >> > taking a step back, the session-c1.scope directive is definitely not >> > wanted and we should drop it. We should also use a custom PAM name >> > setup. If we do that, is the service file otherwise good for >> > guaranteeing: >> > >> > - a full user session setup (I presume we want it), specifically >> > XDG_RUNTIME_DIR being set up >> > >> > - running exclusively on a VT that is made current >> >> This really depends on how weston sets up a VT. I really don't know >> Weston and what it does. > > Weston doesn't set up the VT, we rely on the systemd unit directives to > set it up. > > Weston calls sd_pid_get_session(getpid()), sd_session_get_seat(), and wants > sd_session_get_vt() to succeed and give a VT number. Then it connects > to logind, wants TakeControl to succeed, and calls Activate. It uses > TakeDevice to open the DRM KMS device and input devices. I think that's > the start-up sequence, it also listens on signals from logind etc. > >> > - access to DRM and input devices via logind >> >> So, I can't comment on Weston really. > > No worries, that was more of a general question about whether the > example unit file was making any unwarranted assumptions. > >> Here are brief (and possibly slightly out-of-date, but probably not) >> notes on how to write display managers with logind: >> >> https://www.freedesktop.org/wiki/Software/systemd/writing-display-managers/ > > Thanks, I had a quick read through. We expect the systemd unit to also > set up PAM, Weston itself does not touch PAM. > >> > so that all these are already in place by the time the Weston process >> > is started? >> > >> > As you can see from Martyn below, the first issue that prevented Weston >> > from running was that XDG_RUNTIME_DIR was not set. Furthermore, this >> > failure did not occur always, sometimes things just worked as we >> > expected. >> >> So, as long as weston runs from a PAM enabled service (and its PAM >> snippet pulls in pam_systemd) all should be good and race-free: as the >> PAM session is set up XDG_RUNTIME_DIR will be made available and the >> systemd --user instance is invoked in the background. > > This is exactly what we attempted with the User and PAMName directive, > and it turned out to be racy somehow. > >> What currently is not supported is to run things independently of any >> session, i.e. run weston as systemd --user service with nothing that >> creates a session in the first place. In that case XDG_RUNTIME_DIR >> will not be set up, and no devices are made available to weston... > > Weston never was a --user service. > > As far as I know, there was also nothing that would manually attempt to > start user@.service, the only trigger for that were the User and PAMName > directives in the system weston.service. > >> > > > > > +# Set up a full user session for the user, required by Weston. >> > > > > > +PAMName=login >> > > > > >> > > > > Piggy-backing on "login" is a bad idea. "login" is a text tool, and >> > > > > thus the PAM rules for it usually pull in some TTY specific PAM >> > > > > modules. YOu shoudl really use your own PAM fragment here, and >> > > > > configure only the bits you need. >> > > > >> > > >> > > Oh, so could it just be that we needed something other than >> > > "PAMName=login"? >> > >> > What are they key bits in the PAM configuration we must have, and are >> > there any often used bits we must not have to avoid the race Martyn >> > describes? >> >> well, pam_systemd needs to be pulled in from it, that's the most >> important thing. A PAM snippet that pulls in pam_systemd means you get >> a session allcoated in logind, which in turn sets up XDG_RUNTIME_DIR >> for you. > > Yes, it was, but apparently somewhere in the PAM stack or something it > calls there was a race, which sometimes let Weston to start before > XDG_RUNTIME_DIR environment variable was set, causing weston to fail. > > We all here were quite baffled on what could even be racing, unless it > is possible that the weston process gets started in parallel with the > PAM setup done by the User/PAMName in weston.service. We assumed that > all the setup described in the systemd unit file would be guaranteed to > complete before the actual process gets started. > > It seems our and your expectations are aligned. Maybe we should just > forget about that race, remove the hacks that tried to work around it, > and see if anyone ever sees the failure again. Maybe it was something > very special on that one system alone. > > > Thanks, > pq
The approach that you and Pekka most recently put on record here: * User=foo * PAMName=weston with a /etc/pam.d/weston that just does minimal stuff (enforce the account exists and then execute pam_systemd.so for the session phase) works well for me. One thing I can't figure out though: using PAMName= causes the service process's journal entries emitted by regular stdout and stderr not to be visible with 'journalctl -u weston.service' anymore. Only the messages coming internally from systemd ("Started Weston." and similar) show in that journal. I've tacked in StandardOutput=journal and StandardError=journal to compensate for the StandardInput=tty-fail. The messages do make it across to journald; you can view them with 'journalctl /usr/bin/weston'. But somehow they're not associated with the system unit weston.service anymore. Does using the PAMName= directive cause the stdout/stderr messages to be reassigned to a user-session unit or something? Thanks, Matt _______________________________________________ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel