Debian Stable is still using glibc 2.13, which doesn't provide the setns().
So we detect this and provide a tiny wrapper that issues the setns syscall
towards the kernel.
signal(7) provides a list of functions which may be called from a
signal handler. Other functions, which only call those functions and
don't access global memory and are reentrant are also safe.
sd_j_sendv was mostly OK, but would call mkostemp and writev in a
fallback path, which are unsafe.
Being able to call sd_j_sendv in a async-signal-safe way is important
because it allows it be used in signal handlers.
Safety is achieved by replacing mkostemp with open(O_TMPFILE) and an
open-coded writev replacement which uses write. Unfortunately,
O_TMPFILE is only available on kernels >= 3.11. When O_TMPFILE is
unavailable, an open-coded mkostemp is used.
https://bugzilla.gnome.org/show_bug.cgi?id=722889
A session-device is a device that is bound to a seat and used by a
session-controller to run the session. This currently includes DRM, fbdev
and evdev devices. A session-device can be created via RequestDevice() on
the dbus API of the session. You can drop it via ReleaseDevice() again.
Once the session is destroyed or you drop control of the session, all
session-devices are automatically destroyed.
Session devices follow the session "active" state. A device can be
active/running or inactive/paused. Whenever a session is not the active
session, no session-device of it can be active. That is, if a session is
not in foreground, all session-devices are paused.
Whenever a session becomes active, all devices are resumed/activated by
logind. If it fails, a device may stay paused.
With every session-device you request, you also get a file-descriptor
back. logind keeps a copy of this fd and uses kernel specific calls to
pause/resume the file-descriptors. For example, a DRM fd is muted
by logind as long as a given session is not active. Hence, the fd of the
application is also muted. Once the session gets active, logind unmutes
the fd and the application will get DRM access again.
This, however, requires kernel support. DRM devices provide DRM-Master for
synchronization, evdev devices have EVIOCREVOKE (pending on
linux-input-ML). fbdev devices do not provide such synchronization methods
(and never will).
Note that for evdev devices, we call EVIOCREVOKE once a session gets
inactive. However, this cannot be undone (the fd is still valid but mostly
unusable). So we reopen a new fd once the session is activated and send it
together with the ResumeDevice() signal.
With this infrastructure in place, compositors can now run without
CAP_SYS_ADMIN (that is, without being root). They use RequestControl() to
acquire a session and listen for devices via udev_monitor. For every
device they want to open, they call RequestDevice() on logind. This
returns a fd which they can use now. They no longer have to open the
devices themselves or call any privileged ioctls. This is all done by
logind.
Session-switches are still bound to VTs. Hence, compositors will get
notified via the usual VT mechanisms and can cleanup their state. Once the
VT switch is acknowledged as usual, logind will get notified via sysfs and
pause the old-session's devices and resume the devices of the new session.
To allow using this infrastructure with systems without VTs, we provide
notification signals. logind sends PauseDevice("force") dbus signals to
the current session controller for every device that it pauses. And it
sends ResumeDevice signals for every device that it resumes. For
seats with VTs this is sent _after_ the VT switch is acknowledged. Because
the compositor already acknowledged that it cleaned-up all devices.
However, for seats without VTs, this is used to notify the active
compositor that the session is about to be deactivated. That is, logind
sends PauseDevice("force") for each active device and then performs the
session-switch. The session-switch changes the "Active" property of the
session which can be monitored by the compositor. The new session is
activated and the ResumeDevice events are sent.
For seats without VTs, this is a forced session-switch. As this is not
backwards-compatible (xserver actually crashes, weston drops the related
devices, ..) we also provide an acknowledged session-switch. Note that
this is never used for sessions with VTs. You use the acknowledged
VT-switch on these seats.
An acknowledged session switch sends PauseDevice("pause") instead of
PauseDevice("force") to the active session. It schedules a short timeout
and waits for the session to acknowledge each of them with
PauseDeviceComplete(). Once all are acknowledged, or the session ran out
of time, a PauseDevice("force") is sent for all remaining active devices
and the session switch is performed.
Note that this is only partially implemented, yet, as we don't allow
multi-session without VTs, yet. A follow up commit will hook it up and
implemented the acknowledgements+timeout.
The implementation is quite simple. We use major/minor exclusively to
identify devices on the bus. On RequestDevice() we retrieve the
udev_device from the major/minor and search for an existing "Device"
object. If no exists, we create it. This guarantees us that we are
notified whenever the device changes seats or is removed.
We create a new SessionDevice object and link it to the related Session
and Device. Session->devices is a hashtable to lookup SessionDevice
objects via major/minor. Device->session_devices is a linked list so we
can release all linked session-devices once a device vanishes.
Now we only have to hook this up in seat_set_active() so we correctly
change device states during session-switches. As mentioned earlier, these
are forced state-changes as VTs are currently used exclusively for
multi-session implementations.
Everything else are hooks to release all session-devices once the
controller changes or a session is closed or removed.
The affected files in this patch had inconsistent use of tabs vs. spaces
for indentation, and this patch eliminates the stray tabs.
Also, the opening brace of sigchld_hdl() in activate.c was moved so the
opening braces are consistent throughout the file.
Network file systems generally do not offer inotify() that would work
across the network. We hence cannot rely on inotify() exclusiely in
those case. Provide an API to determine these cases, and suggest doing
manual regular rechecks.
Note that this is not complete yet, as we need to rescan journal dirs on
network file systems explicitly to find new/removed files
#pragma once has been "un-deprecated" in gcc since 3.3, and is widely supported
in other compilers.
I've been using and maintaining (rebasing) this patch for a while now, as
it annoyed me to see #ifndef fooblahfoo, etc all over the place,
almost arrogant about the annoyance of having to define all these names to
perform a commen but neccicary functionality, when a completely superior
alternative exists.
I havn't sent it till now, cause its kindof a style change, and it is bad
voodoo to mess with style that has been established by more established
editors. So feel free to lambast me as a crazy bafoon.
v2 - preserve externally used headers
Lennart Poettering
Holger Schurig
Tom Gundersen
Zbigniew Jędrzejewski-Szmek
David Herrmann
Michael Marineau
Shawn Landden
Jason St. John
Chengwei Yang
Aleksander Morgado
Michael Olbrich
Eelco Dolstra
Auke Kok
Andreas Schwab
Kay Sievers