sd-booted: update sd_booted() check a bit

Previously we were testing whether /sys/fs/cgroup/systemd/ was a mount
point. This might be problematic however, when the cgroup trees are bind
mounted into a container from the host (which should be absolutely
valid), which might create the impression that the container was running
systemd, but only the host actually is.

Replace this by a check for the existance of the directory
/run/systemd/system/, which should work unconditionally, since /run can
never be a bind mount but *must* be a tmpfs on systemd systems, which is
flushed at boots. This means that data in /run always reflects
information about the current boot, and only of the local container,
which makes it the perfect choice for a check like this.

(As side effect this is nice to Ubuntu people who now use logind with
the systemd cgroup hierarchy, where the old sd_booted() check misdetects
systemd, even though they still run legacy Upstart.)

1 files changed, 5 insertions, 1 deletions

diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
index 42cdc6dc52..ce10be944a 100644
--- a/src/core/mount-setup.c
+++ b/src/core/mount-setup.c
@@ -440,7 +440,11 @@ int mount_setup(bool loaded_policy) {
                 if (mount(NULL, "/", NULL, MS_REC|MS_SHARED, NULL) < 0)
                         log_warning("Failed to set up the root directory for shared mount propagation: %m");
 
-        /* Create a few directories we always want around */
+        /* Create a few directories we always want around, Note that
+         * sd_booted() checks for /run/systemd/system, so this mkdir
+         * really needs to stay for good, otherwise software that
+         * copied sd-daemon.c into their sources will misdetect
+         * systemd. */
         mkdir_label("/run/systemd", 0755);
         mkdir_label("/run/systemd/system", 0755);