imp: cgroup: fix relative cgroup paths in containers

Problem: Relative paths for cgroups in /proc/self/cgroup may contain
leading `/..` when the IMP is run inside some containers. This is
because the path is relative to the container's mount point, not the
actual mount point of the cgroupfs.

Just strip the leading /.. from paths during discovery of the current
cgroup path from `/proc/self/cgroup`. This should only apply in
containers and thus CI.

src/imp/cgroup.c

@@ -44,6 +44,13 @@
 #include "cgroup.h"
 #include "imp_log.h"
 
+static char *remove_leading_dotdot (char *relpath)
+{
+    while (strncmp (relpath, "/..", 3) == 0)
+        relpath += 3;
+    return relpath;
+}
+
 /*
  *  Look up the current cgroup relative path from /proc/self/cgroup.
  *
@@ -80,6 +87,11 @@ static int cgroup_init_path (struct cgroup_info *cgroup)
         /* Nullify subsys, relpath is already nul-terminated at newline */
         *(relpath++) = '\0';
 
+        /* Remove leading /.. in relpath. This could be due to cgroup
+         * mounted in a container.
+         */
+        relpath = remove_leading_dotdot (relpath);
+
         /*  If unified cgroups are being used, then stop when we find
          *   subsys="". Otherwise stop at subsys="name=systemd":
          */