mount-setup.c: fix handling of symlink Smack labelling in cgroup setup

The code introduced in f8c1a81c51 (= elogind 227) failed for me with:
  Failed to copy smack label from net_cls to /sys/fs/cgroup/net_cls: No such file or directory

There is no need for a symlink in this case because source and target
are identical. The symlink() call is allowed to fail when the target
already exists. When that happens, copying the Smack label must be
skipped.

But the code also failed when there is a symlink, like "cpu ->
cpu,cpuacct", because mac_smack_copy() got called with
src="cpu,cpuacct" which fails to find the entry because the current
directory is not inside /sys/fs/cgroup. The absolute path to the existing
entry must be used instead.

1 files changed, 10 insertions, 5 deletions

diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
index 94598e998..310eff9fc 100644
--- a/src/core/mount-setup.c
+++ b/src/core/mount-setup.c
@@ -319,13 +319,18 @@ int mount_cgroup_controllers(char ***join_controllers) {
                                         return log_oom();
 
                                 r = symlink(options, t);
-                                if (r < 0 && errno != EEXIST)
-                                        return log_error_errno(errno, "Failed to create symlink %s: %m", t);
+                                if (r >= 0) {
 #ifdef SMACK_RUN_LABEL
-                                r = mac_smack_copy(t, options);
-                                if (r < 0 && r != -EOPNOTSUPP)
-                                        return log_error_errno(r, "Failed to copy smack label from %s to %s: %m", options, t);
+                                        _cleanup_free_ char *src;
+                                        src = strappend("/sys/fs/cgroup/", options);
+                                        if (!src)
+                                                return log_oom();
+                                        r = mac_smack_copy(t, src);
+                                        if (r < 0 && r != -EOPNOTSUPP)
+                                                return log_error_errno(r, "Failed to copy smack label from %s to %s: %m", src, t);
 #endif
+                                } else if (errno != EEXIST)
+                                        return log_error_errno(errno, "Failed to create symlink %s: %m", t);
                         }
                 }
         }