cgroup: make cgroup_path() not print double slashes

While reimplementing cgroup_path(), 65dff759d2 ("cgroup: fix
cgroup_path() vs rename() race") introduced a bug where the path of a
non-root cgroup would have two slahses at the beginning, which is
caused by treating the root cgroup which has the name '/' like
non-root cgroups.

 $ grep systemd /proc/self/cgroup
 1:name=systemd://user/root/1

Fix it by special casing root cgroup case and not looping over it in
the normal path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 678a22c75fdb..faf55f59646b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1811,11 +1811,17 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
         int ret = -ENAMETOOLONG;
         char *start;
 
+        if (!cgrp->parent) {
+                if (strlcpy(buf, "/", buflen) >= buflen)
+                        return -ENAMETOOLONG;
+                return 0;
+        }
+
         start = buf + buflen - 1;
         *start = '\0';
 
         rcu_read_lock();
-        while (cgrp) {
+        do {
                 const char *name = cgroup_name(cgrp);
                 int len;
 
@@ -1824,15 +1830,12 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
                         goto out;
                 memcpy(start, name, len);
 
-                if (!cgrp->parent)
-                        break;
-
                 if (--start < buf)
                         goto out;
                 *start = '/';
 
                 cgrp = cgrp->parent;
-        }
+        } while (cgrp->parent);
         ret = 0;
         memmove(buf, start, buf + buflen - start);
 out: