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>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 678a22c75fdbe3975dac991927296a9dce33c00c..faf55f59646b099c46a394b87bb2bf6f1ec8e517 100644 (file)
--- 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: