[RFC PATCH bpf-next 6/9] cgroup: add v1 support to cgroup_get_from_id()

[Yosry Ahmed]

The current implementation of cgroup_get_from_id() only searches the
default hierarchy for the given id. Make it compatible with cgroup v1 by
looking through all the roots instead.

cgrp_dfl_root should be the first element in the list so there shouldn't
be a performance impact for cgroup v2 users (in the case of a valid id).

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 kernel/cgroup/cgroup.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index af703cfcb9d2..12700cd21973 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5970,10 +5970,16 @@ void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
  */
 struct cgroup *cgroup_get_from_id(u64 id)
 {
-	struct kernfs_node *kn;
+	struct kernfs_node *kn = NULL;
 	struct cgroup *cgrp = NULL;
+	struct cgroup_root *root;
+
+	for_each_root(root) {
+		kn = kernfs_find_and_get_node_by_id(root->kf_root, id);
+		if (kn)
+			break;
+	}
 
-	kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
 	if (!kn)
 		goto out;
 
-- 
2.36.0.512.ge40c2bad7a-goog