path: root/kernel/cgroup.c
diff options
authorSerge E. Hallyn <>2016-05-09 09:59:55 -0500
committerTejun Heo <>2016-05-09 12:15:03 -0400
commit4f41fc59620fcedaa97cbdf3d7d2956d80fcd922 (patch)
tree4082950fe6b016fc30366d8c26982261c3d874fe /kernel/cgroup.c
parente99ed4de1731f79868b00a7ff9f479a55a4799a7 (diff)
cgroup, kernfs: make mountinfo show properly scoped path for cgroup namespaces
Patch summary: When showing a cgroupfs entry in mountinfo, show the path of the mount root dentry relative to the reader's cgroup namespace root. Short explanation (courtesy of mkerrisk): If we create a new cgroup namespace, then we want both /proc/self/cgroup and /proc/self/mountinfo to show cgroup paths that are correctly virtualized with respect to the cgroup mount point. Previous to this patch, /proc/self/cgroup shows the right info, but /proc/self/mountinfo does not. Long version: When a uid 0 task which is in freezer cgroup /a/b, unshares a new cgroup namespace, and then mounts a new instance of the freezer cgroup, the new mount will be rooted at /a/b. The root dentry field of the mountinfo entry will show '/a/b'. cat > /tmp/do1 << EOF mount -t cgroup -o freezer freezer /mnt grep freezer /proc/self/mountinfo EOF unshare -Gm bash /tmp/do1 > 330 160 0:34 / /sys/fs/cgroup/freezer rw,nosuid,nodev,noexec,relatime - cgroup cgroup rw,freezer > 355 133 0:34 /a/b /mnt rw,relatime - cgroup freezer rw,freezer The task's freezer cgroup entry in /proc/self/cgroup will simply show '/': grep freezer /proc/self/cgroup 9:freezer:/ If instead the same task simply bind mounts the /a/b cgroup directory, the resulting mountinfo entry will again show /a/b for the dentry root. However in this case the task will find its own cgroup at /mnt/a/b, not at /mnt: mount --bind /sys/fs/cgroup/freezer/a/b /mnt 130 25 0:34 /a/b /mnt rw,nosuid,nodev,noexec,relatime shared:21 - cgroup cgroup rw,freezer In other words, there is no way for the task to know, based on what is in mountinfo, which cgroup directory is its own. Example (by mkerrisk): First, a little script to save some typing and verbiage: echo -e "\t/proc/self/cgroup:\t$(cat /proc/self/cgroup | grep freezer)" cat /proc/self/mountinfo | grep freezer | awk '{print "\tmountinfo:\t\t" $4 "\t" $5}' Create cgroup, place this shell into the cgroup, and look at the state of the /proc files: 2653 2653 # Our shell 14254 # cat(1) /proc/self/cgroup: 10:freezer:/a/b mountinfo: / /sys/fs/cgroup/freezer Create a shell in new cgroup and mount namespaces. The act of creating a new cgroup namespace causes the process's current cgroups directories to become its cgroup root directories. (Here, I'm using my own version of the "unshare" utility, which takes the same options as the util-linux version): Look at the state of the /proc files: /proc/self/cgroup: 10:freezer:/ mountinfo: / /sys/fs/cgroup/freezer The third entry in /proc/self/cgroup (the pathname of the cgroup inside the hierarchy) is correctly virtualized w.r.t. the cgroup namespace, which is rooted at /a/b in the outer namespace. However, the info in /proc/self/mountinfo is not for this cgroup namespace, since we are seeing a duplicate of the mount from the old mount namespace, and the info there does not correspond to the new cgroup namespace. However, trying to create a new mount still doesn't show us the right information in mountinfo: # propagating to other mountns /proc/self/cgroup: 7:freezer:/ mountinfo: /a/b /mnt/freezer The act of creating a new cgroup namespace caused the process's current freezer directory, "/a/b", to become its cgroup freezer root directory. In other words, the pathname directory of the directory within the newly mounted cgroup filesystem should be "/", but mountinfo wrongly shows us "/a/b". The consequence of this is that the process in the cgroup namespace cannot correctly construct the pathname of its cgroup root directory from the information in /proc/PID/mountinfo. With this patch, the dentry root field in mountinfo is shown relative to the reader's cgroup namespace. So the same steps as above: /proc/self/cgroup: 10:freezer:/a/b mountinfo: / /sys/fs/cgroup/freezer /proc/self/cgroup: 10:freezer:/ mountinfo: /../.. /sys/fs/cgroup/freezer /proc/self/cgroup: 10:freezer:/ mountinfo: / /mnt/freezer cgroup.clone_children freezer.parent_freezing freezer.state tasks cgroup.procs freezer.self_freezing notify_on_release 3164 2653 # First shell that placed in this cgroup 3164 # Shell started by 'unshare' 14197 # cat(1) Signed-off-by: Serge Hallyn <> Tested-by: Michael Kerrisk <> Acked-by: Michael Kerrisk <> Signed-off-by: Tejun Heo <>
Diffstat (limited to 'kernel/cgroup.c')
1 files changed, 63 insertions, 0 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 909a7d31ffd3..afea39eb7649 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1215,6 +1215,41 @@ static void cgroup_destroy_root(struct cgroup_root *root)
+ * look up cgroup associated with current task's cgroup namespace on the
+ * specified hierarchy
+ */
+static struct cgroup *
+current_cgns_cgroup_from_root(struct cgroup_root *root)
+ struct cgroup *res = NULL;
+ struct css_set *cset;
+ lockdep_assert_held(&css_set_lock);
+ rcu_read_lock();
+ cset = current->nsproxy->cgroup_ns->root_cset;
+ if (cset == &init_css_set) {
+ res = &root->cgrp;
+ } else {
+ struct cgrp_cset_link *link;
+ list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
+ struct cgroup *c = link->cgrp;
+ if (c->root == root) {
+ res = c;
+ break;
+ }
+ }
+ }
+ rcu_read_unlock();
+ BUG_ON(!res);
+ return res;
/* look up cgroup associated with given css_set on the specified hierarchy */
static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
struct cgroup_root *root)
@@ -1593,6 +1628,33 @@ static int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
return 0;
+static int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
+ struct kernfs_root *kf_root)
+ int len = 0, ret = 0;
+ char *buf = NULL;
+ struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
+ struct cgroup *ns_cgroup;
+ buf = kmalloc(PATH_MAX, GFP_KERNEL);
+ if (!buf)
+ return -ENOMEM;
+ spin_lock_bh(&css_set_lock);
+ ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
+ len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
+ spin_unlock_bh(&css_set_lock);
+ if (len >= PATH_MAX)
+ len = -ERANGE;
+ else if (len > 0) {
+ seq_escape(sf, buf, " \t\n\\");
+ len = 0;
+ }
+ kfree(buf);
+ return len;
static int cgroup_show_options(struct seq_file *seq,
struct kernfs_root *kf_root)
@@ -5433,6 +5495,7 @@ static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
.mkdir = cgroup_mkdir,
.rmdir = cgroup_rmdir,
.rename = cgroup_rename,
+ .show_path = cgroup_show_path,
static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)