This patch enables cgroup mounting inside userns when a process
as appropriate privileges. The cgroup filesystem mounted is
rooted at the cgroupns-root. Thus, in a container-setup, only
the hierarchy under the cgroupns-root is exposed inside the container.
This allows container management tools to run inside the containers
without depending on any global state.
In order to support this, a new kernfs api is added to lookup the
dentry for the cgroupns-root.
Signed-off-by: Aditya Kali <adityakali@xxxxxxxxxx>
---
fs/kernfs/mount.c | 48
++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/kernfs.h | 2 ++
kernel/cgroup.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 95 insertions(+), 1 deletion(-)
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index f973ae9..efe5e15 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -62,6 +62,54 @@ struct kernfs_root *kernfs_root_from_sb(struct
super_block *sb)
return NULL;
}
+/**
+ * kernfs_obtain_root - get a dentry for the given kernfs_node
+ * @sb: the kernfs super_block
+ * @kn: kernfs_node for which a dentry is needed
+ *
+ * This can used used by callers which want to mount only a part of the
kernfs
+ * as root of the filesystem.
+ */
+struct dentry *kernfs_obtain_root(struct super_block *sb,
+ struct kernfs_node *kn)
+{
+ struct dentry *dentry;
+ struct inode *inode;
+
+ BUG_ON(sb->s_op != &kernfs_sops);
+
+ /* inode for the given kernfs_node should already exist. */
+ inode = ilookup(sb, kn->ino);
+ if (!inode) {
+ pr_debug("kernfs: could not get inode for '");
+ pr_cont_kernfs_path(kn);
+ pr_cont("'.\n");
+ return ERR_PTR(-EINVAL);
+ }
+
+ /* instantiate and link root dentry */
+ dentry = d_obtain_root(inode);
+ if (!dentry) {
+ pr_debug("kernfs: could not get dentry for '");
+ pr_cont_kernfs_path(kn);
+ pr_cont("'.\n");
+ return ERR_PTR(-ENOMEM);
+ }
+
+ /* If this is a new dentry, set it up. We need kernfs_mutex because this
+ * may be called by callers other than kernfs_fill_super. */
+ mutex_lock(&kernfs_mutex);
+ if (!dentry->d_fsdata) {
+ kernfs_get(kn);
+ dentry->d_fsdata = kn;
+ } else {
+ WARN_ON(dentry->d_fsdata != kn);
+ }
+ mutex_unlock(&kernfs_mutex);
+
+ return dentry;
+}
+
static int kernfs_fill_super(struct super_block *sb, unsigned long magic)
{
struct kernfs_super_info *info = kernfs_info(sb);
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 3c2be75..b9538e0 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -274,6 +274,8 @@ void kernfs_put(struct kernfs_node *kn);
struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry);
struct kernfs_root *kernfs_root_from_sb(struct super_block *sb);
+struct dentry *kernfs_obtain_root(struct super_block *sb,
+ struct kernfs_node *kn);
struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
unsigned int flags, void *priv);
void kernfs_destroy_root(struct kernfs_root *root);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7e5d597..8008c4c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1389,6 +1389,14 @@ static int parse_cgroupfs_options(char *data,
struct cgroup_sb_opts *opts)
return -ENOENT;
}
+ /* If inside a non-init cgroup namespace, only allow default hierarchy
+ * to be mounted.
+ */
+ if ((current->nsproxy->cgroup_ns != &init_cgroup_ns) &&
+ !(opts->flags & CGRP_ROOT_SANE_BEHAVIOR)) {
+ return -EINVAL;
+ }
+
if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
pr_warn("sane_behavior: this is still under development and its
behaviors will change, proceed at your own risk\n");
if (nr_opts != 1) {
@@ -1581,6 +1589,15 @@ static void init_cgroup_root(struct cgroup_root
*root,
set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags);
}
+struct dentry *cgroupns_get_root(struct super_block *sb,
+ struct cgroup_namespace *ns)
+{
+ struct dentry *nsdentry;
+
+ nsdentry = kernfs_obtain_root(sb, ns->root_cgrp->kn);
+ return nsdentry;
+}
+
static int cgroup_setup_root(struct cgroup_root *root, unsigned int
ss_mask)
{
LIST_HEAD(tmp_links);
@@ -1685,6 +1702,14 @@ static struct dentry *cgroup_mount(struct
file_system_type *fs_type,
int ret;
int i;
bool new_sb;
+ struct cgroup_namespace *ns =
+ get_cgroup_ns(current->nsproxy->cgroup_ns);
+
+ /* Check if the caller has permission to mount. */
+ if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
+ put_cgroup_ns(ns);
+ return ERR_PTR(-EPERM);
+ }
/*
* The first time anyone tries to mount a cgroup, enable the list
@@ -1817,11 +1842,28 @@ out_free:
kfree(opts.release_agent);
kfree(opts.name);
- if (ret)
+ if (ret) {
+ put_cgroup_ns(ns);
return ERR_PTR(ret);
+ }
dentry = kernfs_mount(fs_type, flags, root->kf_root,
CGROUP_SUPER_MAGIC, &new_sb);
+
+ if (!IS_ERR(dentry) && (root == &cgrp_dfl_root)) {
+ /* If this mount is for the default hierarchy in non-init cgroup
+ * namespace, then instead of root cgroup's dentry, we return
+ * the dentry corresponding to the cgroupns->root_cgrp.
+ */
+ if (ns != &init_cgroup_ns) {
+ struct dentry *nsdentry;
+
+ nsdentry = cgroupns_get_root(dentry->d_sb, ns);
+ dput(dentry);
+ dentry = nsdentry;
+ }
+ }
+
if (IS_ERR(dentry) || !new_sb)
cgroup_put(&root->cgrp);
@@ -1834,6 +1876,7 @@ out_free:
deactivate_super(pinned_sb);
}
+ put_cgroup_ns(ns);
return dentry;
}
@@ -1862,6 +1905,7 @@ static struct file_system_type cgroup_fs_type = {
.name = "cgroup",
.mount = cgroup_mount,
.kill_sb = cgroup_kill_sb,
+ .fs_flags = FS_USERNS_MOUNT,
};
static struct kobject *cgroup_kobj;
--
2.1.0.rc2.206.gedb03e5
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