v4.19.13 snapshot.
diff --git a/fs/nsfs.c b/fs/nsfs.c
new file mode 100644
index 0000000..60702d6
--- /dev/null
+++ b/fs/nsfs.c
@@ -0,0 +1,283 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/mount.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/proc_ns.h>
+#include <linux/magic.h>
+#include <linux/ktime.h>
+#include <linux/seq_file.h>
+#include <linux/user_namespace.h>
+#include <linux/nsfs.h>
+#include <linux/uaccess.h>
+
+static struct vfsmount *nsfs_mnt;
+
+static long ns_ioctl(struct file *filp, unsigned int ioctl,
+ unsigned long arg);
+static const struct file_operations ns_file_operations = {
+ .llseek = no_llseek,
+ .unlocked_ioctl = ns_ioctl,
+};
+
+static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)
+{
+ struct inode *inode = d_inode(dentry);
+ const struct proc_ns_operations *ns_ops = dentry->d_fsdata;
+
+ return dynamic_dname(dentry, buffer, buflen, "%s:[%lu]",
+ ns_ops->name, inode->i_ino);
+}
+
+static void ns_prune_dentry(struct dentry *dentry)
+{
+ struct inode *inode = d_inode(dentry);
+ if (inode) {
+ struct ns_common *ns = inode->i_private;
+ atomic_long_set(&ns->stashed, 0);
+ }
+}
+
+const struct dentry_operations ns_dentry_operations =
+{
+ .d_prune = ns_prune_dentry,
+ .d_delete = always_delete_dentry,
+ .d_dname = ns_dname,
+};
+
+static void nsfs_evict(struct inode *inode)
+{
+ struct ns_common *ns = inode->i_private;
+ clear_inode(inode);
+ ns->ops->put(ns);
+}
+
+static void *__ns_get_path(struct path *path, struct ns_common *ns)
+{
+ struct vfsmount *mnt = nsfs_mnt;
+ struct dentry *dentry;
+ struct inode *inode;
+ unsigned long d;
+
+ rcu_read_lock();
+ d = atomic_long_read(&ns->stashed);
+ if (!d)
+ goto slow;
+ dentry = (struct dentry *)d;
+ if (!lockref_get_not_dead(&dentry->d_lockref))
+ goto slow;
+ rcu_read_unlock();
+ ns->ops->put(ns);
+got_it:
+ path->mnt = mntget(mnt);
+ path->dentry = dentry;
+ return NULL;
+slow:
+ rcu_read_unlock();
+ inode = new_inode_pseudo(mnt->mnt_sb);
+ if (!inode) {
+ ns->ops->put(ns);
+ return ERR_PTR(-ENOMEM);
+ }
+ inode->i_ino = ns->inum;
+ inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
+ inode->i_flags |= S_IMMUTABLE;
+ inode->i_mode = S_IFREG | S_IRUGO;
+ inode->i_fop = &ns_file_operations;
+ inode->i_private = ns;
+
+ dentry = d_alloc_pseudo(mnt->mnt_sb, &empty_name);
+ if (!dentry) {
+ iput(inode);
+ return ERR_PTR(-ENOMEM);
+ }
+ d_instantiate(dentry, inode);
+ dentry->d_flags |= DCACHE_RCUACCESS;
+ dentry->d_fsdata = (void *)ns->ops;
+ d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);
+ if (d) {
+ d_delete(dentry); /* make sure ->d_prune() does nothing */
+ dput(dentry);
+ cpu_relax();
+ return ERR_PTR(-EAGAIN);
+ }
+ goto got_it;
+}
+
+void *ns_get_path_cb(struct path *path, ns_get_path_helper_t *ns_get_cb,
+ void *private_data)
+{
+ struct ns_common *ns;
+ void *ret;
+
+again:
+ ns = ns_get_cb(private_data);
+ if (!ns)
+ return ERR_PTR(-ENOENT);
+
+ ret = __ns_get_path(path, ns);
+ if (IS_ERR(ret) && PTR_ERR(ret) == -EAGAIN)
+ goto again;
+ return ret;
+}
+
+struct ns_get_path_task_args {
+ const struct proc_ns_operations *ns_ops;
+ struct task_struct *task;
+};
+
+static struct ns_common *ns_get_path_task(void *private_data)
+{
+ struct ns_get_path_task_args *args = private_data;
+
+ return args->ns_ops->get(args->task);
+}
+
+void *ns_get_path(struct path *path, struct task_struct *task,
+ const struct proc_ns_operations *ns_ops)
+{
+ struct ns_get_path_task_args args = {
+ .ns_ops = ns_ops,
+ .task = task,
+ };
+
+ return ns_get_path_cb(path, ns_get_path_task, &args);
+}
+
+int open_related_ns(struct ns_common *ns,
+ struct ns_common *(*get_ns)(struct ns_common *ns))
+{
+ struct path path = {};
+ struct file *f;
+ void *err;
+ int fd;
+
+ fd = get_unused_fd_flags(O_CLOEXEC);
+ if (fd < 0)
+ return fd;
+
+ while (1) {
+ struct ns_common *relative;
+
+ relative = get_ns(ns);
+ if (IS_ERR(relative)) {
+ put_unused_fd(fd);
+ return PTR_ERR(relative);
+ }
+
+ err = __ns_get_path(&path, relative);
+ if (IS_ERR(err) && PTR_ERR(err) == -EAGAIN)
+ continue;
+ break;
+ }
+ if (IS_ERR(err)) {
+ put_unused_fd(fd);
+ return PTR_ERR(err);
+ }
+
+ f = dentry_open(&path, O_RDONLY, current_cred());
+ path_put(&path);
+ if (IS_ERR(f)) {
+ put_unused_fd(fd);
+ fd = PTR_ERR(f);
+ } else
+ fd_install(fd, f);
+
+ return fd;
+}
+EXPORT_SYMBOL_GPL(open_related_ns);
+
+static long ns_ioctl(struct file *filp, unsigned int ioctl,
+ unsigned long arg)
+{
+ struct user_namespace *user_ns;
+ struct ns_common *ns = get_proc_ns(file_inode(filp));
+ uid_t __user *argp;
+ uid_t uid;
+
+ switch (ioctl) {
+ case NS_GET_USERNS:
+ return open_related_ns(ns, ns_get_owner);
+ case NS_GET_PARENT:
+ if (!ns->ops->get_parent)
+ return -EINVAL;
+ return open_related_ns(ns, ns->ops->get_parent);
+ case NS_GET_NSTYPE:
+ return ns->ops->type;
+ case NS_GET_OWNER_UID:
+ if (ns->ops->type != CLONE_NEWUSER)
+ return -EINVAL;
+ user_ns = container_of(ns, struct user_namespace, ns);
+ argp = (uid_t __user *) arg;
+ uid = from_kuid_munged(current_user_ns(), user_ns->owner);
+ return put_user(uid, argp);
+ default:
+ return -ENOTTY;
+ }
+}
+
+int ns_get_name(char *buf, size_t size, struct task_struct *task,
+ const struct proc_ns_operations *ns_ops)
+{
+ struct ns_common *ns;
+ int res = -ENOENT;
+ const char *name;
+ ns = ns_ops->get(task);
+ if (ns) {
+ name = ns_ops->real_ns_name ? : ns_ops->name;
+ res = snprintf(buf, size, "%s:[%u]", name, ns->inum);
+ ns_ops->put(ns);
+ }
+ return res;
+}
+
+struct file *proc_ns_fget(int fd)
+{
+ struct file *file;
+
+ file = fget(fd);
+ if (!file)
+ return ERR_PTR(-EBADF);
+
+ if (file->f_op != &ns_file_operations)
+ goto out_invalid;
+
+ return file;
+
+out_invalid:
+ fput(file);
+ return ERR_PTR(-EINVAL);
+}
+
+static int nsfs_show_path(struct seq_file *seq, struct dentry *dentry)
+{
+ struct inode *inode = d_inode(dentry);
+ const struct proc_ns_operations *ns_ops = dentry->d_fsdata;
+
+ seq_printf(seq, "%s:[%lu]", ns_ops->name, inode->i_ino);
+ return 0;
+}
+
+static const struct super_operations nsfs_ops = {
+ .statfs = simple_statfs,
+ .evict_inode = nsfs_evict,
+ .show_path = nsfs_show_path,
+};
+static struct dentry *nsfs_mount(struct file_system_type *fs_type,
+ int flags, const char *dev_name, void *data)
+{
+ return mount_pseudo(fs_type, "nsfs:", &nsfs_ops,
+ &ns_dentry_operations, NSFS_MAGIC);
+}
+static struct file_system_type nsfs = {
+ .name = "nsfs",
+ .mount = nsfs_mount,
+ .kill_sb = kill_anon_super,
+};
+
+void __init nsfs_init(void)
+{
+ nsfs_mnt = kern_mount(&nsfs);
+ if (IS_ERR(nsfs_mnt))
+ panic("can't set nsfs up\n");
+ nsfs_mnt->mnt_sb->s_flags &= ~SB_NOUSER;
+}