fs: keep an index of current mount namespaces

In order to allow for listmount() to be used on different namespaces we
need a way to lookup a mount ns by its id.  Keep a rbtree of the current
!anonymous mount name spaces indexed by ID that we can use to look up
the namespace.

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/e5fdd78a90f5b00a75bd893962a70f52a2c015cd.1719243756.git.josef@toxicpanda.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/mount.h b/fs/mount.h
index 4a42fc68f4cca94770fe5131cb0fcd264b9a820d..5b980c262c0c3aa477647b7f6e9e1e8feac054b7 100644 (file)
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -16,6 +16,8 @@ struct mnt_namespace {
        u64 event;
        unsigned int            nr_mounts; /* # of mounts in the namespace */
        unsigned int            pending_mounts;
+       struct rb_node          mnt_ns_tree_node; /* node in the mnt_ns_tree */
+       refcount_t              passive; /* number references not pinning @mounts */
 } __randomize_layout;
 
 struct mnt_pcp {

diff --git a/fs/namespace.c b/fs/namespace.c
index 352e8c90b9f7a7aefe10055a371c41d0578f3476..eebe9d912a719a3f966007fc5094ea25a96b9456 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -78,6 +78,8 @@ static struct kmem_cache *mnt_cache __ro_after_init;
 static DECLARE_RWSEM(namespace_sem);
 static HLIST_HEAD(unmounted);  /* protected by namespace_sem */
 static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
+static DEFINE_RWLOCK(mnt_ns_tree_lock);
+static struct rb_root mnt_ns_tree = RB_ROOT; /* protected by mnt_ns_tree_lock */
 
 struct mount_kattr {
        unsigned int attr_set;
@@ -103,6 +105,109 @@ EXPORT_SYMBOL_GPL(fs_kobj);
  */
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
 
+static int mnt_ns_cmp(u64 seq, const struct mnt_namespace *ns)
+{
+       u64 seq_b = ns->seq;
+
+       if (seq < seq_b)
+               return -1;
+       if (seq > seq_b)
+               return 1;
+       return 0;
+}
+
+static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
+{
+       if (!node)
+               return NULL;
+       return rb_entry(node, struct mnt_namespace, mnt_ns_tree_node);
+}
+
+static bool mnt_ns_less(struct rb_node *a, const struct rb_node *b)
+{
+       struct mnt_namespace *ns_a = node_to_mnt_ns(a);
+       struct mnt_namespace *ns_b = node_to_mnt_ns(b);
+       u64 seq_a = ns_a->seq;
+
+       return mnt_ns_cmp(seq_a, ns_b) < 0;
+}
+
+static void mnt_ns_tree_add(struct mnt_namespace *ns)
+{
+       guard(write_lock)(&mnt_ns_tree_lock);
+       rb_add(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_less);
+}
+
+static void mnt_ns_release(struct mnt_namespace *ns)
+{
+       lockdep_assert_not_held(&mnt_ns_tree_lock);
+
+       /* keep alive for {list,stat}mount() */
+       if (refcount_dec_and_test(&ns->passive)) {
+               put_user_ns(ns->user_ns);
+               kfree(ns);
+       }
+}
+DEFINE_FREE(mnt_ns_release, struct mnt_namespace *, if (_T) mnt_ns_release(_T))
+
+static void mnt_ns_tree_remove(struct mnt_namespace *ns)
+{
+       /* remove from global mount namespace list */
+       if (!is_anon_ns(ns)) {
+               guard(write_lock)(&mnt_ns_tree_lock);
+               rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
+       }
+
+       mnt_ns_release(ns);
+}
+
+/*
+ * Returns the mount namespace which either has the specified id, or has the
+ * next smallest id afer the specified one.
+ */
+static struct mnt_namespace *mnt_ns_find_id_at(u64 mnt_ns_id)
+{
+       struct rb_node *node = mnt_ns_tree.rb_node;
+       struct mnt_namespace *ret = NULL;
+
+       lockdep_assert_held(&mnt_ns_tree_lock);
+
+       while (node) {
+               struct mnt_namespace *n = node_to_mnt_ns(node);
+
+               if (mnt_ns_id <= n->seq) {
+                       ret = node_to_mnt_ns(node);
+                       if (mnt_ns_id == n->seq)
+                               break;
+                       node = node->rb_left;
+               } else {
+                       node = node->rb_right;
+               }
+       }
+       return ret;
+}
+
+/*
+ * Lookup a mount namespace by id and take a passive reference count. Taking a
+ * passive reference means the mount namespace can be emptied if e.g., the last
+ * task holding an active reference exits. To access the mounts of the
+ * namespace the @namespace_sem must first be acquired. If the namespace has
+ * already shut down before acquiring @namespace_sem, {list,stat}mount() will
+ * see that the mount rbtree of the namespace is empty.
+ */
+static struct mnt_namespace *lookup_mnt_ns(u64 mnt_ns_id)
+{
+       struct mnt_namespace *ns;
+
+       guard(read_lock)(&mnt_ns_tree_lock);
+       ns = mnt_ns_find_id_at(mnt_ns_id);
+       if (!ns || ns->seq != mnt_ns_id)
+               return NULL;
+
+       refcount_inc(&ns->passive);
+       return ns;
+}
+
 static inline void lock_mount_hash(void)
 {
        write_seqlock(&mount_lock);
@@ -3733,8 +3838,7 @@ static void free_mnt_ns(struct mnt_namespace *ns)
        if (!is_anon_ns(ns))
                ns_free_inum(&ns->ns);
        dec_mnt_namespaces(ns->ucounts);
-       put_user_ns(ns->user_ns);
-       kfree(ns);
+       mnt_ns_tree_remove(ns);
 }
 
 /*
@@ -3773,7 +3877,9 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool a
        if (!anon)
                new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
        refcount_set(&new_ns->ns.count, 1);
+       refcount_set(&new_ns->passive, 1);
        new_ns->mounts = RB_ROOT;
+       RB_CLEAR_NODE(&new_ns->mnt_ns_tree_node);
        init_waitqueue_head(&new_ns->poll);
        new_ns->user_ns = get_user_ns(user_ns);
        new_ns->ucounts = ucounts;
@@ -3850,6 +3956,7 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
                while (p->mnt.mnt_root != q->mnt.mnt_root)
                        p = next_mnt(skip_mnt_tree(p), old);
        }
+       mnt_ns_tree_add(new_ns);
        namespace_unlock();
 
        if (rootmnt)
@@ -5205,6 +5312,8 @@ static void __init init_mount_tree(void)
 
        set_fs_pwd(current->fs, &root);
        set_fs_root(current->fs, &root);
+
+       mnt_ns_tree_add(ns);
 }
 
 void __init mnt_init(void)