linux/fs/pnode.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  linux/fs/pnode.c
 *
 * (C) Copyright IBM Corporation 2005.
 *	Author : Ram Pai (linuxram@us.ibm.com)
 */
#include <linux/mnt_namespace.h>
#include <linux/mount.h>
#include <linux/fs.h>
#include <linux/nsproxy.h>
#include <uapi/linux/mount.h>
#include "internal.h"
#include "pnode.h"

/* return the next shared peer mount of @p */
static inline struct mount *next_peer(struct mount *p)
{
	return list_entry(p->mnt_share.next, struct mount, mnt_share);
}

static inline struct mount *first_slave(struct mount *p)
{
	return list_entry(p->mnt_slave_list.next, struct mount, mnt_slave);
}

static inline struct mount *next_slave(struct mount *p)
{
	return list_entry(p->mnt_slave.next, struct mount, mnt_slave);
}

static struct mount *get_peer_under_root(struct mount *mnt,
					 struct mnt_namespace *ns,
					 const struct path *root)
{
	struct mount *m = mnt;

	do {
		/* Check the namespace first for optimization */
		if (m->mnt_ns == ns && is_path_reachable(m, m->mnt.mnt_root, root))
			return m;

		m = next_peer(m);
	} while (m != mnt);

	return NULL;
}

/*
 * Get ID of closest dominating peer group having a representative
 * under the given root.
 *
 * Caller must hold namespace_sem
 */
int get_dominating_id(struct mount *mnt, const struct path *root)
{
	struct mount *m;

	for (m = mnt->mnt_master; m != NULL; m = m->mnt_master) {
		struct mount *d = get_peer_under_root(m, mnt->mnt_ns, root);
		if (d)
			return d->mnt_group_id;
	}

	return 0;
}

static int do_make_slave(struct mount *mnt)
{
	struct mount *master, *slave_mnt;

	if (list_empty(&mnt->mnt_share)) {
		if (IS_MNT_SHARED(mnt)) {
			mnt_release_group_id(mnt);
			CLEAR_MNT_SHARED(mnt);
		}
		master = mnt->mnt_master;
		if (!master) {
			struct list_head *p = &mnt->mnt_slave_list;
			while (!list_empty(p)) {
				slave_mnt = list_first_entry(p,
						struct mount, mnt_slave);
				list_del_init(&slave_mnt->mnt_slave);
				slave_mnt->mnt_master = NULL;
			}
			return 0;
		}
	} else {
		struct mount *m;
		/*
		 * slave 'mnt' to a peer mount that has the
		 * same root dentry. If none is available then
		 * slave it to anything that is available.
		 */
		for (m = master = next_peer(mnt); m != mnt; m = next_peer(m)) {
			if (m->mnt.mnt_root == mnt->mnt.mnt_root) {
				master = m;
				break;
			}
		}
		list_del_init(&mnt->mnt_share);
		mnt->mnt_group_id = 0;
		CLEAR_MNT_SHARED(mnt);
	}
	list_for_each_entry(slave_mnt, &mnt->mnt_slave_list, mnt_slave)
		slave_mnt->mnt_master = master;
	list_move(&mnt->mnt_slave, &master->mnt_slave_list);
	list_splice(&mnt->mnt_slave_list, master->mnt_slave_list.prev);
	INIT_LIST_HEAD(&mnt->mnt_slave_list);
	mnt->mnt_master = master;
	return 0;
}

/*
 * vfsmount lock must be held for write
 */
void change_mnt_propagation(struct mount *mnt, int type)
{
	if (type == MS_SHARED) {
		set_mnt_shared(mnt);
		return;
	}
	do_make_slave(mnt);
	if (type != MS_SLAVE) {
		list_del_init(&mnt->mnt_slave);
		mnt->mnt_master = NULL;
		if (type == MS_UNBINDABLE)
			mnt->mnt.mnt_flags |= MNT_UNBINDABLE;
		else
			mnt->mnt.mnt_flags &= ~MNT_UNBINDABLE;
	}
}

static struct mount *__propagation_next(struct mount *m,
					 struct mount *origin)
{
	while (1) {
		struct mount *master = m->mnt_master;

		if (master == origin->mnt_master) {
			struct mount *next = next_peer(m);
			return (next == origin) ? NULL : next;
		} else if (m->mnt_slave.next != &master->mnt_slave_list)
			return next_slave(m);

		/* back at master */
		m = master;
	}
}

/*
 * get the next mount in the propagation tree.
 * @m: the mount seen last
 * @origin: the original mount from where the tree walk initiated
 *
 * Note that peer groups form contiguous segments of slave lists.
 * We rely on that in get_source() to be able to find out if
 * vfsmount found while iterating with propagation_next() is
 * a peer of one we'd found earlier.
 */
static struct mount *propagation_next(struct mount *m,
					 struct mount *origin)
{
	/* are there any slaves of this mount? */
	if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
		return first_slave(m);

	return __propagation_next(m, origin);
}

static struct mount *skip_propagation_subtree(struct mount *m,
						struct mount *origin)
{
	/*
	 * Advance m past everything that gets propagation from it.
	 */
	struct mount *p = __propagation_next(m, origin);

	while (p && peers(m, p))
		p = __propagation_next(p, origin);

	return p;
}

static struct mount *next_group(struct mount *m, struct mount *origin)
{
	while (1) {
		while (1) {
			struct mount *next;
			if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
				return first_slave(m);
			next = next_peer(m);
			if (m->mnt_group_id == origin->mnt_group_id) {
				if (next == origin)
					return NULL;
			} else if (m->mnt_slave.next != &next->mnt_slave)
				break;
			m = next;
		}
		/* m is the last peer */
		while (1) {
			struct mount *master = m->mnt_master;
			if (m->mnt_slave.next != &master->mnt_slave_list)
				return next_slave(m);
			m = next_peer(master);
			if (master->mnt_group_id == origin->mnt_group_id)
				break;
			if (master->mnt_slave.next == &m->mnt_slave)
				break;
			m = master;
		}
		if (m == origin)
			return NULL;
	}
}

/* all accesses are serialized by namespace_sem */
static struct mount *last_dest, *first_source, *last_source, *dest_master;
static struct hlist_head *list;

static int propagate_one(struct mount *m, struct mountpoint *dest_mp)
{
	struct mount *child;
	int type;
	/* skip ones added by this propagate_mnt() */
	if (IS_MNT_NEW(m))
		return 0;
	/* skip if mountpoint isn't visible in m */
	if (!is_subdir(dest_mp->m_dentry, m->mnt.mnt_root))
		return 0;
	/* skip if m is in the anon_ns */
	if (is_anon_ns(m->mnt_ns))
		return 0;

	if (peers(m, last_dest)) {
		type = CL_MAKE_SHARED;
	} else {
		struct mount *n, *p;
		bool done;
		for (n = m; ; n = p) {
			p = n->mnt_master;
			if (p == dest_master || IS_MNT_MARKED(p))
				break;
		}
		do {
			struct mount *parent = last_source->mnt_parent;
			if (peers(last_source, first_source))
				break;
			done = parent->mnt_master == p;
			if (done && peers(n, parent))
				break;
			last_source = last_source->mnt_master;
		} while (!done);

		type = CL_SLAVE;
		/* beginning of peer group among the slaves? */
		if (IS_MNT_SHARED(m))
			type |= CL_MAKE_SHARED;
	}
		
	child = copy_tree(last_source, last_source->mnt.mnt_root, type);
	if (IS_ERR(child))
		return PTR_ERR(child);
	read_seqlock_excl(&mount_lock);
	mnt_set_mountpoint(m, dest_mp, child);
	if (m->mnt_master != dest_master)
		SET_MNT_MARK(m->mnt_master);
	read_sequnlock_excl(&mount_lock);
	last_dest = m;
	last_source = child;
	hlist_add_head(&child->mnt_hash, list);
	return count_mounts(m->mnt_ns, child);
}

/*
 * mount 'source_mnt' under the destination 'dest_mnt' at
 * dentry 'dest_dentry'. And propagate that mount to
 * all the peer and slave mounts of 'dest_mnt'.
 * Link all the new mounts into a propagation tree headed at
 * source_mnt. Also link all the new mounts using ->mnt_list
 * headed at source_mnt's ->mnt_list
 *
 * @dest_mnt: destination mount.
 * @dest_dentry: destination dentry.
 * @source_mnt: source mount.
 * @tree_list : list of heads of trees to be attached.
 */
int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
		    struct mount *source_mnt, struct hlist_head *tree_list)
{
	struct mount *m, *n;
	int ret = 0;

	/*
	 * we don't want to bother passing tons of arguments to
	 * propagate_one(); everything is serialized by namespace_sem,
	 * so globals will do just fine.
	 */
	last_dest = dest_mnt;
	first_source = source_mnt;
	last_source = source_mnt;
	list = tree_list;
	dest_master = dest_mnt->mnt_master;

	/* all peers of dest_mnt, except dest_mnt itself */
	for (n = next_peer(dest_mnt); n != dest_mnt; n = next_peer(n)) {
		ret = propagate_one(n, dest_mp);
		if (ret)
			goto out;
	}

	/* all slave groups */
	for (m = next_group(dest_mnt, dest_mnt); m;
			m = next_group(m, dest_mnt)) {
		/* everything in that slave group */
		n = m;
		do {
			ret = propagate_one(n, dest_mp);
			if (ret)
				goto out;
			n = next_peer(n);
		} while (n != m);
	}
out:
	read_seqlock_excl(&mount_lock);
	hlist_for_each_entry(n, tree_list, mnt_hash) {
		m = n->mnt_parent;
		if (m->mnt_master != dest_mnt->mnt_master)
			CLEAR_MNT_MARK(m->mnt_master);
	}
	read_sequnlock_excl(&mount_lock);
	return ret;
}

static struct mount *find_topper(struct mount *mnt)
{
	/* If there is exactly one mount covering mnt completely return it. */
	struct mount *child;

	if (!list_is_singular(&mnt->mnt_mounts))
		return NULL;

	child = list_first_entry(&mnt->mnt_mounts, struct mount, mnt_child);
	if (child->mnt_mountpoint != mnt->mnt.mnt_root)
		return NULL;

	return child;
}

/*
 * return true if the refcount is greater than count
 */
static inline int do_refcount_check(struct mount *mnt, int count)
{
	return mnt_get_count(mnt) > count;
}

/**
 * propagation_would_overmount - check whether propagation from @from
 *                               would overmount @to
 * @from: shared mount
 * @to:   mount to check
 * @mp:   future mountpoint of @to on @from
 *
 * If @from propagates mounts to @to, @from and @to must either be peers
 * or one of the masters in the hierarchy of masters of @to must be a
 * peer of @from.
 *
 * If the root of the @to mount is equal to the future mountpoint @mp of
 * the @to mount on @from then @to will be overmounted by whatever is
 * propagated to it.
 *
 * Context: This function expects namespace_lock() to be held and that
 *          @mp is stable.
 * Return: If @from overmounts @to, true is returned, false if not.
 */
bool propagation_would_overmount(const struct mount *from,
				 const struct mount *to,
				 const struct mountpoint *mp)
{
	if (!IS_MNT_SHARED(from))
		return false;

	if (to->mnt.mnt_root != mp->m_dentry)
		return false;

	for (const struct mount *m = to; m; m = m->mnt_master) {
		if (peers(from, m))
			return true;
	}

	return false;
}

/*
 * check if the mount 'mnt' can be unmounted successfully.
 * @mnt: the mount to be checked for unmount
 * NOTE: unmounting 'mnt' would naturally propagate to all
 * other mounts its parent propagates to.
 * Check if any of these mounts that **do not have submounts**
 * have more references than 'refcnt'. If so return busy.
 *
 * vfsmount lock must be held for write
 */
int propagate_mount_busy(struct mount *mnt, int refcnt)
{
	struct mount *m, *child, *topper;
	struct mount *parent = mnt->mnt_parent;

	if (mnt == parent)
		return do_refcount_check(mnt, refcnt);

	/*
	 * quickly check if the current mount can be unmounted.
	 * If not, we don't have to go checking for all other
	 * mounts
	 */
	if (!list_empty(&mnt->mnt_mounts) || do_refcount_check(mnt, refcnt))
		return 1;

	for (m = propagation_next(parent, parent); m;
	     		m = propagation_next(m, parent)) {
		int count = 1;
		child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
		if (!child)
			continue;

		/* Is there exactly one mount on the child that covers
		 * it completely whose reference should be ignored?
		 */
		topper = find_topper(child);
		if (topper)
			count += 1;
		else if (!list_empty(&child->mnt_mounts))
			continue;

		if (do_refcount_check(child, count))
			return 1;
	}
	return 0;
}

/*
 * Clear MNT_LOCKED when it can be shown to be safe.
 *
 * mount_lock lock must be held for write
 */
void propagate_mount_unlock(struct mount *mnt)
{
	struct mount *parent = mnt->mnt_parent;
	struct mount *m, *child;

	BUG_ON(parent == mnt);

	for (m = propagation_next(parent, parent); m;
			m = propagation_next(m, parent)) {
		child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
		if (child)
			child->mnt.mnt_flags &= ~MNT_LOCKED;
	}
}

static inline bool is_candidate(struct mount *m)
{
	return m->mnt.mnt_flags & MNT_UMOUNT_CANDIDATE;
}

static inline bool will_be_unmounted(struct mount *m)
{
	return m->mnt.mnt_flags & MNT_UMOUNT;
}

static void umount_one(struct mount *m, struct list_head *to_umount)
{
	m->mnt.mnt_flags |= MNT_UMOUNT;
	list_del_init(&m->mnt_child);
	move_from_ns(m, to_umount);
}

static void remove_from_candidate_list(struct mount *m)
{
	m->mnt.mnt_flags &= ~(MNT_MARKED | MNT_UMOUNT_CANDIDATE);
	list_del_init(&m->mnt_list);
}

static void gather_candidates(struct list_head *set,
			      struct list_head *candidates)
{
	struct mount *m, *p, *q;

	list_for_each_entry(m, set, mnt_list) {
		if (is_candidate(m))
			continue;
		m->mnt.mnt_flags |= MNT_UMOUNT_CANDIDATE;
		p = m->mnt_parent;
		q = propagation_next(p, p);
		while (q) {
			struct mount *child = __lookup_mnt(&q->mnt,
							   m->mnt_mountpoint);
			if (child) {
				/*
				 * We might've already run into this one.  That
				 * must've happened on earlier iteration of the
				 * outer loop; in that case we can skip those
				 * parents that get propagation from q - there
				 * will be nothing new on those as well.
				 */
				if (is_candidate(child)) {
					q = skip_propagation_subtree(q, p);
					continue;
				}
				child->mnt.mnt_flags |= MNT_UMOUNT_CANDIDATE;
				if (!will_be_unmounted(child))
					list_add(&child->mnt_list, candidates);
			}
			q = propagation_next(q, p);
		}
	}
	list_for_each_entry(m, set, mnt_list)
		m->mnt.mnt_flags &= ~MNT_UMOUNT_CANDIDATE;
}

/*
 * We know that some child of @m can't be unmounted.  In all places where the
 * chain of descent of @m has child not overmounting the root of parent,
 * the parent can't be unmounted either.
 */
static void trim_ancestors(struct mount *m)
{
	struct mount *p;

	for (p = m->mnt_parent; is_candidate(p); m = p, p = p->mnt_parent) {
		if (IS_MNT_MARKED(m))	// all candidates beneath are overmounts
			return;
		SET_MNT_MARK(m);
		if (m != p->overmount)
			p->mnt.mnt_flags &= ~MNT_UMOUNT_CANDIDATE;
	}
}

/*
 * Find and exclude all umount candidates forbidden by @m
 * (see Documentation/filesystems/propagate_umount.txt)
 * If we can immediately tell that @m is OK to unmount (unlocked
 * and all children are already committed to unmounting) commit
 * to unmounting it.
 * Only @m itself might be taken from the candidates list;
 * anything found by trim_ancestors() is marked non-candidate
 * and left on the list.
 */
static void trim_one(struct mount *m, struct list_head *to_umount)
{
	bool remove_this = false, found = false, umount_this = false;
	struct mount *n;

	if (!is_candidate(m)) { // trim_ancestors() left it on list
		remove_from_candidate_list(m);
		return;
	}

	list_for_each_entry(n, &m->mnt_mounts, mnt_child) {
		if (!is_candidate(n)) {
			found = true;
			if (n != m->overmount) {
				remove_this = true;
				break;
			}
		}
	}
	if (found) {
		trim_ancestors(m);
	} else if (!IS_MNT_LOCKED(m) && list_empty(&m->mnt_mounts)) {
		remove_this = true;
		umount_this = true;
	}
	if (remove_this) {
		remove_from_candidate_list(m);
		if (umount_this)
			umount_one(m, to_umount);
	}
}

static void handle_locked(struct mount *m, struct list_head *to_umount)
{
	struct mount *cutoff = m, *p;

	if (!is_candidate(m)) { // trim_ancestors() left it on list
		remove_from_candidate_list(m);
		return;
	}
	for (p = m; is_candidate(p); p = p->mnt_parent) {
		remove_from_candidate_list(p);
		if (!IS_MNT_LOCKED(p))
			cutoff = p->mnt_parent;
	}
	if (will_be_unmounted(p))
		cutoff = p;
	while (m != cutoff) {
		umount_one(m, to_umount);
		m = m->mnt_parent;
	}
}

/*
 * @m is not to going away, and it overmounts the top of a stack of mounts
 * that are going away.  We know that all of those are fully overmounted
 * by the one above (@m being the topmost of the chain), so @m can be slid
 * in place where the bottom of the stack is attached.
 *
 * NOTE: here we temporarily violate a constraint - two mounts end up with
 * the same parent and mountpoint; that will be remedied as soon as we
 * return from propagate_umount() - its caller (umount_tree()) will detach
 * the stack from the parent it (and now @m) is attached to.  umount_tree()
 * might choose to keep unmounted pieces stuck to each other, but it always
 * detaches them from the mounts that remain in the tree.
 */
static void reparent(struct mount *m)
{
	struct mount *p = m;
	struct mountpoint *mp;

	do {
		mp = p->mnt_mp;
		p = p->mnt_parent;
	} while (will_be_unmounted(p));

	mnt_change_mountpoint(p, mp, m);
	mnt_notify_add(m);
}

/**
 * propagate_umount - apply propagation rules to the set of mounts for umount()
 * @set: the list of mounts to be unmounted.
 *
 * Collect all mounts that receive propagation from the mount in @set and have
 * no obstacles to being unmounted.  Add these additional mounts to the set.
 *
 * See Documentation/filesystems/propagate_umount.txt if you do anything in
 * this area.
 *
 * Locks held:
 * mount_lock (write_seqlock), namespace_sem (exclusive).
 */
void propagate_umount(struct list_head *set)
{
	struct mount *m, *p;
	LIST_HEAD(to_umount);	// committed to unmounting
	LIST_HEAD(candidates);	// undecided umount candidates

	// collect all candidates
	gather_candidates(set, &candidates);

	// reduce the set until it's non-shifting
	list_for_each_entry_safe(m, p, &candidates, mnt_list)
		trim_one(m, &to_umount);

	// ... and non-revealing
	while (!list_empty(&candidates)) {
		m = list_first_entry(&candidates,struct mount, mnt_list);
		handle_locked(m, &to_umount);
	}

	// now to_umount consists of all acceptable candidates
	// deal with reparenting of remaining overmounts on those
	list_for_each_entry(m, &to_umount, mnt_list) {
		if (m->overmount)
			reparent(m->overmount);
	}

	// and fold them into the set
	list_splice_tail_init(&to_umount, set);
}