Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

1 files changed, 687 insertions, 0 deletions

diff --git a/fs/pnode.c b/fs/pnode.c
new file mode 100644
index 000000000..5d91c3e58
--- /dev/null
+++ b/fs/pnode.c
@@ -0,0 +1,687 @@
+// 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 hlist_entry(p->mnt_slave_list.first, struct mount, mnt_slave);
+}
+
+static inline struct mount *next_slave(struct mount *p)
+{
+	return hlist_entry(p->mnt_slave.next, struct mount, mnt_slave);
+}
+
+/* locks: namespace_shared && is_mounted(mnt) */
+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.
+ *
+ * locks: namespace_shared
+ */
+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 inline bool will_be_unmounted(struct mount *m)
+{
+	return m->mnt.mnt_flags & MNT_UMOUNT;
+}
+
+static void transfer_propagation(struct mount *mnt, struct mount *to)
+{
+	struct hlist_node *p = NULL, *n;
+	struct mount *m;
+
+	hlist_for_each_entry_safe(m, n, &mnt->mnt_slave_list, mnt_slave) {
+		m->mnt_master = to;
+		if (!to)
+			hlist_del_init(&m->mnt_slave);
+		else
+			p = &m->mnt_slave;
+	}
+	if (p)
+		hlist_splice_init(&mnt->mnt_slave_list, p, &to->mnt_slave_list);
+}
+
+/*
+ * EXCL[namespace_sem]
+ */
+void change_mnt_propagation(struct mount *mnt, int type)
+{
+	struct mount *m = mnt->mnt_master;
+
+	if (type == MS_SHARED) {
+		set_mnt_shared(mnt);
+		return;
+	}
+	if (IS_MNT_SHARED(mnt)) {
+		if (list_empty(&mnt->mnt_share)) {
+			mnt_release_group_id(mnt);
+		} else {
+			m = next_peer(mnt);
+			list_del_init(&mnt->mnt_share);
+			mnt->mnt_group_id = 0;
+		}
+		CLEAR_MNT_SHARED(mnt);
+		transfer_propagation(mnt, m);
+	}
+	hlist_del_init(&mnt->mnt_slave);
+	if (type == MS_SLAVE) {
+		mnt->mnt_master = m;
+		if (m)
+			hlist_add_head(&mnt->mnt_slave, &m->mnt_slave_list);
+	} else {
+		mnt->mnt_master = NULL;
+		if (type == MS_UNBINDABLE)
+			mnt->mnt_t_flags |= T_UNBINDABLE;
+		else
+			mnt->mnt_t_flags &= ~T_UNBINDABLE;
+	}
+}
+
+static struct mount *trace_transfers(struct mount *m)
+{
+	while (1) {
+		struct mount *next = next_peer(m);
+
+		if (next != m) {
+			list_del_init(&m->mnt_share);
+			m->mnt_group_id = 0;
+			m->mnt_master = next;
+		} else {
+			if (IS_MNT_SHARED(m))
+				mnt_release_group_id(m);
+			next = m->mnt_master;
+		}
+		hlist_del_init(&m->mnt_slave);
+		CLEAR_MNT_SHARED(m);
+		SET_MNT_MARK(m);
+
+		if (!next || !will_be_unmounted(next))
+			return next;
+		if (IS_MNT_MARKED(next))
+			return next->mnt_master;
+		m = next;
+	}
+}
+
+static void set_destinations(struct mount *m, struct mount *master)
+{
+	struct mount *next;
+
+	while ((next = m->mnt_master) != master) {
+		m->mnt_master = master;
+		m = next;
+	}
+}
+
+void bulk_make_private(struct list_head *set)
+{
+	struct mount *m;
+
+	list_for_each_entry(m, set, mnt_list)
+		if (!IS_MNT_MARKED(m))
+			set_destinations(m, trace_transfers(m));
+
+	list_for_each_entry(m, set, mnt_list) {
+		transfer_propagation(m, m->mnt_master);
+		m->mnt_master = NULL;
+		CLEAR_MNT_MARK(m);
+	}
+}
+
+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)
+			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) && !hlist_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) && !hlist_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)
+				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;
+	}
+}
+
+static bool need_secondary(struct mount *m, struct mountpoint *dest_mp)
+{
+	/* skip ones added by this propagate_mnt() */
+	if (IS_MNT_NEW(m))
+		return false;
+	/* skip if mountpoint isn't visible in m */
+	if (!is_subdir(dest_mp->m_dentry, m->mnt.mnt_root))
+		return false;
+	/* skip if m is in the anon_ns */
+	if (is_anon_ns(m->mnt_ns))
+		return false;
+	return true;
+}
+
+static struct mount *find_master(struct mount *m,
+				struct mount *last_copy,
+				struct mount *original)
+{
+	struct mount *p;
+
+	// ascend until there's a copy for something with the same master
+	for (;;) {
+		p = m->mnt_master;
+		if (!p || IS_MNT_MARKED(p))
+			break;
+		m = p;
+	}
+	while (!peers(last_copy, original)) {
+		struct mount *parent = last_copy->mnt_parent;
+		if (parent->mnt_master == p) {
+			if (!peers(parent, m))
+				last_copy = last_copy->mnt_master;
+			break;
+		}
+		last_copy = last_copy->mnt_master;
+	}
+	return last_copy;
+}
+
+/**
+ * propagate_mnt() - create secondary copies for tree attachment
+ * @dest_mnt:    destination mount.
+ * @dest_mp:     destination mountpoint.
+ * @source_mnt:  source mount.
+ * @tree_list:   list of secondaries to be attached.
+ *
+ * Create secondary copies for attaching a tree with root @source_mnt
+ * at mount @dest_mnt with mountpoint @dest_mp.  Link all new mounts
+ * into a propagation graph.  Set mountpoints for all secondaries,
+ * link their roots into @tree_list via ->mnt_hash.
+ */
+int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
+		  struct mount *source_mnt, struct hlist_head *tree_list)
+{
+	struct mount *m, *n, *copy, *this;
+	int err = 0, type;
+
+	if (dest_mnt->mnt_master)
+		SET_MNT_MARK(dest_mnt->mnt_master);
+
+	/* iterate over peer groups, depth first */
+	for (m = dest_mnt; m && !err; m = next_group(m, dest_mnt)) {
+		if (m == dest_mnt) { // have one for dest_mnt itself
+			copy = source_mnt;
+			type = CL_MAKE_SHARED;
+			n = next_peer(m);
+			if (n == m)
+				continue;
+		} else {
+			type = CL_SLAVE;
+			/* beginning of peer group among the slaves? */
+			if (IS_MNT_SHARED(m))
+				type |= CL_MAKE_SHARED;
+			n = m;
+		}
+		do {
+			if (!need_secondary(n, dest_mp))
+				continue;
+			if (type & CL_SLAVE) // first in this peer group
+				copy = find_master(n, copy, source_mnt);
+			this = copy_tree(copy, copy->mnt.mnt_root, type);
+			if (IS_ERR(this)) {
+				err = PTR_ERR(this);
+				break;
+			}
+			scoped_guard(mount_locked_reader)
+				mnt_set_mountpoint(n, dest_mp, this);
+			if (n->mnt_master)
+				SET_MNT_MARK(n->mnt_master);
+			copy = this;
+			hlist_add_head(&this->mnt_hash, tree_list);
+			err = count_mounts(n->mnt_ns, this);
+			if (err)
+				break;
+			type = CL_MAKE_SHARED;
+		} while ((n = next_peer(n)) != m);
+	}
+
+	hlist_for_each_entry(n, tree_list, mnt_hash) {
+		m = n->mnt_parent;
+		if (m->mnt_master)
+			CLEAR_MNT_MARK(m->mnt_master);
+	}
+	if (dest_mnt->mnt_master)
+		CLEAR_MNT_MARK(dest_mnt->mnt_master);
+	return err;
+}
+
+/*
+ * 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 *parent = mnt->mnt_parent;
+
+	/*
+	 * 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;
+
+	if (mnt == parent)
+		return 0;
+
+	for (struct mount *m = propagation_next(parent, parent); m;
+	     		m = propagation_next(m, parent)) {
+		struct list_head *head;
+		struct mount *child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
+
+		if (!child)
+			continue;
+
+		head = &child->mnt_mounts;
+		if (!list_empty(head)) {
+			/*
+			 * a mount that covers child completely wouldn't prevent
+			 * it being pulled out; any other would.
+			 */
+			if (!list_is_singular(head) || !child->overmount)
+				continue;
+		}
+		if (do_refcount_check(child, 1))
+			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_t_flags & T_UMOUNT_CANDIDATE;
+}
+
+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);
+	list_add_tail(&m->mnt_list, to_umount);
+}
+
+static void remove_from_candidate_list(struct mount *m)
+{
+	m->mnt_t_flags &= ~(T_MARKED | T_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_t_flags |= T_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_t_flags |= T_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_t_flags &= ~T_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_t_flags &= ~T_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 surviving overmounts on those
+	list_for_each_entry(m, &to_umount, mnt_list) {
+		struct mount *over = m->overmount;
+		if (over && !will_be_unmounted(over))
+			reparent(over);
+	}
+
+	// and fold them into the set
+	list_splice_tail_init(&to_umount, set);
+}