Andrei Vagin <avagin@xxxxxxxxxxxxx> writes:
> Hi Eric,
>
> I found one issue about this patch. Take a look at the next script. The
> idea is simple, we call mount twice and than we call umount twice, but
> the second umount fails.
Definitely an odd. I will take a look.
Eric
>
> [root@fc24 ~]# cat m.sh
> #!/bin/sh
>
> set -x -e
> mount -t tmpfs xxx /mnt
> mkdir -p /mnt/1
> mkdir -p /mnt/2
> mount --bind /mnt/1 /mnt/1
> mount --make-shared /mnt/1
> mount --bind /mnt/1 /mnt/2
> mkdir -p /mnt/1/1
> for i in `seq 2`; do
> mount --bind /mnt/1/1 /mnt/1/1
> done
> for i in `seq 2`; do
> umount /mnt/1/1 || {
> cat /proc/self/mountinfo | grep xxx
> exit 1
> }
> done
>
> [root@fc24 ~]# unshare -Urm ./m.sh
> + mount -t tmpfs xxx /mnt
> + mkdir -p /mnt/1
> + mkdir -p /mnt/2
> + mount --bind /mnt/1 /mnt/1
> + mount --make-shared /mnt/1
> + mount --bind /mnt/1 /mnt/2
> + mkdir -p /mnt/1/1
> ++ seq 2
> + for i in '`seq 2`'
> + mount --bind /mnt/1/1 /mnt/1/1
> + for i in '`seq 2`'
> + mount --bind /mnt/1/1 /mnt/1/1
> ++ seq 2
> + for i in '`seq 2`'
> + umount /mnt/1/1
> + for i in '`seq 2`'
> + umount /mnt/1/1
> umount: /mnt/1/1: not mounted
> + cat /proc/self/mountinfo
> + grep xxx
> 147 116 0:42 / /mnt rw,relatime - tmpfs xxx rw
> 148 147 0:42 /1 /mnt/1 rw,relatime shared:65 - tmpfs xxx rw
> 149 147 0:42 /1 /mnt/2 rw,relatime shared:65 - tmpfs xxx rw
> 157 149 0:42 /1/1 /mnt/2/1 rw,relatime shared:65 - tmpfs xxx rw
> + exit 1
>
> And you can see that /mnt/2 contains a mount, but it should not.
>
> Thanks,
> Andrei
>
> On Thu, Jan 05, 2017 at 10:04:14AM +1300, Eric W. Biederman wrote:
>>
>> Ever since mount propagation was introduced in cases where a mount in
>> propagated to parent mount mountpoint pair that is already in use the
>> code has placed the new mount behind the old mount in the mount hash
>> table.
>>
>> This implementation detail is problematic as it allows creating
>> arbitrary length mount hash chains.
>>
>> Furthermore it invalidates the constraint maintained elsewhere in the
>> mount code that a parent mount and a mountpoint pair will have exactly
>> one mount upon them. Making it hard to deal with and to talk about
>> this special case in the mount code.
>>
>> Modify mount propagation to notice when there is already a mount at
>> the parent mount and mountpoint where a new mount is propagating to
>> and place that preexisting mount on top of the new mount.
>>
>> Modify unmount propagation to notice when a mount that is being
>> unmounted has another mount on top of it (and no other children), and
>> to replace the unmounted mount with the mount on top of it.
>>
>> Move the MNT_UMUONT test from __lookup_mnt_last into
>> __propagate_umount as that is the only call of __lookup_mnt_last where
>> MNT_UMOUNT may be set on any mount visible in the mount hash table.
>>
>> These modifications allow:
>> - __lookup_mnt_last to be removed.
>> - attach_shadows to be renamed __attach_mnt and the it's shadow
>> handling to be removed.
>> - commit_tree to be simplified
>> - copy_tree to be simplified
>>
>> The result is an easier to understand tree of mounts that does not
>> allow creation of arbitrary length hash chains in the mount hash table.
>>
>> v2: Updated to mnt_change_mountpoint to not call dput or mntput
>> and instead to decrement the counts directly. It is guaranteed
>> that there will be other references when mnt_change_mountpoint is
>> called so this is safe.
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Fixes: b90fa9ae8f51 ("[PATCH] shared mount handling: bind and rbind")
>> Tested-by: Andrei Vagin <avagin@xxxxxxxxxxxxx>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>
>> Since the last version some of you may have seen I have modified
>> my implementation of mnt_change_mountpoint so that it no longer calls
>> mntput or dput but instead relies on the knowledge that it can not
>> possibly have the last reference to the mnt and dentry of interest.
>> This avoids code checking tools from complaining bitterly.
>>
>> This is on top of my previous patch that sorts out locking of the
>> mountpoint hash table. After time giving ample time for review I intend
>> to push this and the previous bug fix to Linus.
>>
>> fs/mount.h | 1 -
>> fs/namespace.c | 110 +++++++++++++++++++++++++++++++--------------------------
>> fs/pnode.c | 27 ++++++++++----
>> fs/pnode.h | 2 ++
>> 4 files changed, 82 insertions(+), 58 deletions(-)
>>
>> diff --git a/fs/mount.h b/fs/mount.h
>> index 2c856fc47ae3..2826543a131d 100644
>> --- a/fs/mount.h
>> +++ b/fs/mount.h
>> @@ -89,7 +89,6 @@ static inline int is_mounted(struct vfsmount *mnt)
>> }
>>
>> extern struct mount *__lookup_mnt(struct vfsmount *, struct dentry *);
>> -extern struct mount *__lookup_mnt_last(struct vfsmount *, struct dentry *);
>>
>> extern int __legitimize_mnt(struct vfsmount *, unsigned);
>> extern bool legitimize_mnt(struct vfsmount *, unsigned);
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 487ba30bb5c6..91ccfb73f0e0 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -637,28 +637,6 @@ struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
>> }
>>
>> /*
>> - * find the last mount at @dentry on vfsmount @mnt.
>> - * mount_lock must be held.
>> - */
>> -struct mount *__lookup_mnt_last(struct vfsmount *mnt, struct dentry *dentry)
>> -{
>> - struct mount *p, *res = NULL;
>> - p = __lookup_mnt(mnt, dentry);
>> - if (!p)
>> - goto out;
>> - if (!(p->mnt.mnt_flags & MNT_UMOUNT))
>> - res = p;
>> - hlist_for_each_entry_continue(p, mnt_hash) {
>> - if (&p->mnt_parent->mnt != mnt || p->mnt_mountpoint != dentry)
>> - break;
>> - if (!(p->mnt.mnt_flags & MNT_UMOUNT))
>> - res = p;
>> - }
>> -out:
>> - return res;
>> -}
>> -
>> -/*
>> * lookup_mnt - Return the first child mount mounted at path
>> *
>> * "First" means first mounted chronologically. If you create the
>> @@ -878,6 +856,13 @@ void mnt_set_mountpoint(struct mount *mnt,
>> hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);
>> }
>>
>> +static void __attach_mnt(struct mount *mnt, struct mount *parent)
>> +{
>> + hlist_add_head_rcu(&mnt->mnt_hash,
>> + m_hash(&parent->mnt, mnt->mnt_mountpoint));
>> + list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
>> +}
>> +
>> /*
>> * vfsmount lock must be held for write
>> */
>> @@ -886,28 +871,45 @@ static void attach_mnt(struct mount *mnt,
>> struct mountpoint *mp)
>> {
>> mnt_set_mountpoint(parent, mp, mnt);
>> - hlist_add_head_rcu(&mnt->mnt_hash, m_hash(&parent->mnt, mp->m_dentry));
>> - list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
>> + __attach_mnt(mnt, parent);
>> }
>>
>> -static void attach_shadowed(struct mount *mnt,
>> - struct mount *parent,
>> - struct mount *shadows)
>> +void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct mount *mnt)
>> {
>> - if (shadows) {
>> - hlist_add_behind_rcu(&mnt->mnt_hash, &shadows->mnt_hash);
>> - list_add(&mnt->mnt_child, &shadows->mnt_child);
>> - } else {
>> - hlist_add_head_rcu(&mnt->mnt_hash,
>> - m_hash(&parent->mnt, mnt->mnt_mountpoint));
>> - list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
>> - }
>> + struct mountpoint *old_mp = mnt->mnt_mp;
>> + struct dentry *old_mountpoint = mnt->mnt_mountpoint;
>> + struct mount *old_parent = mnt->mnt_parent;
>> +
>> + list_del_init(&mnt->mnt_child);
>> + hlist_del_init(&mnt->mnt_mp_list);
>> + hlist_del_init_rcu(&mnt->mnt_hash);
>> +
>> + attach_mnt(mnt, parent, mp);
>> +
>> + put_mountpoint(old_mp);
>> +
>> + /*
>> + * Safely avoid even the suggestion this code might sleep or
>> + * lock the mount hash by taking avantage of the knowlege that
>> + * mnt_change_mounpoint will not release the final reference
>> + * to a mountpoint.
>> + *
>> + * During mounting, another mount will continue to use the old
>> + * mountpoint and during unmounting, the old mountpoint will
>> + * continue to exist until namespace_unlock which happens well
>> + * after mnt_change_mountpoint.
>> + */
>> + spin_lock(&old_mountpoint->d_lock);
>> + old_mountpoint->d_lockref.count--;
>> + spin_unlock(&old_mountpoint->d_lock);
>> +
>> + mnt_add_count(old_parent, -1);
>> }
>>
>> /*
>> * vfsmount lock must be held for write
>> */
>> -static void commit_tree(struct mount *mnt, struct mount *shadows)
>> +static void commit_tree(struct mount *mnt)
>> {
>> struct mount *parent = mnt->mnt_parent;
>> struct mount *m;
>> @@ -925,7 +927,7 @@ static void commit_tree(struct mount *mnt, struct mount *shadows)
>> n->mounts += n->pending_mounts;
>> n->pending_mounts = 0;
>>
>> - attach_shadowed(mnt, parent, shadows);
>> + __attach_mnt(mnt, parent);
>> touch_mnt_namespace(n);
>> }
>>
>> @@ -1764,7 +1766,6 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
>> continue;
>>
>> for (s = r; s; s = next_mnt(s, r)) {
>> - struct mount *t = NULL;
>> if (!(flag & CL_COPY_UNBINDABLE) &&
>> IS_MNT_UNBINDABLE(s)) {
>> s = skip_mnt_tree(s);
>> @@ -1786,14 +1787,7 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
>> goto out;
>> lock_mount_hash();
>> list_add_tail(&q->mnt_list, &res->mnt_list);
>> - mnt_set_mountpoint(parent, p->mnt_mp, q);
>> - if (!list_empty(&parent->mnt_mounts)) {
>> - t = list_last_entry(&parent->mnt_mounts,
>> - struct mount, mnt_child);
>> - if (t->mnt_mp != p->mnt_mp)
>> - t = NULL;
>> - }
>> - attach_shadowed(q, parent, t);
>> + attach_mnt(q, parent, p->mnt_mp);
>> unlock_mount_hash();
>> }
>> }
>> @@ -1992,10 +1986,18 @@ static int attach_recursive_mnt(struct mount *source_mnt,
>> {
>> HLIST_HEAD(tree_list);
>> struct mnt_namespace *ns = dest_mnt->mnt_ns;
>> + struct mountpoint *smp;
>> struct mount *child, *p;
>> struct hlist_node *n;
>> int err;
>>
>> + /* Preallocate a mountpoint in case the new mounts need
>> + * to be tucked under other mounts.
>> + */
>> + smp = get_mountpoint(source_mnt->mnt.mnt_root);
>> + if (IS_ERR(smp))
>> + return PTR_ERR(smp);
>> +
>> /* Is there space to add these mounts to the mount namespace? */
>> if (!parent_path) {
>> err = count_mounts(ns, source_mnt);
>> @@ -2022,17 +2024,22 @@ static int attach_recursive_mnt(struct mount *source_mnt,
>> touch_mnt_namespace(source_mnt->mnt_ns);
>> } else {
>> mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
>> - commit_tree(source_mnt, NULL);
>> + commit_tree(source_mnt);
>> }
>>
>> hlist_for_each_entry_safe(child, n, &tree_list, mnt_hash) {
>> - struct mount *q;
>> hlist_del_init(&child->mnt_hash);
>> - q = __lookup_mnt_last(&child->mnt_parent->mnt,
>> - child->mnt_mountpoint);
>> - commit_tree(child, q);
>> + if (child->mnt.mnt_root == smp->m_dentry) {
>> + struct mount *q;
>> + q = __lookup_mnt(&child->mnt_parent->mnt,
>> + child->mnt_mountpoint);
>> + if (q)
>> + mnt_change_mountpoint(child, smp, q);
>> + }
>> + commit_tree(child);
>> }
>> unlock_mount_hash();
>> + put_mountpoint(smp);
>>
>> return 0;
>>
>> @@ -2046,6 +2053,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
>> cleanup_group_ids(source_mnt, NULL);
>> out:
>> ns->pending_mounts = 0;
>> + put_mountpoint(smp);
>> return err;
>> }
>>
>> diff --git a/fs/pnode.c b/fs/pnode.c
>> index 06a793f4ae38..eb4331240fd1 100644
>> --- a/fs/pnode.c
>> +++ b/fs/pnode.c
>> @@ -327,6 +327,9 @@ int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
>> */
>> static inline int do_refcount_check(struct mount *mnt, int count)
>> {
>> + struct mount *topper = __lookup_mnt(&mnt->mnt, mnt->mnt.mnt_root);
>> + if (topper)
>> + count++;
>> return mnt_get_count(mnt) > count;
>> }
>>
>> @@ -359,7 +362,7 @@ int propagate_mount_busy(struct mount *mnt, int refcnt)
>>
>> for (m = propagation_next(parent, parent); m;
>> m = propagation_next(m, parent)) {
>> - child = __lookup_mnt_last(&m->mnt, mnt->mnt_mountpoint);
>> + child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
>> if (child && list_empty(&child->mnt_mounts) &&
>> (ret = do_refcount_check(child, 1)))
>> break;
>> @@ -381,7 +384,7 @@ void propagate_mount_unlock(struct mount *mnt)
>>
>> for (m = propagation_next(parent, parent); m;
>> m = propagation_next(m, parent)) {
>> - child = __lookup_mnt_last(&m->mnt, mnt->mnt_mountpoint);
>> + child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
>> if (child)
>> child->mnt.mnt_flags &= ~MNT_LOCKED;
>> }
>> @@ -399,9 +402,11 @@ static void mark_umount_candidates(struct mount *mnt)
>>
>> for (m = propagation_next(parent, parent); m;
>> m = propagation_next(m, parent)) {
>> - struct mount *child = __lookup_mnt_last(&m->mnt,
>> + struct mount *child = __lookup_mnt(&m->mnt,
>> mnt->mnt_mountpoint);
>> - if (child && (!IS_MNT_LOCKED(child) || IS_MNT_MARKED(m))) {
>> + if (!child || (child->mnt.mnt_flags & MNT_UMOUNT))
>> + continue;
>> + if (!IS_MNT_LOCKED(child) || IS_MNT_MARKED(m)) {
>> SET_MNT_MARK(child);
>> }
>> }
>> @@ -420,8 +425,8 @@ static void __propagate_umount(struct mount *mnt)
>>
>> for (m = propagation_next(parent, parent); m;
>> m = propagation_next(m, parent)) {
>> -
>> - struct mount *child = __lookup_mnt_last(&m->mnt,
>> + struct mount *topper;
>> + struct mount *child = __lookup_mnt(&m->mnt,
>> mnt->mnt_mountpoint);
>> /*
>> * umount the child only if the child has no children
>> @@ -430,6 +435,16 @@ static void __propagate_umount(struct mount *mnt)
>> if (!child || !IS_MNT_MARKED(child))
>> continue;
>> CLEAR_MNT_MARK(child);
>> +
>> + /* If there is exactly one mount covering all of child
>> + * replace child with that mount.
>> + */
>> + topper = __lookup_mnt(&child->mnt, child->mnt.mnt_root);
>> + if (topper &&
>> + (child->mnt_mounts.next == &topper->mnt_child) &&
>> + (topper->mnt_child.next == &child->mnt_mounts))
>> + mnt_change_mountpoint(child->mnt_parent, child->mnt_mp, topper);
>> +
>> if (list_empty(&child->mnt_mounts)) {
>> list_del_init(&child->mnt_child);
>> child->mnt.mnt_flags |= MNT_UMOUNT;
>> diff --git a/fs/pnode.h b/fs/pnode.h
>> index 550f5a8b4fcf..dc87e65becd2 100644
>> --- a/fs/pnode.h
>> +++ b/fs/pnode.h
>> @@ -49,6 +49,8 @@ int get_dominating_id(struct mount *mnt, const struct path *root);
>> unsigned int mnt_get_count(struct mount *mnt);
>> void mnt_set_mountpoint(struct mount *, struct mountpoint *,
>> struct mount *);
>> +void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp,
>> + struct mount *mnt);
>> struct mount *copy_tree(struct mount *, struct dentry *, int);
>> bool is_path_reachable(struct mount *, struct dentry *,
>> const struct path *root);
>> --
>> 2.10.1
>>
