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    SubjectRe: [RFC][PATCH 1/3] vfs: Keep a list of mounts on a mount point
    On Fri, Oct 04, 2013 at 03:42:37PM -0700, Eric W. Biederman wrote:
    >
    > Signed-off-by: Eric W. Biederman <ebiederman@twitter.com>
    > ---
    > fs/mount.h | 2 ++
    > fs/namespace.c | 5 +++++
    > 2 files changed, 7 insertions(+), 0 deletions(-)
    >
    > diff --git a/fs/mount.h b/fs/mount.h
    > index 64a858143ff9..e4342b8dfab1 100644
    > --- a/fs/mount.h
    > +++ b/fs/mount.h
    > @@ -21,6 +21,7 @@ struct mnt_pcp {
    > struct mountpoint {
    > struct list_head m_hash;
    > struct dentry *m_dentry;
    > + struct list_head m_list;
    > int m_count;
    > };
    >
    > @@ -47,6 +48,7 @@ struct mount {
    > struct mount *mnt_master; /* slave is on master->mnt_slave_list */
    > struct mnt_namespace *mnt_ns; /* containing namespace */
    > struct mountpoint *mnt_mp; /* where is it mounted */
    > + struct list_head mnt_mp_list; /* list mounts with the same mountpoint */
    > #ifdef CONFIG_FSNOTIFY
    > struct hlist_head mnt_fsnotify_marks;
    > __u32 mnt_fsnotify_mask;
    > diff --git a/fs/namespace.c b/fs/namespace.c
    > index da5c49483430..d092964fe7f9 100644
    > --- a/fs/namespace.c
    > +++ b/fs/namespace.c
    > @@ -197,6 +197,7 @@ static struct mount *alloc_vfsmnt(const char *name)
    > INIT_LIST_HEAD(&mnt->mnt_share);
    > INIT_LIST_HEAD(&mnt->mnt_slave_list);
    > INIT_LIST_HEAD(&mnt->mnt_slave);
    > + INIT_LIST_HEAD(&mnt->mnt_mp_list);
    > #ifdef CONFIG_FSNOTIFY
    > INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
    > #endif
    > @@ -636,6 +637,7 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry)
    > mp->m_dentry = dentry;
    > mp->m_count = 1;
    > list_add(&mp->m_hash, chain);
    > + INIT_LIST_HEAD(&mp->m_list);
    > return mp;
    > }
    >
    > @@ -691,6 +693,7 @@ static void detach_mnt(struct mount *mnt, struct path *old_path)
    > list_del_init(&mnt->mnt_hash);
    > put_mountpoint(mnt->mnt_mp);
    > mnt->mnt_mp = NULL;
    > + list_del_init(&mnt->mnt_mp_list);

    Should be done *before* put_mountpoint(), for obvious reasons.

    And a BUG_ON(!list_empty(&mp->m_list)) in put_mountpoint() for good measure (and
    no, WARN_ON() is not better here, since use-after-free is definitely worse than
    a BUG).

    > }
    >
    > /*
    > @@ -705,6 +708,7 @@ void mnt_set_mountpoint(struct mount *mnt,
    > child_mnt->mnt_mountpoint = dget(mp->m_dentry);
    > child_mnt->mnt_parent = mnt;
    > child_mnt->mnt_mp = mp;
    > + list_add_tail(&child_mnt->mnt_mp_list, &mp->m_list);
    > }
    >
    > /*
    > @@ -1193,6 +1197,7 @@ void umount_tree(struct mount *mnt, int propagate)
    > p->mnt_parent->mnt_ghosts++;
    > put_mountpoint(p->mnt_mp);
    > p->mnt_mp = NULL;
    > + list_del_init(&mnt->mnt_mp_list);

    Ditto.

    > }
    > change_mnt_propagation(p, MS_PRIVATE);
    > }
    > --
    > 1.7.5.4
    >


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