[PATCH 2/8] vfs - add path_is_mountpoint() helper

Ian Kent
2016-10-11T05:34:04Z

From: Ian Kent <ikent@redhat.com>

d_mountpoint() can only be used reliably to establish if a dentry is
not mounted in any namespace. It isn't aware of the possibility there
may be multiple mounts using a given dentry that may be in a different
namespace.

Add helper functions, path_is_mountpoint() and an rcu version , that
checks if a struct path is a mountpoint for this case.

Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Omar Sandoval <osandov@osandov.com>
---
fs/namespace.c | 43 +++++++++++++++++++++++++++++++++++++++++++
include/linux/fs.h | 2 ++
2 files changed, 45 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index ff1cd14..5ef9618 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1160,6 +1160,49 @@ struct vfsmount *mntget(struct vfsmount *mnt)
}
EXPORT_SYMBOL(mntget);

+static bool __path_is_mountpoint(struct path *path)
+{
+ struct mount *mount;
+ struct vfsmount *mnt;
+ unsigned seq;
+
+ do {
+ seq = read_seqbegin(&mount_lock);
+ mount = __lookup_mnt(path->mnt, path->dentry);
+ mnt = mount ? &mount->mnt : NULL;
+ } while (mnt &&
+ !(mnt->mnt_flags & MNT_SYNC_UMOUNT) &&
+ read_seqretry(&mount_lock, seq));
+
+ return mnt != NULL;
+}
+
+/* Check if path is a mount in current namespace */
+bool path_is_mountpoint(struct path *path)
+{
+ bool res;
+
+ if (!d_mountpoint(path->dentry))
+ return 0;
+
+ rcu_read_lock();
+ res = __path_is_mountpoint(path);
+ rcu_read_unlock();
+
+ return res;
+}
+EXPORT_SYMBOL(path_is_mountpoint);
+
+/* Check if path is a mount in current namespace */
+bool path_is_mountpoint_rcu(struct path *path)
+{
+ if (!d_mountpoint(path->dentry))
+ return 0;
+
+ return __path_is_mountpoint(path);
+}
+EXPORT_SYMBOL(path_is_mountpoint_rcu);
+
struct vfsmount *mnt_clone_internal(struct path *path)
{
struct mount *p;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 09a8c41..deaf08b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2142,6 +2142,8 @@ extern int vfs_ustat(dev_t, struct kstatfs *);
extern int freeze_super(struct super_block *super);
extern int thaw_super(struct super_block *super);
extern bool our_mnt(struct vfsmount *mnt);
+extern bool path_is_mountpoint(struct path *);
+extern bool path_is_mountpoint_rcu(struct path *);

extern int current_umask(void);