vfs: add path_is_mountpoint() helper

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(), that checks if a struct path
is a mountpoint for this case.

Link: http://lkml.kernel.org/r/20161011053358.27645.9729.stgit@pluto.themaw.net
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/mount.h b/fs/mount.h
index d2e25d7b64b3073c28f37a339b31ac75d275c6f4..2c856fc47ae309ed88c3ca59302cb1ba660142f2 100644 (file)
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -94,6 +94,12 @@ extern struct mount *__lookup_mnt_last(struct vfsmount *, struct dentry *);
 extern int __legitimize_mnt(struct vfsmount *, unsigned);
 extern bool legitimize_mnt(struct vfsmount *, unsigned);
 
+static inline bool __path_is_mountpoint(const struct path *path)
+{
+       struct mount *m = __lookup_mnt(path->mnt, path->dentry);
+       return m && likely(!(m->mnt.mnt_flags & MNT_SYNC_UMOUNT));
+}
+
 extern void __detach_mounts(struct dentry *dentry);
 
 static inline void detach_mounts(struct dentry *dentry)

diff --git a/fs/namespace.c b/fs/namespace.c
index e6c234b1a6456c1364bcc06b029f3cc4a155d5bd..7a73383e836574059d3503a2509633504ca7ed6e 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1159,6 +1159,35 @@ struct vfsmount *mntget(struct vfsmount *mnt)
 }
 EXPORT_SYMBOL(mntget);
 
+/* path_is_mountpoint() - Check if path is a mount in the current
+ *                          namespace.
+ *
+ *  d_mountpoint() can only be used reliably to establish if a dentry is
+ *  not mounted in any namespace and that common case is handled inline.
+ *  d_mountpoint() isn't aware of the possibility there may be multiple
+ *  mounts using a given dentry in a different namespace. This function
+ *  checks if the passed in path is a mountpoint rather than the dentry
+ *  alone.
+ */
+bool path_is_mountpoint(const struct path *path)
+{
+       unsigned seq;
+       bool res;
+
+       if (!d_mountpoint(path->dentry))
+               return false;
+
+       rcu_read_lock();
+       do {
+               seq = read_seqbegin(&mount_lock);
+               res = __path_is_mountpoint(path);
+       } while (read_seqretry(&mount_lock, seq));
+       rcu_read_unlock();
+
+       return res;
+}
+EXPORT_SYMBOL(path_is_mountpoint);
+
 struct vfsmount *mnt_clone_internal(struct path *path)
 {
        struct mount *p;

diff --git a/include/linux/mount.h b/include/linux/mount.h
index 1172cce949a4c36226fea3b1c64c66d6b01c313e..5b6dd004bfdcb347f326636c16e96d1776c7243a 100644 (file)
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -98,4 +98,6 @@ extern dev_t name_to_dev_t(const char *name);
 
 extern unsigned int sysctl_mount_max;
 
+extern bool path_is_mountpoint(const struct path *path);
+
 #endif /* _LINUX_MOUNT_H */