[PATCH review 18/19] vfs: Handle mounts whose parents are unreachable from their mountpoint

["Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>]

- Add a mount flag MNT_UNREACHABLE_PARENT to mark mounts that can not
  reach their parent mount's root from their mountpoint.

- In follup_up and follow_up_rcu don't follow up if the current
  mount's mountpoint can not reach the parent mount's root.

- In prepend_path and it's callers in the d_path family don't follow
  to the parent mount if the current mount's mountpoint can not reach
  the parent mount's root.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/dcache.c           | 3 ++-
 fs/namei.c            | 4 ++--
 include/linux/mount.h | 1 +
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index e07eb03f6de6..cae4a42c1846 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2893,7 +2893,8 @@ restart:
 		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
 			struct mount *parent = ACCESS_ONCE(mnt->mnt_parent);
 			/* Global root? */
-			if (mnt != parent) {
+			if ((mnt != parent) &&
+			    !(mnt->mnt.mnt_flags & MNT_UNREACHABLE_PARENT)) {
 				dentry = ACCESS_ONCE(mnt->mnt_mountpoint);
 				mnt = parent;
 				vfsmnt = &mnt->mnt;
diff --git a/fs/namei.c b/fs/namei.c
index ec09c90089a9..31a9c9a5787d 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -938,7 +938,7 @@ static int follow_up_rcu(struct path *path)
 	struct dentry *mountpoint;
 
 	parent = mnt->mnt_parent;
-	if (&parent->mnt == path->mnt)
+	if ((mnt->mnt.mnt_flags & MNT_UNREACHABLE_PARENT) || (parent == mnt))
 		return 0;
 	mountpoint = mnt->mnt_mountpoint;
 	path->dentry = mountpoint;
@@ -964,7 +964,7 @@ int follow_up(struct path *path)
 
 	read_seqlock_excl(&mount_lock);
 	parent = mnt->mnt_parent;
-	if (parent == mnt) {
+	if ((mnt->mnt.mnt_flags & MNT_UNREACHABLE_PARENT) || (parent == mnt)) {
 		read_sequnlock_excl(&mount_lock);
 		return 0;
 	}
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 778d7cf65b9a..3249da1af130 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -63,6 +63,7 @@ struct mnt_namespace;
 #define MNT_MARKED		0x4000000
 #define MNT_UMOUNT		0x8000000
 #define MNT_VIOLATED		0x10000000
+#define MNT_UNREACHABLE_PARENT	0x20000000
 
 struct vfsmount {
 	struct dentry *mnt_root;	/* root of the mounted tree */
-- 
2.2.1

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