[PATCH 2/4] vfs: check unlinked ancestors before mount

[Miklos Szeredi <miklos@szeredi.hu>]

From: Miklos Szeredi <mszeredi@suse.cz>

We check submounts before doing d_drop() on a non-empty directory dentry in
NFS (have_submounts()).  But we do not exclude a racing mount.  Nor do we
prevent mounts to be added to the disconnected subtree using relative paths
after the d_drop().

This patch fixes these issues by checking for unlinked (unhashed, non-root)
ancestors before proceeding with the mount.  This is done after setting
DCACHE_MOUNTED on the soon-to-be mountpoint and with the rename seqlock
taken for write.  This ensures that the only one of have_submounts() or
has_unlinked_ancestor() can succeed.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---
 fs/dcache.c    | 28 ++++++++++++++++++++++++++++
 fs/internal.h  |  1 +
 fs/namespace.c |  9 +++++++++
 3 files changed, 38 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index ba429d9..eae7cc1 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1103,6 +1103,34 @@ rename_retry:
 }
 EXPORT_SYMBOL(have_submounts);
 
+static bool __has_unlinked_ancestor(struct dentry *dentry)
+{
+	struct dentry *this;
+
+	for (this = dentry; !IS_ROOT(this); this = this->d_parent) {
+		if (d_unhashed(this))
+			return true;
+	}
+	return false;
+}
+
+/*
+ * Called by mount code to check if the mountpoint is reachable (e.g. NFS can
+ * unhash a directory dentry and then the complete subtree can become
+ * unreachable).
+ */
+bool has_unlinked_ancestor(struct dentry *dentry)
+{
+	bool found;
+
+	/* Need exclusion wrt. check_submounts_and_drop() */
+	write_seqlock(&rename_lock);
+	found = __has_unlinked_ancestor(dentry);
+	write_sequnlock(&rename_lock);
+
+	return found;
+}
+
 /*
  * Search the dentry child list of the specified parent,
  * and move any unused dentries to the end of the unused
diff --git a/fs/internal.h b/fs/internal.h
index 7c5f01c..d232355 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -126,6 +126,7 @@ extern int invalidate_inodes(struct super_block *, bool);
  * dcache.c
  */
 extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
+extern bool has_unlinked_ancestor(struct dentry *dentry);
 
 /*
  * read_write.c
diff --git a/fs/namespace.c b/fs/namespace.c
index 7b1ca9b..bb92a9c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -634,6 +634,15 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry)
 	}
 	dentry->d_flags |= DCACHE_MOUNTED;
 	spin_unlock(&dentry->d_lock);
+
+	if (has_unlinked_ancestor(dentry)) {
+		spin_lock(&dentry->d_lock);
+		dentry->d_flags &= ~DCACHE_MOUNTED;
+		spin_unlock(&dentry->d_lock);
+		kfree(mp);
+		return ERR_PTR(-ENOENT);
+	}
+
 	mp->m_dentry = dentry;
 	mp->m_count = 1;
 	list_add(&mp->m_hash, chain);
-- 
1.8.1.4