From: Miklos Szeredi <mszeredi@xxxxxxx>
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@xxxxxxx>
---
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
--
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