This one actually has a slight chance of working.
Thanks,
Miklos
---
Subject: vfs: check unlinked ancestors before mount
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
check_submounts_and_drop() or has_unlinked_ancestor() can succeed.
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
CC: David Howells <dhowells@xxxxxxxxxx>
CC: Steven Whitehouse <swhiteho@xxxxxxxxxx>
CC: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/dcache.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
fs/internal.h | 1 +
fs/namespace.c | 11 +++++------
3 files changed, 58 insertions(+), 6 deletions(-)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1159,6 +1159,58 @@ int have_submounts(struct dentry *parent
}
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) {
+ int is_unhashed;
+
+ /* Need exclusion wrt. check_submounts_and_drop() */
+ spin_lock(&this->d_lock);
+ is_unhashed = d_unhashed(this);
+ spin_unlock(&this->d_lock);
+
+ if (is_unhashed)
+ return true;
+ }
+ return false;
+}
+
+/*
+ * Called by mount code to set a mountpoint and check if the mountpoint is
+ * reachable (e.g. NFS can unhash a directory dentry and then the complete
+ * subtree can become unreachable).
+ *
+ * Only one of check_submounts_and_drop() and d_set_mounted() must succeed. For
+ * this reason take rename_lock and d_lock on dentry and ancestors.
+ */
+int d_set_mounted(struct dentry *dentry)
+{
+ int ret = -ENOENT;
+
+ write_seqlock(&rename_lock);
+ spin_lock(&dentry->d_lock);
+ if (d_unlinked(dentry)) {
+ spin_unlock(&dentry->d_lock);
+ goto out;
+ }
+
+ dentry->d_flags |= DCACHE_MOUNTED;
+ spin_unlock(&dentry->d_lock);
+
+ if (__has_unlinked_ancestor(dentry->d_parent)) {
+ spin_lock(&dentry->d_lock);
+ dentry->d_flags &= ~DCACHE_MOUNTED;
+ spin_unlock(&dentry->d_lock);
+ goto out;
+ }
+ ret = 0;
+out:
+ write_sequnlock(&rename_lock);
+ return ret;
+}
+
/*
* Search the dentry child list of the specified parent,
* and move any unused dentries to the end of the unused
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -126,6 +126,7 @@ extern int invalidate_inodes(struct supe
* dcache.c
*/
extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
+extern int d_set_mounted(struct dentry *dentry);
/*
* read_write.c
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -611,6 +611,7 @@ static struct mountpoint *new_mountpoint
{
struct list_head *chain = mountpoint_hashtable + hash(NULL, dentry);
struct mountpoint *mp;
+ int ret;
list_for_each_entry(mp, chain, m_hash) {
if (mp->m_dentry == dentry) {
@@ -626,14 +627,12 @@ static struct mountpoint *new_mountpoint
if (!mp)
return ERR_PTR(-ENOMEM);
- spin_lock(&dentry->d_lock);
- if (d_unlinked(dentry)) {
- spin_unlock(&dentry->d_lock);
+ ret = d_set_mounted(dentry);
+ if (ret) {
kfree(mp);
- return ERR_PTR(-ENOENT);
+ return ERR_PTR(ret);
}
- dentry->d_flags |= DCACHE_MOUNTED;
- spin_unlock(&dentry->d_lock);
+
mp->m_dentry = dentry;
mp->m_count = 1;
list_add(&mp->m_hash, chain);
--
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