[PATCH 13/19] VFS: lock dentry for ->revalidate to avoid races with rename etc

[NeilBrown <neilb@xxxxxxx>]

When we call ->revalidate we want to be sure we are revalidating the
expected name.  As a shared lock on i_rwsem no longer prevents renames
we need to lock the dentry and ensure it still has the expected name.

So pass parent name to d_revalidate() and be prepared to retry the
lookup if it returns -EAGAIN.

Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
 fs/namei.c | 49 ++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 11 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 145ae07f9b8c..3a107d6098be 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -957,12 +957,24 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry)
 }
 
 static inline int d_revalidate(struct inode *dir, const struct qstr *name,
-			       struct dentry *dentry, unsigned int flags)
+			       struct dentry *dentry, unsigned int flags,
+			       struct dentry *base, const struct qstr *last)
 {
-	if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE))
-		return dentry->d_op->d_revalidate(dir, name, dentry, flags);
-	else
+	int status;
+
+	if (!unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE))
 		return 1;
+
+	if (dentry->d_flags & LOOKUP_RCU) {
+		if (!d_update_trylock(dentry, base, last))
+			return -ECHILD;
+	} else {
+		if (!d_update_lock(dentry, base, last, I_MUTEX_NORMAL))
+			return -EAGAIN;
+	}
+	status = dentry->d_op->d_revalidate(dir, name, dentry, flags);
+	d_update_unlock(dentry);
+	return status;
 }
 
 /**
@@ -1686,13 +1698,18 @@ static struct dentry *lookup_dcache(const struct qstr *name,
 				    struct dentry *dir,
 				    unsigned int flags)
 {
-	struct dentry *dentry = d_lookup(dir, name);
+	struct dentry *dentry;
+again:
+	dentry = d_lookup(dir, name);
 	if (dentry) {
-		int error = d_revalidate(dir->d_inode, name, dentry, flags);
+		int error = d_revalidate(dir->d_inode, name, dentry, flags, dir, name);
 		if (unlikely(error <= 0)) {
 			if (!error)
 				d_invalidate(dentry);
 			dput(dentry);
+			if (error == -EAGAIN)
+				/* raced with rename etc */
+				goto again;
 			return ERR_PTR(error);
 		}
 	}
@@ -1915,6 +1932,7 @@ static struct dentry *lookup_fast(struct nameidata *nd)
 	 * of a false negative due to a concurrent rename, the caller is
 	 * going to fall back to non-racy lookup.
 	 */
+again:
 	if (nd->flags & LOOKUP_RCU) {
 		dentry = __d_lookup_rcu(parent, &nd->last, &nd->next_seq);
 		if (unlikely(!dentry)) {
@@ -1930,7 +1948,7 @@ static struct dentry *lookup_fast(struct nameidata *nd)
 		if (read_seqcount_retry(&parent->d_seq, nd->seq))
 			return ERR_PTR(-ECHILD);
 
-		status = d_revalidate(nd->inode, &nd->last, dentry, nd->flags);
+		status = d_revalidate(nd->inode, &nd->last, dentry, nd->flags, parent, &nd->last);
 		if (likely(status > 0))
 			return dentry;
 		if (!try_to_unlazy_next(nd, dentry))
@@ -1938,17 +1956,19 @@ static struct dentry *lookup_fast(struct nameidata *nd)
 		if (status == -ECHILD)
 			/* we'd been told to redo it in non-rcu mode */
 			status = d_revalidate(nd->inode, &nd->last,
-					      dentry, nd->flags);
+					      dentry, nd->flags, parent, &nd->last);
 	} else {
 		dentry = __d_lookup(parent, &nd->last);
 		if (unlikely(!dentry))
 			return NULL;
-		status = d_revalidate(nd->inode, &nd->last, dentry, nd->flags);
+		status = d_revalidate(nd->inode, &nd->last, dentry, nd->flags, parent, &nd->last);
 	}
 	if (unlikely(status <= 0)) {
 		if (!status)
 			d_invalidate(dentry);
 		dput(dentry);
+		if (status == -EAGAIN)
+			goto again;
 		return ERR_PTR(status);
 	}
 	return dentry;
@@ -1970,7 +1990,7 @@ static struct dentry *__lookup_slow(const struct qstr *name,
 	if (IS_ERR(dentry))
 		return dentry;
 	if (unlikely(!d_in_lookup(dentry))) {
-		int error = d_revalidate(inode, name, dentry, flags);
+		int error = d_revalidate(inode, name, dentry, flags, dir, name);
 		if (unlikely(error <= 0)) {
 			if (!error) {
 				d_invalidate(dentry);
@@ -1978,6 +1998,8 @@ static struct dentry *__lookup_slow(const struct qstr *name,
 				goto again;
 			}
 			dput(dentry);
+			if (error == -EAGAIN)
+				goto again;
 			dentry = ERR_PTR(error);
 		}
 	} else {
@@ -3777,6 +3799,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 		return ERR_PTR(-ENOENT);
 
 	file->f_mode &= ~FMODE_CREATED;
+again:
 	dentry = d_lookup(dir, &nd->last);
 	for (;;) {
 		if (!dentry) {
@@ -3787,9 +3810,13 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 		if (d_in_lookup(dentry))
 			break;
 
-		error = d_revalidate(dir_inode, &nd->last, dentry, nd->flags);
+		error = d_revalidate(dir_inode, &nd->last, dentry, nd->flags, dir, &nd->last);
 		if (likely(error > 0))
 			break;
+		if (error == -EAGAIN) {
+			dput(dentry);
+			goto again;
+		}
 		if (error)
 			goto out_dput;
 		d_invalidate(dentry);
-- 
2.47.1