[PATCH 08/13] ovl: Simplify ovl_lookup_real_one()

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

The primary purpose of this patch is to remove the locking from
ovl_lookup_real_one() as part of centralising all locking of directories
for name operations.

The locking here isn't needed.  By performing consistency tests after
the lookup we can be sure that the result of the lookup was valid at
least for a moment, which is all the original code promised.

lookup_noperm_unlocked() is used for the lookup and it will take the
lock if needed only where it is needed.

Also:
 - don't take a reference to real->d_parent.  The parent is
   only use for a pointer comparison, and no reference is needed for
   that.
 - Several "if" statements have a "goto" followed by "else" - the
   else isn't needed: the following statement can directly follow
   the "if" as a new statement
 - Use a consistent pattern of setting "err" before performing a test
   and possibly going to "fail".
 - remove the "out" label (now that we don't need to dput(parent) or
   unlock) and simply return from fail:.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/overlayfs/export.c | 61 ++++++++++++++++++-------------------------
 1 file changed, 26 insertions(+), 35 deletions(-)

diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
index 83f80fdb1567..dcd28ffc4705 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -359,59 +359,50 @@ static struct dentry *ovl_lookup_real_one(struct dentry *connected,
 					  struct dentry *real,
 					  const struct ovl_layer *layer)
 {
-	struct inode *dir = d_inode(connected);
-	struct dentry *this, *parent = NULL;
+	struct dentry *this;
 	struct name_snapshot name;
 	int err;
 
 	/*
-	 * Lookup child overlay dentry by real name. The dir mutex protects us
-	 * from racing with overlay rename. If the overlay dentry that is above
-	 * real has already been moved to a parent that is not under the
-	 * connected overlay dir, we return -ECHILD and restart the lookup of
-	 * connected real path from the top.
+	 * @connected is a directory in the overlay and @real is an object
+	 * on @layer which is expected to be a child of @connected.
+	 * The goal is to return a dentry from the overlay which corresponds
+	 * to @real.  This is done by looking up the name from @real in
+	 * @connected and checking that the result meets expectations.
+	 *
+	 * Return %-ECHILD if the parent of @real no-longer corresponds to
+	 * @connected, and %-ESTALE if the dentry found by lookup doesn't
+	 * correspond to @real.
 	 */
-	inode_lock_nested(dir, I_MUTEX_PARENT);
-	err = -ECHILD;
-	parent = dget_parent(real);
-	if (ovl_dentry_real_at(connected, layer->idx) != parent)
-		goto fail;
 
-	/*
-	 * We also need to take a snapshot of real dentry name to protect us
-	 * from racing with underlying layer rename. In this case, we don't
-	 * care about returning ESTALE, only from dereferencing a free name
-	 * pointer because we hold no lock on the real dentry.
-	 */
 	take_dentry_name_snapshot(&name, real);
-	/*
-	 * No idmap handling here: it's an internal lookup.
-	 */
-	this = lookup_noperm(&name.name, connected);
+	this = lookup_noperm_unlocked(&name.name, connected);
 	release_dentry_name_snapshot(&name);
+
+	err = -ECHILD;
+	if (ovl_dentry_real_at(connected, layer->idx) != real->d_parent)
+		goto fail;
+
 	err = PTR_ERR(this);
-	if (IS_ERR(this)) {
+	if (IS_ERR(this))
 		goto fail;
-	} else if (!this || !this->d_inode) {
-		dput(this);
-		err = -ENOENT;
+
+	err = -ENOENT;
+	if (!this || !this->d_inode)
 		goto fail;
-	} else if (ovl_dentry_real_at(this, layer->idx) != real) {
-		dput(this);
-		err = -ESTALE;
+
+	err = -ESTALE;
+	if (ovl_dentry_real_at(this, layer->idx) != real)
 		goto fail;
-	}
 
-out:
-	dput(parent);
-	inode_unlock(dir);
 	return this;
 
 fail:
 	pr_warn_ratelimited("failed to lookup one by real (%pd2, layer=%d, connected=%pd2, err=%i)\n",
 			    real, layer->idx, connected, err);
-	this = ERR_PTR(err);
-	goto out;
+	if (!IS_ERR(this))
+		dput(this);
+	return ERR_PTR(err);
 }
 
 static struct dentry *ovl_lookup_real(struct super_block *sb,
-- 
2.50.0.107.gf914562f5916.dirty