libfs: massage path_from_stashed() to allow custom stashing behavior

* Add a callback to struct stashed_operations so it's possible to
  implement custom behavior for pidfs and allow for it to return errors.

* Teach stashed_dentry_get() to handle error pointers.

Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-2-98f3456fd552@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/internal.h

@@ -322,12 +322,15 @@ struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns);
 struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap);
 void mnt_idmap_put(struct mnt_idmap *idmap);
 struct stashed_operations {
+	struct dentry *(*stash_dentry)(struct dentry **stashed,
+				       struct dentry *dentry);
 	void (*put_data)(void *data);
 	int (*init_inode)(struct inode *inode, void *data);
 };
 int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
 		      struct path *path);
 void stashed_dentry_prune(struct dentry *dentry);
+struct dentry *stash_dentry(struct dentry **stashed, struct dentry *dentry);
 struct dentry *stashed_dentry_get(struct dentry **stashed);
 /**
  * path_mounted - check whether path is mounted

fs/libfs.c

@@ -2128,6 +2128,8 @@ struct dentry *stashed_dentry_get(struct dentry **stashed)
 	dentry = rcu_dereference(*stashed);
 	if (!dentry)
 		return NULL;
+	if (IS_ERR(dentry))
+		return dentry;
 	if (!lockref_get_not_dead(&dentry->d_lockref))
 		return NULL;
 	return dentry;
@@ -2176,8 +2178,7 @@ static struct dentry *prepare_anon_dentry(struct dentry **stashed,
 	return dentry;
 }
 
-static struct dentry *stash_dentry(struct dentry **stashed,
-				   struct dentry *dentry)
+struct dentry *stash_dentry(struct dentry **stashed, struct dentry *dentry)
 {
 	guard(rcu)();
 	for (;;) {
@@ -2218,12 +2219,15 @@ static struct dentry *stash_dentry(struct dentry **stashed,
 int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
 		      struct path *path)
 {
-	struct dentry *dentry;
+	struct dentry *dentry, *res;
 	const struct stashed_operations *sops = mnt->mnt_sb->s_fs_info;
 
 	/* See if dentry can be reused. */
-	path->dentry = stashed_dentry_get(stashed);
-	if (path->dentry) {
+	res = stashed_dentry_get(stashed);
+	if (IS_ERR(res))
+		return PTR_ERR(res);
+	if (res) {
+		path->dentry = res;
 		sops->put_data(data);
 		goto out_path;
 	}
@@ -2234,8 +2238,17 @@ int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
 		return PTR_ERR(dentry);
 
 	/* Added a new dentry. @data is now owned by the filesystem. */
-	path->dentry = stash_dentry(stashed, dentry);
-	if (path->dentry != dentry)
+	if (sops->stash_dentry)
+		res = sops->stash_dentry(stashed, dentry);
+	else
+		res = stash_dentry(stashed, dentry);
+	if (IS_ERR(res)) {
+		dput(dentry);
+		return PTR_ERR(res);
+	}
+	path->dentry = res;
+	/* A dentry was reused. */
+	if (res != dentry)
 		dput(dentry);
 
 out_path: