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>

diff --git a/fs/internal.h b/fs/internal.h
index 393f6c5c24f6b87ede28ca72764e8d50e0972f65..22ba066d1dbaf7cb4b84222b9df85538df1fc67f 100644 (file)
--- a/fs/internal.h
+++ b/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

diff --git a/fs/libfs.c b/fs/libfs.c
index 9ea0ecc325a81c93a55e584824417789e8d1aea0..3541e22c87b5de52b410be96f158221201b61725 100644 (file)
--- a/fs/libfs.c
+++ b/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: