VFS/nfsd/cachefiles/ovl: introduce start_removing() and end_removing()

start_removing() is similar to start_creating() but will only return a
positive dentry with the expectation that it will be removed.  This is
used by nfsd, cachefiles, and overlayfs.  They are changed to also use
end_removing() to terminate the action begun by start_removing().  This
is a simple alias for end_dirop().

Apart from changes to the error paths, as we no longer need to unlock on
a lookup error, an effect on callers is that they don't need to test if
the found dentry is positive or negative - they can be sure it is
positive.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-6-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: neilbrown

fs/cachefiles/namei.c

@@ -260,6 +260,7 @@ static int cachefiles_unlink(struct cachefiles_cache *cache,
  * - File backed objects are unlinked
  * - Directory backed objects are stuffed into the graveyard for userspace to
  *   delete
+ * On entry dir must be locked.  It will be unlocked on exit.
  */
 int cachefiles_bury_object(struct cachefiles_cache *cache,
 			   struct cachefiles_object *object,
@@ -274,28 +275,30 @@ int cachefiles_bury_object(struct cachefiles_cache *cache,
 
 	_enter(",'%pd','%pd'", dir, rep);
 
+	/* end_removing() will dput() @rep but we need to keep
+	 * a ref, so take one now.  This also stops the dentry
+	 * being negated when unlinked which we need.
+	 */
+	dget(rep);
+
 	if (rep->d_parent != dir) {
-		inode_unlock(d_inode(dir));
+		end_removing(rep);
 		_leave(" = -ESTALE");
 		return -ESTALE;
 	}
 
 	/* non-directories can just be unlinked */
 	if (!d_is_dir(rep)) {
-		dget(rep); /* Stop the dentry being negated if it's only pinned
-			    * by a file struct.
-			    */
 		ret = cachefiles_unlink(cache, object, dir, rep, why);
-		dput(rep);
+		end_removing(rep);
 
-		inode_unlock(d_inode(dir));
 		_leave(" = %d", ret);
 		return ret;
 	}
 
 	/* directories have to be moved to the graveyard */
 	_debug("move stale object to graveyard");
-	inode_unlock(d_inode(dir));
+	end_removing(rep);
 
 try_again:
 	/* first step is to make up a grave dentry in the graveyard */
@@ -749,26 +752,20 @@ static struct dentry *cachefiles_lookup_for_cull(struct cachefiles_cache *cache,
 	struct dentry *victim;
 	int ret = -ENOENT;
 
-	inode_lock_nested(d_inode(dir), I_MUTEX_PARENT);
+	victim = start_removing(&nop_mnt_idmap, dir, &QSTR(filename));
 
-	victim = lookup_one(&nop_mnt_idmap, &QSTR(filename), dir);
 	if (IS_ERR(victim))
 		goto lookup_error;
-	if (d_is_negative(victim))
-		goto lookup_put;
 	if (d_inode(victim)->i_flags & S_KERNEL_FILE)
 		goto lookup_busy;
 	return victim;
 
 lookup_busy:
 	ret = -EBUSY;
-lookup_put:
-	inode_unlock(d_inode(dir));
-	dput(victim);
+	end_removing(victim);
 	return ERR_PTR(ret);
 
 lookup_error:
-	inode_unlock(d_inode(dir));
 	ret = PTR_ERR(victim);
 	if (ret == -ENOENT)
 		return ERR_PTR(-ESTALE); /* Probably got retired by the netfs */
@@ -816,18 +813,17 @@ int cachefiles_cull(struct cachefiles_cache *cache, struct dentry *dir,
 
 	ret = cachefiles_bury_object(cache, NULL, dir, victim,
 				     FSCACHE_OBJECT_WAS_CULLED);
+	dput(victim);
 	if (ret < 0)
 		goto error;
 
 	fscache_count_culled();
-	dput(victim);
 	_leave(" = 0");
 	return 0;
 
 error_unlock:
-	inode_unlock(d_inode(dir));
+	end_removing(victim);
 error:
-	dput(victim);
 	if (ret == -ENOENT)
 		return -ESTALE; /* Probably got retired by the netfs */
 

fs/namei.c

@@ -3248,6 +3248,33 @@ struct dentry *start_creating(struct mnt_idmap *idmap, struct dentry *parent,
 }
 EXPORT_SYMBOL(start_creating);
 
+/**
+ * start_removing - prepare to remove a given name with permission checking
+ * @idmap:  idmap of the mount
+ * @parent: directory in which to find the name
+ * @name:   the name to be removed
+ *
+ * Locks are taken and a lookup in performed prior to removing
+ * an object from a directory.  Permission checking (MAY_EXEC) is performed
+ * against @idmap.
+ *
+ * If the name doesn't exist, an error is returned.
+ *
+ * end_removing() should be called when removal is complete, or aborted.
+ *
+ * Returns: a positive dentry, or an error.
+ */
+struct dentry *start_removing(struct mnt_idmap *idmap, struct dentry *parent,
+			      struct qstr *name)
+{
+	int err = lookup_one_common(idmap, name, parent);
+
+	if (err)
+		return ERR_PTR(err);
+	return start_dirop(parent, name, 0);
+}
+EXPORT_SYMBOL(start_removing);
+
 #ifdef CONFIG_UNIX98_PTYS
 int path_pts(struct path *path)
 {

fs/nfsd/nfs4recover.c

@@ -324,20 +324,12 @@ nfsd4_unlink_clid_dir(char *name, struct nfsd_net *nn)
 	dprintk("NFSD: nfsd4_unlink_clid_dir. name %s\n", name);
 
 	dir = nn->rec_file->f_path.dentry;
-	inode_lock_nested(d_inode(dir), I_MUTEX_PARENT);
-	dentry = lookup_one(&nop_mnt_idmap, &QSTR(name), dir);
-	if (IS_ERR(dentry)) {
-		status = PTR_ERR(dentry);
-		goto out_unlock;
-	}
-	status = -ENOENT;
-	if (d_really_is_negative(dentry))
-		goto out;
+	dentry = start_removing(&nop_mnt_idmap, dir, &QSTR(name));
+	if (IS_ERR(dentry))
+		return PTR_ERR(dentry);
+
 	status = vfs_rmdir(&nop_mnt_idmap, d_inode(dir), dentry);
-out:
-	dput(dentry);
-out_unlock:
-	inode_unlock(d_inode(dir));
+	end_removing(dentry);
 	return status;
 }
 

fs/nfsd/vfs.c

@@ -2044,7 +2044,7 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 {
 	struct dentry	*dentry, *rdentry;
 	struct inode	*dirp;
-	struct inode	*rinode;
+	struct inode	*rinode = NULL;
 	__be32		err;
 	int		host_err;
 
@@ -2063,24 +2063,21 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 
 	dentry = fhp->fh_dentry;
 	dirp = d_inode(dentry);
-	inode_lock_nested(dirp, I_MUTEX_PARENT);
 
-	rdentry = lookup_one(&nop_mnt_idmap, &QSTR_LEN(fname, flen), dentry);
+	rdentry = start_removing(&nop_mnt_idmap, dentry, &QSTR_LEN(fname, flen));
+
 	host_err = PTR_ERR(rdentry);
 	if (IS_ERR(rdentry))
-		goto out_unlock;
+		goto out_drop_write;
 
-	if (d_really_is_negative(rdentry)) {
-		dput(rdentry);
-		host_err = -ENOENT;
-		goto out_unlock;
-	}
-	rinode = d_inode(rdentry);
 	err = fh_fill_pre_attrs(fhp);
 	if (err != nfs_ok)
 		goto out_unlock;
 
+	rinode = d_inode(rdentry);
+	/* Prevent truncation until after locks dropped */
 	ihold(rinode);
+
 	if (!type)
 		type = d_inode(rdentry)->i_mode & S_IFMT;
 
@@ -2102,10 +2099,10 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 	}
 	fh_fill_post_attrs(fhp);
 
-	inode_unlock(dirp);
-	if (!host_err)
+out_unlock:
+	end_removing(rdentry);
+	if (!err && !host_err)
 		host_err = commit_metadata(fhp);
-	dput(rdentry);
 	iput(rinode);    /* truncate the inode here */
 
 out_drop_write:
@@ -2123,9 +2120,6 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 	}
 out:
 	return err != nfs_ok ? err : nfserrno(host_err);
-out_unlock:
-	inode_unlock(dirp);
-	goto out_drop_write;
 }
 
 /*

fs/overlayfs/dir.c

@@ -866,17 +866,17 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir,
 			goto out;
 	}
 
-	inode_lock_nested(dir, I_MUTEX_PARENT);
-	upper = ovl_lookup_upper(ofs, dentry->d_name.name, upperdir,
-				 dentry->d_name.len);
+	upper = ovl_start_removing_upper(ofs, upperdir,
+					 &QSTR_LEN(dentry->d_name.name,
+						   dentry->d_name.len));
 	err = PTR_ERR(upper);
 	if (IS_ERR(upper))
-		goto out_unlock;
+		goto out_dput;
 
 	err = -ESTALE;
 	if ((opaquedir && upper != opaquedir) ||
 	    (!opaquedir && !ovl_matches_upper(dentry, upper)))
-		goto out_dput_upper;
+		goto out_unlock;
 
 	if (is_dir)
 		err = ovl_do_rmdir(ofs, dir, upper);
@@ -892,10 +892,9 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir,
 	 */
 	if (!err)
 		d_drop(dentry);
-out_dput_upper:
-	dput(upper);
 out_unlock:
-	inode_unlock(dir);
+	end_removing(upper);
+out_dput:
 	dput(opaquedir);
 out:
 	return err;

fs/overlayfs/overlayfs.h

@@ -423,6 +423,14 @@ static inline struct dentry *ovl_start_creating_upper(struct ovl_fs *ofs,
 			      parent, name);
 }
 
+static inline struct dentry *ovl_start_removing_upper(struct ovl_fs *ofs,
+						      struct dentry *parent,
+						      struct qstr *name)
+{
+	return start_removing(ovl_upper_mnt_idmap(ofs),
+			      parent, name);
+}
+
 static inline bool ovl_open_flags_need_copy_up(int flags)
 {
 	if (!flags)

include/linux/namei.h

@@ -90,6 +90,8 @@ struct dentry *lookup_one_positive_killable(struct mnt_idmap *idmap,
 
 struct dentry *start_creating(struct mnt_idmap *idmap, struct dentry *parent,
 			      struct qstr *name);
+struct dentry *start_removing(struct mnt_idmap *idmap, struct dentry *parent,
+			      struct qstr *name);
 
 /**
  * end_creating - finish action started with start_creating
@@ -121,6 +123,22 @@ static inline void end_creating(struct dentry *child, struct dentry *parent)
 		end_dirop(child);
 }
 
+/**
+ * end_removing - finish action started with start_removing
+ * @child:  dentry returned by start_removing()
+ * @parent: dentry given to start_removing()
+ *
+ * Unlock and release the child.
+ *
+ * This is identical to end_dirop().  It can be passed the result of
+ * start_removing() whether that was successful or not, but it not needed
+ * if start_removing() failed.
+ */
+static inline void end_removing(struct dentry *child)
+{
+	end_dirop(child);
+}
+
 extern int follow_down_one(struct path *);
 extern int follow_down(struct path *path, unsigned int flags);
 extern int follow_up(struct path *);