fs: prepare for adding LSM blob to backing_file

In preparation to adding LSM blob to backing_file struct, factor out
helpers init_backing_file() and backing_file_free().

Cc: stable@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-unionfs@vger.kernel.org
Cc: linux-erofs@lists.ozlabs.org
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
[PM: use the term "LSM blob", fix comment style to match file]
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: amir73il

fs/file_table.c

@@ -66,15 +66,20 @@ void backing_file_set_user_path(struct file *f, const struct path *path)
 }
 EXPORT_SYMBOL_GPL(backing_file_set_user_path);
 
+static inline void backing_file_free(struct backing_file *ff)
+{
+	path_put(&ff->user_path);
+	kmem_cache_free(bfilp_cachep, ff);
+}
+
 static inline void file_free(struct file *f)
 {
 	security_file_free(f);
 	if (likely(!(f->f_mode & FMODE_NOACCOUNT)))
 		percpu_counter_dec(&nr_files);
 	put_cred(f->f_cred);
 	if (unlikely(f->f_mode & FMODE_BACKING)) {
-		path_put(backing_file_user_path(f));
-		kmem_cache_free(bfilp_cachep, backing_file(f));
+		backing_file_free(backing_file(f));
 	} else {
 		kmem_cache_free(filp_cachep, f);
 	}
@@ -283,6 +288,12 @@ struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred)
 	return f;
 }
 
+static int init_backing_file(struct backing_file *ff)
+{
+	memset(&ff->user_path, 0, sizeof(ff->user_path));
+	return 0;
+}
+
 /*
  * Variant of alloc_empty_file() that allocates a backing_file container
  * and doesn't check and modify nr_files.
@@ -305,7 +316,14 @@ struct file *alloc_empty_backing_file(int flags, const struct cred *cred)
 		return ERR_PTR(error);
 	}
 
+	/* The f_mode flags must be set before fput(). */
 	ff->file.f_mode |= FMODE_BACKING | FMODE_NOACCOUNT;
+	error = init_backing_file(ff);
+	if (unlikely(error)) {
+		fput(&ff->file);
+		return ERR_PTR(error);
+	}
+
 	return &ff->file;
 }
 EXPORT_SYMBOL_GPL(alloc_empty_backing_file);