v9fs: fix error handling in v9fs_session_init()

On failure, v9fs_session_init() returns with the v9fs_session_info
struct partially initialized and expects the caller to invoke
v9fs_session_close() to clean it up; however, it doesn't track whether
the bdi is initialized or not and curiously invokes bdi_destroy() in
both vfs_session_init() failure path too.

A. If v9fs_session_init() fails before the bdi is initialized, the
   follow-up v9fs_session_close() will invoke bdi_destroy() on an
   uninitialized bdi.

B. If v9fs_session_init() fails after the bdi is initialized,
   bdi_destroy() will be called twice on the same bdi - once in the
   failure path of v9fs_session_init() and then by
   v9fs_session_close().

A is broken no matter what.  B used to be okay because bdi_destroy()
allowed being invoked multiple times on the same bdi, which BTW was
broken in its own way - if bdi_destroy() was invoked on an initialiezd
but !registered bdi, it'd fail to free percpu counters.  Since
f0054bb1e1 ("writeback: move backing_dev_info->wb_lock and
->worklist into bdi_writeback"), this no longer work - bdi_destroy()
on an initialized but not registered bdi works correctly but multiple
invocations of bdi_destroy() is no longer allowed.

The obvious culprit here is v9fs_session_init()'s odd and broken error
behavior.  It should simply clean up after itself on failures.  This
patch makes the following updates to v9fs_session_init().

* @rc -> @retval error return propagation removed.  It didn't serve
  any purpose.  Just use @rc.

* Move addition to v9fs_sessionlist to the end of the function so that
  incomplete sessions are not put on the list or iterated and error
  path doesn't have to worry about it.

* Update error handling so that it cleans up after itself.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/9p/v9fs.c

@@ -320,31 +320,21 @@ fail_option_alloc:
 struct p9_fid *v9fs_session_init(struct v9fs_session_info *v9ses,
 		  const char *dev_name, char *data)
 {
-	int retval = -EINVAL;
 	struct p9_fid *fid;
-	int rc;
+	int rc = -ENOMEM;
 
 	v9ses->uname = kstrdup(V9FS_DEFUSER, GFP_KERNEL);
 	if (!v9ses->uname)
-		return ERR_PTR(-ENOMEM);
+		goto err_names;
 
 	v9ses->aname = kstrdup(V9FS_DEFANAME, GFP_KERNEL);
-	if (!v9ses->aname) {
-		kfree(v9ses->uname);
-		return ERR_PTR(-ENOMEM);
-	}
+	if (!v9ses->aname)
+		goto err_names;
 	init_rwsem(&v9ses->rename_sem);
 
 	rc = bdi_setup_and_register(&v9ses->bdi, "9p");
-	if (rc) {
-		kfree(v9ses->aname);
-		kfree(v9ses->uname);
-		return ERR_PTR(rc);
-	}
-
-	spin_lock(&v9fs_sessionlist_lock);
-	list_add(&v9ses->slist, &v9fs_sessionlist);
-	spin_unlock(&v9fs_sessionlist_lock);
+	if (rc)
+		goto err_names;
 
 	v9ses->uid = INVALID_UID;
 	v9ses->dfltuid = V9FS_DEFUID;
@@ -352,10 +342,9 @@ struct p9_fid *v9fs_session_init(struct v9fs_session_info *v9ses,
 
 	v9ses->clnt = p9_client_create(dev_name, data);
 	if (IS_ERR(v9ses->clnt)) {
-		retval = PTR_ERR(v9ses->clnt);
-		v9ses->clnt = NULL;
+		rc = PTR_ERR(v9ses->clnt);
 		p9_debug(P9_DEBUG_ERROR, "problem initializing 9p client\n");
-		goto error;
+		goto err_bdi;
 	}
 
 	v9ses->flags = V9FS_ACCESS_USER;
@@ -368,10 +357,8 @@ struct p9_fid *v9fs_session_init(struct v9fs_session_info *v9ses,
 	}
 
 	rc = v9fs_parse_options(v9ses, data);
-	if (rc < 0) {
-		retval = rc;
-		goto error;
-	}
+	if (rc < 0)
+		goto err_clnt;
 
 	v9ses->maxdata = v9ses->clnt->msize - P9_IOHDRSZ;
 
@@ -405,10 +392,9 @@ struct p9_fid *v9fs_session_init(struct v9fs_session_info *v9ses,
 	fid = p9_client_attach(v9ses->clnt, NULL, v9ses->uname, INVALID_UID,
 							v9ses->aname);
 	if (IS_ERR(fid)) {
-		retval = PTR_ERR(fid);
-		fid = NULL;
+		rc = PTR_ERR(fid);
 		p9_debug(P9_DEBUG_ERROR, "cannot attach\n");
-		goto error;
+		goto err_clnt;
 	}
 
 	if ((v9ses->flags & V9FS_ACCESS_MASK) == V9FS_ACCESS_SINGLE)
@@ -420,12 +406,20 @@ struct p9_fid *v9fs_session_init(struct v9fs_session_info *v9ses,
 	/* register the session for caching */
 	v9fs_cache_session_get_cookie(v9ses);
 #endif
+	spin_lock(&v9fs_sessionlist_lock);
+	list_add(&v9ses->slist, &v9fs_sessionlist);
+	spin_unlock(&v9fs_sessionlist_lock);
 
 	return fid;
 
-error:
+err_clnt:
+	p9_client_destroy(v9ses->clnt);
+err_bdi:
 	bdi_destroy(&v9ses->bdi);
-	return ERR_PTR(retval);
+err_names:
+	kfree(v9ses->uname);
+	kfree(v9ses->aname);
+	return ERR_PTR(rc);
 }
 
 /**

fs/9p/vfs_super.c

@@ -130,11 +130,7 @@ static struct dentry *v9fs_mount(struct file_system_type *fs_type, int flags,
 	fid = v9fs_session_init(v9ses, dev_name, data);
 	if (IS_ERR(fid)) {
 		retval = PTR_ERR(fid);
-		/*
-		 * we need to call session_close to tear down some
-		 * of the data structure setup by session_init
-		 */
-		goto close_session;
+		goto free_session;
 	}
 
 	sb = sget(fs_type, NULL, v9fs_set_super, flags, v9ses);
@@ -195,8 +191,8 @@ static struct dentry *v9fs_mount(struct file_system_type *fs_type, int flags,
 
 clunk_fid:
 	p9_client_clunk(fid);
-close_session:
 	v9fs_session_close(v9ses);
+free_session:
 	kfree(v9ses);
 	return ERR_PTR(retval);