From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EF809C3A5A1 for ; Mon, 19 Aug 2019 14:17:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CCC3F206BB for ; Mon, 19 Aug 2019 14:17:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726728AbfHSORg (ORCPT ); Mon, 19 Aug 2019 10:17:36 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:43284 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1726636AbfHSORf (ORCPT ); Mon, 19 Aug 2019 10:17:35 -0400 Received: (qmail 2787 invoked by uid 2102); 19 Aug 2019 10:17:34 -0400 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 19 Aug 2019 10:17:34 -0400 Date: Mon, 19 Aug 2019 10:17:34 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: Oliver Neukum cc: Jiri Kosina , USB list Subject: Re: Duplicated code in hiddev_open() In-Reply-To: <1566211268.5663.9.camel@suse.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-usb-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org On Mon, 19 Aug 2019, Oliver Neukum wrote: > Am Freitag, den 16.08.2019, 13:10 -0400 schrieb Alan Stern: > > Oliver and Jiri: > > > > Why is there duplicated code in > > drivers/hid/usbhid/hiddev.c:hiddev_open()? > > > > Line 267: > > /* > > * no need for locking because the USB major number > > * is shared which usbcore guards against disconnect > > */ > > if (list->hiddev->exist) { > > if (!list->hiddev->open++) { > > res = hid_hw_open(hiddev->hid); > > if (res < 0) > > goto bail; > > } > > } else { > > res = -ENODEV; > > goto bail; > > } > > > > Line 286: > > mutex_lock(&hiddev->existancelock); > > if (!list->hiddev->open++) > > if (list->hiddev->exist) { > > struct hid_device *hid = hiddev->hid; > > res = hid_hw_power(hid, PM_HINT_FULLON); > > if (res < 0) > > goto bail_unlock; > > res = hid_hw_open(hid); > > if (res < 0) > > goto bail_normal_power; > > } > > mutex_unlock(&hiddev->existancelock); > > > > The second part can never execute, because the first part ensures that > > list->hiddev->open > 0 by the time the second part runs. > > > > Even more disturbing, why is one of these code sections protected by a > > mutex and the other not? > > I suppose the comment I made back then: > > 079034073faf9 drivers/hid/usbhid/hiddev.c (Oliver Neukum 2008-12-16 10:55:15 +0100 268) * no need for locking because the USB major number > 079034073faf9 drivers/hid/usbhid/hiddev.c (Oliver Neukum 2008-12-16 10:55:15 +0100 269) * is shared which usbcore guards against disconnect > > has ceased to be true, but the section was not removed, as the check > for existance was duplicated. > > > Note: The second section was added in commit 0361a28d3f9a ("HID: > > autosuspend support for USB HID") over ten years ago! > > Yes and I remember how frustrating keyboards were in testing, but > no further details. Indeed. But more importantly for now, how should this be fixed? This may be the culprit in some of the syzbot bug reports (those involving hiddev). Alan Stern