From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6D7613D4137; Wed, 20 May 2026 12:41:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.167.242.64 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779280888; cv=none; b=f/zTV2GJwS3TdOFyQS5sC7zVOMpoDAjDQnuQhIvQg1I00iNgBmQ7VWB9EAk5OaY05E+vQH+m61fV1hhvKPl/UiY0eRwwXBaxmRxgMhkJUtdNU4UOjfZnHRTNg+BXlgmjgy4qhr/KEOSrSwlVz5mxGJxr/PuzpniSvrGkiqKcUk0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779280888; c=relaxed/simple; bh=ZIdqvykTOiP4i2PilrcCyOlToct7pk/2cjaAhm89HP8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=WqYpGXTWqrr2jrHy+6sAzdokvQkqTYpWrmYceV8M61PuQJXbYrm4ICecYXk70ugA5M6C2O4vYO0jeKWXVg05R1chyga8fgOlyLo8nHeZnjgOPLKkcEcEaYn11Q1T6trUiUKppOlqdCmg39Ifd34O8pxNYQ1wcwWeWF6RiwLIh2M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com; spf=pass smtp.mailfrom=ideasonboard.com; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b=RFGm0OB4; arc=none smtp.client-ip=213.167.242.64 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="RFGm0OB4" Received: from killaraus.ideasonboard.com (unknown [IPv6:2a01:cb1d:8f2:800:42d6:38fa:3bdf:70df]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id D4FB4D52; Wed, 20 May 2026 14:41:10 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1779280870; bh=ZIdqvykTOiP4i2PilrcCyOlToct7pk/2cjaAhm89HP8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=RFGm0OB4ADowT9bHxv4NWqr4L35uGYcckU2BRZvI8HI3x9yHR1OMyLdR1vbMsVxSD XqdYqabGsl75oWOjrNkKyZXKTy4C4ZMQOWxad1Nf7mTnFMNs54lAuyWIaVNQ2ZkVyf TvYIQ9d3JQYaYkltuTEo20Yqgm3OVLkj0tEY74mk= Date: Wed, 20 May 2026 14:41:23 +0200 From: Laurent Pinchart To: Sakari Ailus Cc: Hans Verkuil , Guangshuo Li , Mauro Carvalho Chehab , Kees Cook , Ma Ke , linux-media@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] media: v4l2-dev: do not fire driver's release on __video_register_device() failure Message-ID: <20260520124123.GD215344@killaraus.ideasonboard.com> References: <20260520090624.1071139-1-lgs201920130244@gmail.com> <20260520093421.GA215344@killaraus.ideasonboard.com> <14ab929b-f236-48cf-a022-f424fa1e0c1e@kernel.org> <20260520104816.GB215344@killaraus.ideasonboard.com> <00110771-c722-4208-a0e8-48819952e992@kernel.org> Precedence: bulk X-Mailing-List: linux-media@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Wed, May 20, 2026 at 03:02:19PM +0300, Sakari Ailus wrote: > On Wed, May 20, 2026 at 01:26:30PM +0200, Hans Verkuil wrote: > > On 20/05/2026 12:48, Laurent Pinchart wrote: > > > On Wed, May 20, 2026 at 12:01:46PM +0200, Hans Verkuil wrote: > > >> On 20/05/2026 11:34, Laurent Pinchart wrote: > > >>> On Wed, May 20, 2026 at 05:06:24PM +0800, Guangshuo Li wrote: > > >>>> video_register_device() / __video_register_device() registers vdev->dev > > >>>> with device_register(). Before the call the video core sets > > >>>> > > >>>> vdev->dev.release = v4l2_device_release; > > >>>> > > >>>> v4l2_device_release() invokes vdev->release(vdev) as its last step, and > > >>>> the driver's vdev->release hook is commonly video_device_release(), which > > >>>> kfree()s the vdev that the driver allocated with video_device_alloc(). > > >>>> > > >>>> When device_register() fails inside __video_register_device() the core > > >>>> does > > >>>> > > >>>> put_device(&vdev->dev); > > >>>> return ret; > > >>>> > > >>>> which drops the only reference and fires the v4l2_device_release() > > >>>> chain: > > >>>> > > >>>> __video_register_device() > > >>>> device_register() -> -E* > > >>>> put_device(&vdev->dev) > > >>>> -> v4l2_device_release() > > >>>> -> vdev->release(vdev) > > >>>> -> video_device_release(vdev) /* kfree(vdev), free #1 */ > > >>>> > > >>>> video_register_device() returns the error to the driver. Drivers that > > >>>> follow the documented ownership contract release vdev on their own error > > >>>> path, e.g. > > >>>> > > >>>> driver_probe() > > >>>> if (video_register_device(vdev, ...)) > > >>>> goto err_release_vdev; > > >>>> ... > > >>>> err_release_vdev: > > >>>> video_device_release(vdev); /* free #2 -- DOUBLE FREE */ > > >>>> > > >>>> This is the contract documented in > > >>>> Documentation/driver-api/media/v4l2-dev.rst: the driver owns vdev and > > >>>> is responsible for releasing it if video_register_device() fails. As > > >>>> Hans Verkuil pointed out, the right place to fix this is the v4l2 core > > >>>> rather than every individual driver, because drivers are expected to > > >>>> follow the documented ownership contract. > > >>>> > > >>>> Neutralise vdev->release around put_device() in the device_register() > > >>>> failure path so the device core cleanup does not run the driver's > > >>>> release hook. The driver-supplied release is restored before returning > > >>>> so the caller can release vdev according to the documented contract. > > >>>> Successful registration is unchanged, so the normal teardown sequence > > >>>> continues to call the driver's release hook and free vdev exactly once on > > >>>> unregister. > > >>>> > > >>>> Fixes: 2a934fdb01db ("media: v4l2-dev: fix error handling in __video_register_device()") > > >>>> Signed-off-by: Guangshuo Li > > >>>> --- > > >>>> drivers/media/v4l2-core/v4l2-dev.c | 5 +++++ > > >>>> 1 file changed, 5 insertions(+) > > >>>> > > >>>> diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c > > >>>> index 6ce623a1245a..73648549eb2a 100644 > > >>>> --- a/drivers/media/v4l2-core/v4l2-dev.c > > >>>> +++ b/drivers/media/v4l2-core/v4l2-dev.c > > >>>> @@ -1075,9 +1075,14 @@ int __video_register_device(struct video_device *vdev, > > >>>> mutex_lock(&videodev_lock); > > >>>> ret = device_register(&vdev->dev); > > >>>> if (ret < 0) { > > >>>> + void (*release)(struct video_device *) = vdev->release; > > >>>> + > > >>>> mutex_unlock(&videodev_lock); > > >>>> pr_err("%s: device_register failed\n", __func__); > > >>>> + > > >>>> + vdev->release = video_device_release_empty; > > >>>> put_device(&vdev->dev); > > >>>> + vdev->release = release; > > >>> > > >>> That looks like a big hack. There must be something wrong somewhere else > > >>> in the design. > > >> > > >> There is, unfortunately the design was wrong since the beginning of V4L2. > > >> > > >> Documentation/driver-api/media/v4l2-dev.rst explicitly says that you should use: > > >> > > >> err = video_register_device(vdev, VFL_TYPE_VIDEO, -1); > > >> if (err) { > > >> video_device_release(vdev); /* or kfree(my_vdev); */ > > >> return err; > > >> } > > >> > > >> So everyone does that. Luckily device_register never fails in practice (you probably have > > >> bigger problems if it fails then just a double-free). > > >> > > >> The reality is that we don't handle this failure well at all, and this change wouldn't > > >> help in all cases either. E.g. drivers/media/platform/renesas/renesas-ceu.c actually relies > > >> on the release() callback in that it doesn't call video_device_release(). > > >> > > >> But then it would fail on the v4l2_err(vdev->v4l2_dev, ...) call since vdev would be freed > > >> already. > > >> > > >> There are probably more drivers like that. (drivers/media/i2c/video-i2c.c) > > >> > > >> I'm not sure what is wisdom here. > > >> > > >> See also commit 2a934fdb01db ("media: v4l2-dev: fix error handling in __video_register_device()"), > > >> which is where the put_device was introduced in the first place. > > >> > > >> Perhaps that should be reverted instead? > > > > > > If we want a short term fix I think that would be better. > > > > > > Have you seen > > > https://lore.kernel.org/all/aeCOdWLaVpH-5w8s@hovoldconsulting.com/ ? > > > > I hadn't seen it. Interesting. > > > > > > > > Having an API contract different from device_register() will likely > > > cause issues one way or another. > > > > Looking closely how the driver core works and what commit 2a934fdb01db changed, > > I think this might fix it: > > > > diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c > > index 5516b2bbb08f..6ffd385e880f 100644 > > --- a/drivers/media/v4l2-core/v4l2-dev.c > > +++ b/drivers/media/v4l2-core/v4l2-dev.c > > @@ -1071,25 +1071,27 @@ int __video_register_device(struct video_device *vdev, > > vdev->dev.class = &video_class; > > vdev->dev.devt = MKDEV(VIDEO_MAJOR, vdev->minor); > > vdev->dev.parent = vdev->dev_parent; > > - vdev->dev.release = v4l2_device_release; > > dev_set_name(&vdev->dev, "%s%d", name_base, vdev->num); > > > > - /* Increase v4l2_device refcount */ > > - v4l2_device_get(vdev->v4l2_dev); > > - > > mutex_lock(&videodev_lock); > > ret = device_register(&vdev->dev); > > if (ret < 0) { > > mutex_unlock(&videodev_lock); > > pr_err("%s: device_register failed\n", __func__); > > put_device(&vdev->dev); > > - return ret; > > + goto cleanup; > > } > > + /* Register the release callback that will be called when the last > > + reference to the device goes away. */ > > + vdev->dev.release = v4l2_device_release; > > > > if (nr != -1 && nr != vdev->num && warn_if_nr_in_use) > > pr_warn("%s: requested %s%d, got %s\n", __func__, > > name_base, nr, video_device_node_name(vdev)); > > > > + /* Increase v4l2_device refcount */ > > + v4l2_device_get(vdev->v4l2_dev); > > + > > /* Part 5: Register the entity. */ > > ret = video_register_media_controller(vdev); > > > > This mostly reverts 2a934fdb01db, except that we keep the put_device. > > But when this is called, vdev->dev.release isn't set yet, so it only > > frees the device-related data (device_release in drivers/base/core.c). > > > > Am I missing something? > > I guess this could be workable. This way the caller knows the release > callback won't be called. I think it's a short term hack at best though :-( If the device core requires reference-counting for struct device, with a requirement to call put_device() instead of just freeing the structure, then anything that embeds a struct device should do the same. That means exposing video_put() (which should probably be renamed to video_device_put()) and calling that in the error path, in the caller. We may also want to split video_register_device() into video_device_init() and video_device_add(), but that's a separate question. > Regarding error handling, the return value from > video_register_media_controller() is ignored. It'd probably be good to fix > that in a separate patch though. I could submit one as well. -- Regards, Laurent Pinchart