linux-kernel - Re: [PATCH] media: videobuf2: forbid create_bufs/remove

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Message-ID: <258b9036-697d-48b2-91d6-5fb8ea2f1350@collabora.com>
Date: Mon, 20 Oct 2025 10:30:59 +0200
From: Benjamin Gaignard <benjamin.gaignard@...labora.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>,
 Hans Verkuil <hverkuil+cisco@...nel.org>, linux-media@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: Tomasz Figa <tfiga@...omium.org>,
 Mauro Carvalho Chehab <mchehab@...nel.org>,
 Guennadi Liakhovetski <g.liakhovetski@....de>,
 Hans Verkuil <hverkuil@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] media: videobuf2: forbid create_bufs/remove_bufs when
 legacy fileio is active


Le 20/10/2025 à 10:21, Marek Szyprowski a écrit :
> On 20.10.2025 09:48, Benjamin Gaignard wrote:
>> Le 20/10/2025 à 09:39, Hans Verkuil a écrit :
>>> On 20/10/2025 09:34, Marek Szyprowski wrote:
>>>> On 20.10.2025 09:11, Benjamin Gaignard wrote:
>>>>> Le 16/10/2025 à 13:11, Marek Szyprowski a écrit :
>>>>>> create_bufs and remove_bufs ioctl calls manipulate queue internal
>>>>>> buffer
>>>>>> list, potentially overwriting some pointers used by the legacy fileio
>>>>>> access mode. Simply forbid those calls when fileio is active to
>>>>>> protect
>>>>>> internal queue state between subsequent read/write calls.
>>>>> Hi Marek,
>>>>>
>>>>> I may be wrong but using fileio API and create/remove API at the same
>>>>> time
>>>>> sound incorrect from application point of view, right ? If that not
>>>>> the
>>>>> case maybe we should also add a test in v4l2-compliance.
>>>> Definitely that's incorrect and v4l2-core must forbid such calls. The
>>>> standard reqbufs/qbuf/dqbuf API is also forbidden. Extending
>>>> v4l2-compliance tools is probably a good idea.
>>> Yes, please! A patch is welcome.
>>>
>>>    I also wonder if its a
>>>> good time to add a kernel option to completely disable legacy fileio
>>>> access mode, as it is not really needed for most of the systems
>>>> nowadays.
>>> No, that will break applications. Using read() is very common (and
>>> convenient!)
>>> for MPEG encoders such as the cx18 driver.
>>>
>>> The fileio code is not blocking any new development, it's just there
>>> for those
>>> drivers were it makes sense.
>>>
>>> Regards,
>>>
>>>      Hans
>> I wonder if this patch in useful because when calling
>> vb2_ioctl_create_bufs()
>> it already check in vb2_verify_memory_type() if fileio is used or not.
> Frankly speaking the original report I got was about mixing fileio with
> vb2_ioctl_remove_bufs and that case is indeed not protected.
>
> While analyzing that I've inspected a symmetrical ioctl
> (vb2_ioctl_create_bufs), but it looks I've I missed that a check is in
> vb2_verify_memory_type(). I will remove it in v2 then.

To keep vb2_ioctl_remove_bufs() symmetrical to vb2_ioctl_create_bufs()
we should do in vb2_ioctl_remove_bufs() something like :
res = vb2_verify_memory_type(vdev->queue, vdev->queue->memory, d->type);
instead of vdev->queue->type != d->type.

This way we test fileio too.


>
> Best regards