linux-kernel - Re: [RFC 2/4] media: videobuf2: Replace bufs array by a list

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Message-ID: <20230319233352.GC20234@pendragon.ideasonboard.com>
Date:   Mon, 20 Mar 2023 01:33:52 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Nicolas Dufresne <nicolas.dufresne@...labora.com>
Cc:     Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        tfiga@...omium.org, m.szyprowski@...sung.com, mchehab@...nel.org,
        ming.qian@....com, shijie.qin@....com, eagle.zhou@....com,
        bin.liu@...iatek.com, matthias.bgg@...il.com,
        angelogioacchino.delregno@...labora.com, tiffany.lin@...iatek.com,
        andrew-ct.chen@...iatek.com, yunfei.dong@...iatek.com,
        stanimir.k.varbanov@...il.com, quic_vgarodia@...cinc.com,
        agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        ezequiel@...guardiasur.com.ar, p.zabel@...gutronix.de,
        daniel.almeida@...labora.com, hverkuil-cisco@...all.nl,
        jerbel@...nel.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, kernel@...labora.com
Subject: Re: [RFC 2/4] media: videobuf2: Replace bufs array by a list

On Wed, Mar 15, 2023 at 09:57:51AM -0400, Nicolas Dufresne wrote:
> Le lundi 13 mars 2023 à 20:11 +0200, Laurent Pinchart a écrit :
> > > -	/* Ensure that q->num_buffers+num_buffers is below VB2_MAX_FRAME */
> > > -	num_buffers = min_t(unsigned int, num_buffers,
> > > -			    VB2_MAX_FRAME - q->num_buffers);
> > > -
> > 
> > We can indeed drop this check now, but shouldn't we introduce some kind
> > of resource accounting and limitation ? Otherwise any unpriviledged
> > userspace will be able to starve system memory. This could be
> > implemented on top, as the problem largely exists today already, but I'd
> > like to at least record this in a TODO comment.
> 
> The current limit already isn't work for resource accounting and limitation for
> m2m drivers. You can open a device, allocate 32 buffers, and close that device
> keeping the memory around. And redo this process as many times as you want.

I know, that's why I mentioned that the problem largely exists today
already.

> A TODO is most appropriate, but I would prefer to see this done at a memory
> layer level (rather then v4l2 specific), so that limits and accounting works
> with containers and other sandboxes.

I haven't thought about how this could be implemented, all I know is
that it's about time to tackle this issue, so I would like to at least
record it.

> > I also wonder if we should still limit the number of allocated buffers.
> > The limit could be large, for instance 1024 buffers, and it would be an
> > in-kernel limit that could be increased later if needed. I'm concerned
> > that dropping the limit completely will allow userspace to request
> > UINT_MAX buffers, which may cause integer overflows somewhere. Limiting
> > the number of buffers would avoid extensive review of all the code that
> > deals with counting buffers.
> 

-- 
Regards,

Laurent Pinchart