linux-kernel - Re: [PATCH v3 03/11] media: videobuf2: Remove VB2_MAX

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Date:   Thu, 22 Jun 2023 16:56:29 +0300
From:   Dan Carpenter <dan.carpenter@...aro.org>
To:     Benjamin Gaignard <benjamin.gaignard@...labora.com>
Cc:     mchehab@...nel.org, tfiga@...omium.org, m.szyprowski@...sung.com,
        ming.qian@....com, ezequiel@...guardiasur.com.ar,
        p.zabel@...gutronix.de, gregkh@...uxfoundation.org,
        hverkuil-cisco@...all.nl, nicolas.dufresne@...labora.com,
        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, linux-staging@...ts.linux.dev,
        kernel@...labora.com
Subject: Re: [PATCH v3 03/11] media: videobuf2: Remove VB2_MAX_FRAME limit on
 buffer storage

On Thu, Jun 22, 2023 at 03:13:41PM +0200, Benjamin Gaignard wrote:
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index f1ff7af34a9f..86e1e926fa45 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -455,9 +455,9 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
>  	struct vb2_buffer *vb;
>  	int ret;
>  
> -	/* Ensure that q->num_buffers+num_buffers is below VB2_MAX_FRAME */
> +	/* Ensure that q->num_buffers + num_buffers is UINT_MAX */
>  	num_buffers = min_t(unsigned int, num_buffers,
> -			    VB2_MAX_FRAME - q->num_buffers);
> +			    UINT_MAX - q->num_buffers);

The UINT_MAX limit adds a level of danger.  It would be safer to do what
the vfs layer does for MAX_RW_COUNT and use "INT_MAX - PAGE_SIZE".  That
way you can take size + sizeof() and it's only very rarely going to turn
negative.  Or at least just INT_MAX.  I would keep the VB2_MAX_FRAME and
define it as:

#define VB2_MAX_FRAME (INT_MAX & PAGE_MASK)  /* The mask prevents 85% of integer overflows */

>  
>  	for (buffer = 0; buffer < num_buffers; ++buffer) {
>  		/* Allocate vb2 buffer structures */
> @@ -858,9 +858,9 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>  	/*
>  	 * Make sure the requested values and current defaults are sane.
>  	 */
> -	WARN_ON(q->min_buffers_needed > VB2_MAX_FRAME);
> +	WARN_ON(q->min_buffers_needed > UINT_MAX);

This will trigger a static checker warning because the condition is
impossible.

regards,
dan carpenter