Re: [PATCHv10 22/26] v4l: vb2-dma-contig: fail if user ptr buffer is not correctly aligned

[Tomasz Stanislawski <t.stanislaws@samsung.com>]

Hi Laurent,
Thank you for the review.
Please refer to the comments below.

On 10/11/2012 11:36 PM, Laurent Pinchart wrote:
> Hi Tomasz,
> 
> Thanks for the patch.
> 
> On Wednesday 10 October 2012 16:46:41 Tomasz Stanislawski wrote:
>> From: Marek Szyprowski <m.szyprowski@samsung.com>
>>
>> The DMA transfer must be aligned to a specific value. If userptr is not
>> aligned to DMA requirements then unexpected corruptions of the memory may
>> occur before or after a buffer.  To prevent such situations, all unligned
>> userptr buffers are rejected at VIDIOC_QBUF.
>>
>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
>> Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
>> ---
>>  drivers/media/v4l2-core/videobuf2-dma-contig.c |   12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c
>> b/drivers/media/v4l2-core/videobuf2-dma-contig.c index 2d661fd..571a919
>> 100644
>> --- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
>> +++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
>> @@ -493,6 +493,18 @@ static void *vb2_dc_get_userptr(void *alloc_ctx,
>> unsigned long vaddr, struct vm_area_struct *vma;
>>  	struct sg_table *sgt;
>>  	unsigned long contig_size;
>> +	unsigned long dma_align = dma_get_cache_alignment();
>> +
>> +	/* Only cache aligned DMA transfers are reliable */
>> +	if (!IS_ALIGNED(vaddr | size, dma_align)) {
>> +		pr_debug("user data must be aligned to %lu bytes\n", dma_align);
>> +		return ERR_PTR(-EINVAL);
>> +	}
> 
> Looks good to me.
> 
>> +	if (!size) {
>> +		pr_debug("size is zero\n");
>> +		return ERR_PTR(-EINVAL);
>> +	}
> 
> Can this happen ? The vb2 core already has
> 
>                 /* Check if the provided plane buffer is large enough */
>                 if (planes[plane].length < q->plane_sizes[plane]) {
>                         ret = -EINVAL;
>                         goto err;
>                 }
> 
> Unless queue_setup sets plane_sizes to 0 we can't reach vb2_dc_get_userptr.
> 

Yes.. unfortunately, some drivers set plane_size to 0 at queue_setup.
Especially, if REQBUFS is called before any S_FMT.
Maybe it is just a driver bug.

However, VB2 makes no sanity check if plane_sizes[] is zero.
I was not able to find in Documentation nor code comments
any explicit statement that plane_size cannot be zero.

Therefore I have to reject reject a 0-bytes-long user pointer
at vb2_dc_get_userptr before creating an empty scatterlist
and passing it to the DMA layer.

Regards,
Tomasz Stanislawski

>>  	buf = kzalloc(sizeof *buf, GFP_KERNEL);
>>  	if (!buf)
>