linux-kernel - Re: [PATCH] usb: fix uvc_alloc

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Date:   Tue, 7 Aug 2018 00:58:05 +0000
From:   Nadav Amit <namit@...are.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: fix uvc_alloc_entity() allocation alignment

at 4:58 PM, Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:

> Hi Nadav,
> 
> Thank you for the patch.
> 
> On Monday, 4 June 2018 16:47:13 EEST Nadav Amit wrote:
>> The use of ALIGN() in uvc_alloc_entity() is incorrect, since the size of
>> (entity->pads) is not a power of two. As a stop-gap, until a better
>> solution is adapted, use roundup() instead.
>> 
>> Found by a static assertion. Compile-tested only.
>> 
>> Fixes: 4ffc2d89f38a ("uvcvideo: Register subdevices for each entity")
>> 
>> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
>> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
>> Cc: linux-media@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> 
>> Signed-off-by: Nadav Amit <namit@...are.com>
>> ---
>> drivers/media/usb/uvc/uvc_driver.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/media/usb/uvc/uvc_driver.c
>> b/drivers/media/usb/uvc/uvc_driver.c index 2469b49b2b30..6b989d41c034
>> 100644
>> --- a/drivers/media/usb/uvc/uvc_driver.c
>> +++ b/drivers/media/usb/uvc/uvc_driver.c
>> @@ -909,7 +909,7 @@ static struct uvc_entity *uvc_alloc_entity(u16 type, u8
>> id, unsigned int size;
>> 	unsigned int i;
>> 
>> -	extra_size = ALIGN(extra_size, sizeof(*entity->pads));
>> +	extra_size = roundup(extra_size, sizeof(*entity->pads));
>> 	num_inputs = (type & UVC_TERM_OUTPUT) ? num_pads : num_pads - 1;
>> 	size = sizeof(*entity) + extra_size + sizeof(*entity->pads) * num_pads
>> 	     + num_inputs;
> 
> The purpose of this alignment is to make sure that entity->pads will be 
> properly aligned. In theory the size of uvc_entity should be taken into 
> account too, but the structure contains pointers, so its size should already 
> be properly aligned. This patch thus looks good to me. What made you say it's 
> a stop-gap measure ?

Thanks. It’s been a while. Anyhow, I don’t know how “hot” this code is, but
roundup uses a div operations, so if it is “hot” you may want a different
way to align with lower overhead.

I presume it is not…

Regards,
Nadav