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

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Date:   Tue, 07 Aug 2018 11:59:01 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Nadav Amit <namit@...are.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

Hi Nadav,

On Tuesday, 7 August 2018 03:58:05 EEST Nadav Amit wrote:
> at 4:58 PM, Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:
> > 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…

You're right, it isn't. I'll include this patch in my next pull request for 
v4.20.

-- 
Regards,

Laurent Pinchart