From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from perceval.ideasonboard.com ([95.142.166.194]:36775 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754263AbaC1R7k (ORCPT ); Fri, 28 Mar 2014 13:59:40 -0400 From: Laurent Pinchart To: Guennadi Liakhovetski Cc: Linux Media Mailing List , Hans Verkuil , Mauro Carvalho Chehab Subject: Re: [PATCH] V4L2: fix VIDIOC_CREATE_BUFS in 64- / 32-bit compatibility mode Date: Fri, 28 Mar 2014 19:01:38 +0100 Message-ID: <7890812.mee88PGtyI@avalon> In-Reply-To: References: <22889282.D1rkAPVGhe@avalon> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: Hi Guennadi, On Friday 28 March 2014 18:44:04 Guennadi Liakhovetski wrote: > On Fri, 28 Mar 2014, Laurent Pinchart wrote: > > On Thursday 27 March 2014 22:34:07 Guennadi Liakhovetski wrote: > > > It turns out, that 64-bit compilations sometimes align structs within > > > other structs on 32-bit boundaries, but in other cases alignment is done > > > on 64-bit boundaries, adding padding if necessary. > > > > You make it sound like the behaviour is random, I'm pretty sure it isn't > > :-) > > I didn't mean it was random, I just meant it is not be as simple as "align > always." E.g. if there are only 32-bit fields in the embedded struct, it > won't be aligned, below I explain a bit with pointers. I just don't know > the exact logic, that's used there. The logic is basically that fields are aligned within structures to a multiple of their native access size, and structures are aligned to a multiple of the access size of the largest field. If a structure on a 64-bit systems contains a pointer the pointer field will be aligned to a multiple of 8 bytes within the structure, and instances of the structure will be aligned to multiples of 8 bytes as well. If that structure is embedded inside another structure, it will be placed on an 8 bytes boundary, possibly creating a gap if the fields before the structure don't add up to a multiple of 8 bytes. This is what happens here. > > > This is done, for example when the embedded struct contains a pointer. > > > This is the case with struct v4l2_window, which is embedded into struct > > > v4l2_format, and that one is embedded into struct v4l2_create_buffers. > > > Unlike some other structs, used as a part of the kernel ABI as ioctl() > > > arguments, that are packed, these structs aren't packed. This isn't a > > > problem per se, but it turns out, that the ioctl-compat code for > > > VIDIOC_CREATE_BUFS contains a bug, that triggers in such 64-bit builds. > > > That code wrongly assumes, that in struct v4l2_create_buffers, struct > > > v4l2_format immediately follows the __u32 memory field, which in fact > > > isn't the case. This bug wasn't visible until now, because until > > > recently hardly any applications used this ioctl() and mostly embedded > > > 32-bit only drivers implemented it. This is changing now with addition > > > of this ioctl() to some USB drivers, e.g. UVC. This patch fixes the bug > > > by copying parts of struct v4l2_create_buffers separately. > > > > > > Signed-off-by: Guennadi Liakhovetski > > > --- > > > > > > It's probably too late for 3.14, but maybe after pushing it into 3.15 we > > > have to send it to stable. > > > > > > diff --git a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c > > > b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c index 04b2daf..28f87d7 > > > 100644 > > > --- a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c > > > +++ b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c > > > @@ -213,8 +213,9 @@ static int get_v4l2_format32(struct v4l2_format *kp, > > > struct v4l2_format32 __user static int get_v4l2_create32(struct > > > v4l2_create_buffers *kp, struct v4l2_create_buffers32 __user *up) { > > > > > > if (!access_ok(VERIFY_READ, up, sizeof(struct v4l2_create_buffers32)) > > > || > > > > > > - copy_from_user(kp, up, offsetof(struct v4l2_create_buffers32, > > > format.fmt))) > > > - return -EFAULT; > > > + copy_from_user(kp, up, offsetof(struct v4l2_create_buffers32, > > > format)) || > > > + get_user(kp->format.type, &up->format.type)) > > > + return -EFAULT; > > > > > > return __get_v4l2_format32(&kp->format, &up->format); > > > > > > } > > > > I'm fine with the patch as it is, but wouldn't it be simpler to move the > > get_user() inside the __get_v4l2_format32() function ? You could also then > > remove that call from get_v4l2_format32() as well. > > This would duplicate the call to access_ok(), but it could be done, sure. You don't need to call access_ok() inside __get_v4l2_format32(), both get_v4l2_format32() and get_v4l2_create32() perform an access_ok() check that can be left in place. -- Regards, Laurent Pinchart