Hi Laurent,
On Fri, 28 Mar 2014, Laurent Pinchart wrote:
> Hi Guennadi,
>
> Thank you for the patch.
>
> 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.
> > 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 <g.liakhovetski@xxxxxx>
> > ---
> >
> > 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.
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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