linux-kernel - Re: [PATCH v4 5/8] media: v4l2-core: fix VIDIOC

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Date:   Tue, 26 Nov 2019 15:43:42 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>
Subject: Re: [PATCH v4 5/8] media: v4l2-core: fix VIDIOC_DQEVENT for time64 ABI

On Mon, Nov 25, 2019 at 3:40 PM Hans Verkuil <hverkuil@...all.nl> wrote:
> On 11/11/19 9:38 PM, Arnd Bergmann wrote:

> >       switch (cmd) {
> > +#ifdef CONFIG_COMPAT_32BIT_TIME
> > +     case VIDIOC_DQEVENT_TIME32: {
> > +             struct v4l2_event_time32 ev32;
> > +             struct v4l2_event *ev = parg;
> > +
> > +             memcpy(&ev32, ev, offsetof(struct v4l2_event, timestamp));
> > +             ev32.timestamp.tv_sec = ev->timestamp.tv_sec;
> > +             ev32.timestamp.tv_nsec = ev->timestamp.tv_nsec;
> > +             memcpy(&ev32.id, &ev->id, sizeof(*ev) - offsetof(struct v4l2_event, id));
>
> This looks dangerous: due to 64-bit alignment requirements the
> v4l2_event struct may end with a 4-byte hole at the end of the struct,
> which you do not want to copy to ev32.
>
> I think it is safer to just copy id and reserved separately:
>
>                 ev32.id = ev->id;
>                 memcpy(ev32.reserved, ev->reserved, sizeof(ev->reserved));

Actually I think it's that's also bad: The padding in *ev must already be
cleared here (otherwise there is a leak of stack data in the kernel
already), so  *not* copying the padding requires at least adding a memset
upfront.

I would do the per-member copy like I did for v4l2_buffer in my
other reply:

                struct v4l2_event *ev = parg;
                struct v4l2_event_time32 ev32 = {
                        .type           = ev->type,
                        .pending        = ev->pending,
                        .sequence       = ev->sequence,
                        .timestamp.tv_sec  = ev->timestamp.tv_sec,
                        .timestamp.tv_nsec = ev->timestamp.tv_nsec,
                        .id             = ev->id,
                };

                memcpy(ev32.u, ev->u, sizeof(ev->u));
                memcpy(ev32.reserved, ev->reserved, sizeof(ev->reserved));

                if (copy_to_user(arg, &ev32, sizeof(ev32)))
                        return -EFAULT;

Unfortunately this is a little uglier because it still requires the two
memcpy() for the arrays, but I think it's good enough.

Any other ideas? Let me know if I should do a memset()
plus individual member copy instead.
> > +             if (!(sd->flags & V4L2_SUBDEV_FL_HAS_EVENTS))
> > +                     return -ENOIOCTLCMD;
> > +
> > +             rval = v4l2_event_dequeue(vfh, &ev, file->f_flags & O_NONBLOCK);
> > +
> > +             memcpy(ev32, &ev, offsetof(struct v4l2_event, timestamp));
> > +             ev32->timestamp.tv_sec = ev.timestamp.tv_sec;
> > +             ev32->timestamp.tv_nsec = ev.timestamp.tv_nsec;
> > +             memcpy(&ev32->id, &ev.id,
> > +                    sizeof(ev) - offsetof(struct v4l2_event, id));
>
> Ditto.

Using the corresponding code here as well.

> > +
> >  #define V4L2_EVENT_SUB_FL_SEND_INITIAL               (1 << 0)
> >  #define V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK     (1 << 1)
> >
> > @@ -2486,6 +2515,7 @@ struct v4l2_create_buffers {
> >  #define      VIDIOC_S_DV_TIMINGS     _IOWR('V', 87, struct v4l2_dv_timings)
> >  #define      VIDIOC_G_DV_TIMINGS     _IOWR('V', 88, struct v4l2_dv_timings)
> >  #define      VIDIOC_DQEVENT           _IOR('V', 89, struct v4l2_event)
> > +#define      VIDIOC_DQEVENT_TIME32    _IOR('V', 89, struct v4l2_event_time32)
>
> Shouldn't this be under #ifdef __KERNEL__?
>
> And should this be in the public header at all? media/v4l2-ioctl.h might be a better
> place.

Done.

       Arnd