[Spice-devel] [spice-gtk 1/2] Fix possible multimedia time overflow
Frediano Ziglio
fziglio at redhat.com
Thu Mar 16 10:56:07 UTC 2017
>
> ----- Original Message -----
> > >
> > > Hi
> > >
> > > ----- Original Message -----
> > > > The multimedia time can easily overflow as is encoded in an
> > > > unsigned 32 bit and have a unit of milliseconds so it wrap
> > > > up every 49 days. Use some math that allow the number to overflow
> > > > without issues.
> > > > This could caused the client to stop handling streaming and
> > > > starting only queueing.
> > > >
> > > > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > > > ---
> > > > src/channel-display-gst.c | 11 ++++++-----
> > > > src/channel-display-mjpeg.c | 14 ++++++++------
> > > > src/channel-display-priv.h | 15 +++++++++++++++
> > > > 3 files changed, 29 insertions(+), 11 deletions(-)
> > > >
> > > > This patch should be applied independently on 2/2 as intended to be
> > > > merged upstream as a fix while 2/2 depends on this as it change same
> > > > code portions.
> > > >
> > > > diff --git a/src/channel-display-gst.c b/src/channel-display-gst.c
> > > > index c4190b2..9c62e67 100644
> > > > --- a/src/channel-display-gst.c
> > > > +++ b/src/channel-display-gst.c
> > > > @@ -183,8 +183,9 @@ static void schedule_frame(SpiceGstDecoder
> > > > *decoder)
> > > > }
> > > >
> > > > SpiceStreamDataHeader *op = spice_msg_in_parsed(frame->msg);
> > > > - if (now < op->multi_media_time) {
> > > > - decoder->timer_id = g_timeout_add(op->multi_media_time -
> > > > now,
> > > > + gint32 time_diff = compute_mm_time_diff(op->multi_media_time,
> > > > now);
> > > > + if (time_diff >= 0) {
> > > > + decoder->timer_id = g_timeout_add(time_diff,
> > > > display_frame, decoder);
> > > > } else if (g_queue_get_length(decoder->display_queue) == 1) {
> > > > /* Still attempt to display the least out of date frame so
> > > > the
> > > > @@ -192,8 +193,8 @@ static void schedule_frame(SpiceGstDecoder
> > > > *decoder)
> > > > */
> > > > decoder->timer_id = g_timeout_add(0, display_frame,
> > > > decoder);
> > > > } else {
> > > > - SPICE_DEBUG("%s: rendering too late by %u ms (ts: %u,
> > > > mmtime:
> > > > %u), dropping",
> > > > - __FUNCTION__, now - op->multi_media_time,
> > > > + SPICE_DEBUG("%s: rendering too late by %d ms (ts: %u,
> > > > mmtime:
> > > > %u), dropping",
> > > > + __FUNCTION__, -time_diff,
> > > > op->multi_media_time, now);
> > > > stream_dropped_frame_on_playback(decoder->base.stream);
> > > > g_queue_pop_head(decoder->display_queue);
> > > > @@ -431,7 +432,7 @@ static gboolean
> > > > spice_gst_decoder_queue_frame(VideoDecoder *video_decoder,
> > > > }
> > > >
> > > > SpiceStreamDataHeader *frame_op = spice_msg_in_parsed(frame_msg);
> > > > - if (frame_op->multi_media_time < decoder->last_mm_time) {
> > > > + if (compute_mm_time_diff(frame_op->multi_media_time,
> > > > decoder->last_mm_time) < 0) {
> > > > SPICE_DEBUG("new-frame-time < last-frame-time (%u < %u):"
> > > > " resetting stream, id %u",
> > > > frame_op->multi_media_time,
> > > > diff --git a/src/channel-display-mjpeg.c b/src/channel-display-mjpeg.c
> > > > index 722494e..1b7373b 100644
> > > > --- a/src/channel-display-mjpeg.c
> > > > +++ b/src/channel-display-mjpeg.c
> > > > @@ -195,15 +195,15 @@ static void mjpeg_decoder_schedule(MJpegDecoder
> > > > *decoder)
> > > > do {
> > > > if (frame_msg) {
> > > > SpiceStreamDataHeader *op =
> > > > spice_msg_in_parsed(frame_msg);
> > > > - if (time <= op->multi_media_time) {
> > > > - guint32 d = op->multi_media_time - time;
> > > > + gint32 time_diff =
> > > > compute_mm_time_diff(op->multi_media_time,
> > > > time);
> > > > + if (time_diff >= 0) {
> > > > decoder->cur_frame_msg = frame_msg;
> > > > - decoder->timer_id = g_timeout_add(d,
> > > > mjpeg_decoder_decode_frame, decoder);
> > > > + decoder->timer_id = g_timeout_add(time_diff,
> > > > mjpeg_decoder_decode_frame, decoder);
> > > > break;
> > > > }
> > > >
> > > > - SPICE_DEBUG("%s: rendering too late by %u ms (ts: %u,
> > > > mmtime:
> > > > %u), dropping ",
> > > > - __FUNCTION__, time - op->multi_media_time,
> > > > + SPICE_DEBUG("%s: rendering too late by %d ms (ts: %u,
> > > > mmtime:
> > > > %u), dropping ",
> > > > + __FUNCTION__, -time_diff,
> > > > op->multi_media_time, time);
> > > > stream_dropped_frame_on_playback(decoder->base.stream);
> > > > spice_msg_in_unref(frame_msg);
> > > > @@ -249,7 +249,9 @@ static gboolean
> > > > mjpeg_decoder_queue_frame(VideoDecoder
> > > > *video_decoder,
> > > > SpiceStreamDataHeader *last_op, *frame_op;
> > > > last_op = spice_msg_in_parsed(last_msg);
> > > > frame_op = spice_msg_in_parsed(frame_msg);
> > > > - if (frame_op->multi_media_time < last_op->multi_media_time) {
> > > > + gint32 time_diff =
> > > > + compute_mm_time_diff(frame_op->multi_media_time,
> > > > last_op->multi_media_time);
> > > > + if (time_diff < 0) {
> > > > /* This should really not happen */
> > > > SPICE_DEBUG("new-frame-time < last-frame-time (%u < %u):"
> > > > " resetting stream, id %u",
> > > > diff --git a/src/channel-display-priv.h b/src/channel-display-priv.h
> > > > index b9c08a3..3cd0727 100644
> > > > --- a/src/channel-display-priv.h
> > > > +++ b/src/channel-display-priv.h
> > > > @@ -141,6 +141,21 @@ void
> > > > stream_dropped_frame_on_playback(display_stream
> > > > *st);
> > > > void stream_display_frame(display_stream *st, SpiceMsgIn *frame_msg,
> > > > uint32_t width, uint32_t height, uint8_t *data);
> > > > uint32_t spice_msg_in_frame_data(SpiceMsgIn *frame_msg, uint8_t
> > > > **data);
> > > >
> > > > +/* Compute the difference between 2 multimedia times.
> > > > + * Multimedia times are subject to overflow so the check
> > > > + * for time1 < time2 does not always indicate that time2
> > > > + * happens after time1.
> > > > + * So define a function to compute the difference and
> > > > + * use as documentation for the code.
> > > > + */
> > > > +static inline gint32 compute_mm_time_diff(guint32 time1, guint32
> > > > time2)
> > > > +{
> > > > + /* Fast not fully portable version.
> > > > + * If you are paranoid
> > > > + * (gint32) ((((time1 - time2) & 0xffffffffu) ^ 0x80000000) -
> > > > 0x80000000u)
> > > > + * is more portable */
> > > > + return (gint32) (guint32) (time1 - time2);
> > > > +}
> > >
> > > Fast? I have hard time to understand how that above version would be
> > > faster,
> > > and I hope my compiler is smart enough to optimize that simple math and
> > > type
> > > cast.
> > >
> >
> > The comment document the code, what I said is that the code I wrote is
> > faster than the alternate (in the comment) version.
> > If is confusing I can remove the alternate version.
> >
> > > But what are you trying to compute here? A few examples / tests could
> > > help
> > > to
> > > reason about it.
> > >
> > > Why do you need a helper function if you can simply cast time1 - time2 to
> > > gint32 ?
> > >
> >
> > From the comment:
> >
> > * So define a function to compute the difference and
> > * use as documentation for the code.
> >
> > yes, the guint32 cast is not necessary however if you write in the code
> >
> > if ((gint32) (time1 - time2) < 0)
> >
> > is not clear why this is not the same as
> >
>
> Ok your solution is also fine to me.
>
> I think git blame could give that kind of information, or just a comment
> above the line.
>
yes, but this require to git blame every time you look at the source.
> Obviously we have similar code in mjpeg and gst units, so some refactoring
> would avoid the duplication, not only of the diff.
>
definitively, but there are also multiple arithmetic on the same
source.
> > if (time1 < time2)
> >
> > with the function people reading the code can understand that you need
> > some more effort to compute the difference and will look at the function
> > and documentation. You could document the inline of the function but
> > to make sure to avoid people miscomputing you'll have to have a comment
> > for every computation, easier to have in a single place.
> >
> > > Let's take an example where it overlaps (I don't know if the server
> > > really
> > > handles that).
> > >
> > > "now" is MAXUINT32, and new frame time is say 3: 3 - 4294967295 = 4.
> > > That's
> > > what is expected, no? However we would need to check if the new frame has
> > > a
> >
> > 3 - 4294967295 == 4 is not a portable assumption. Neither is
> > (uint32_t) 3 - (uint32_t) 4294967295 == 4
>
> Ok, I am not an expert in portability, do you have an example where this
> would give different results?
>
If int is 64 bit (uint32_t) 3 - (uint32_t) 4294967295 == -4294967292.
It seems weird but is due to the integral promotion rules.
Similar to
printf("%d\n", (uint16_t) 10 - (uint16_t) 16);
which give -6 (but maybe some people could point out that this result is
not portable either... from my knowledge it is).
> >
> > if time1 == UINT32_MAX and time2 == 3 I expect 4 while if
> > time2 == UINT32_MAX and time2 == 3 (this can happen for different reasons)
> > I expect -4. This make computation easier.
> >
> > I'll add this example.
>
> Yeah, some test cases would be useful to understand the problem and the
> solution.
>
Done
> Thanks
>
> >
> > > ts before the last frame received to check if we overlapped, I think
> > > (otherwise we should consider this frame as a late frame)
> > >
> > >
> >
Frediano
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