[FFmpeg-devel] [PATCH 03/23] avformat/matroskaenc: Use random TrackUID
Andreas Rheinhardt
andreas.rheinhardt at gmail.com
Wed Nov 27 09:25:13 EET 2019
On Mon, Nov 25, 2019 at 2:57 PM Michael Niedermayer <michael at niedermayer.cc>
wrote:
> On Sun, Nov 24, 2019 at 11:05:03PM +0100, Andreas Rheinhardt wrote:
> > On Sun, Nov 24, 2019 at 8:28 PM Michael Niedermayer
> <michael at niedermayer.cc>
> > wrote:
> >
> > > On Sun, Nov 24, 2019 at 09:08:00AM +0000, Andreas Rheinhardt wrote:
> > > > Michael Niedermayer:
> > > > > On Wed, Nov 06, 2019 at 03:49:02AM +0100, Andreas Rheinhardt wrote:
> > > > >> Up until now, the TrackUID of a Matroska track which is supposed
> to be
> > > > >> random was not random at all: It always coincided with the
> TrackNumber
> > > > >> which is usually the 1-based index of the corresponding stream in
> the
> > > > >> array of AVStreams. This has been changed: It is now set via an
> AVLFG
> > > > >> if AVFMT_FLAG_BITEXACT is not set. Otherwise it is set like it is
> set
> > > > >> now (the only change happens if an explicit track number has been
> > > > >> choosen via dash_track_number, because the system used in the
> normal
> > > > >> situation is now used, too). In particular, no FATE tests need to
> be
> > > > >> updated.
> > > > >>
> > > > >> This also fixes a bug in case the dash_track_number option was
> used:
> > > > >> In this case the TrackUID was set to the track number, but the
> tags
> > > were
> > > > >> written with a TagTrackUID simply based upon the index, so that
> the
> > > tags
> > > > >> didn't apply to the track they ought to apply to.
> > > > >>
> > > > >> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> > > > >> ---
> > > > >> mkv_get_uid() might be overkill, but I simply wanted to be sure
> that
> > > > >> there are no collisions.
> > > > >>
> > > > >> libavformat/matroskaenc.c | 65
> > > ++++++++++++++++++++++++++++++++++-----
> > > > >> 1 file changed, 57 insertions(+), 8 deletions(-)
> > > > >>
> > > > >> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> > > > >> index de57e474be..b87d15b7ff 100644
> > > > >> --- a/libavformat/matroskaenc.c
> > > > >> +++ b/libavformat/matroskaenc.c
> > > > >> @@ -94,6 +94,7 @@ typedef struct mkv_cues {
> > > > >> typedef struct mkv_track {
> > > > >> int write_dts;
> > > > >> int has_cue;
> > > > >> + uint32_t uid;
> > > > >> int sample_rate;
> > > > >> int64_t sample_rate_offset;
> > > > >> int64_t last_timestamp;
> > > > >> @@ -1199,8 +1200,7 @@ static int mkv_write_track(AVFormatContext
> *s,
> > > MatroskaMuxContext *mkv,
> > > > >> track = start_ebml_master(pb, MATROSKA_ID_TRACKENTRY, 0);
> > > > >> put_ebml_uint (pb, MATROSKA_ID_TRACKNUMBER,
> > > > >> mkv->is_dash ? mkv->dash_track_number : i +
> 1);
> > > > >> - put_ebml_uint (pb, MATROSKA_ID_TRACKUID,
> > > > >> - mkv->is_dash ? mkv->dash_track_number : i +
> 1);
> > > > >> + put_ebml_uint (pb, MATROSKA_ID_TRACKUID, mkv->tracks[i].uid);
> > > > >> put_ebml_uint (pb, MATROSKA_ID_TRACKFLAGLACING , 0); // no
> > > lacing (yet)
> > > > >>
> > > > >> if ((tag = av_dict_get(st->metadata, "title", NULL, 0)))
> > > > >> @@ -1650,7 +1650,8 @@ static int mkv_write_tags(AVFormatContext
> *s)
> > > > >> if (!mkv_check_tag(st->metadata,
> > > MATROSKA_ID_TAGTARGETS_TRACKUID))
> > > > >> continue;
> > > > >>
> > > > >> - ret = mkv_write_tag(s, st->metadata,
> > > MATROSKA_ID_TAGTARGETS_TRACKUID, i + 1);
> > > > >> + ret = mkv_write_tag(s, st->metadata,
> > > MATROSKA_ID_TAGTARGETS_TRACKUID,
> > > > >> + mkv->tracks[i].uid);
> > > > >> if (ret < 0) return ret;
> > > > >> }
> > > > >>
> > > > >> @@ -1658,13 +1659,15 @@ static int mkv_write_tags(AVFormatContext
> *s)
> > > > >> for (i = 0; i < s->nb_streams; i++) {
> > > > >> AVIOContext *pb;
> > > > >> AVStream *st = s->streams[i];
> > > > >> + mkv_track *track = &mkv->tracks[i];
> > > > >> ebml_master tag_target;
> > > > >> ebml_master tag;
> > > > >>
> > > > >> if (st->codecpar->codec_type ==
> AVMEDIA_TYPE_ATTACHMENT)
> > > > >> continue;
> > > > >>
> > > > >> - mkv_write_tag_targets(s,
> > > MATROSKA_ID_TAGTARGETS_TRACKUID, i + 1, &tag_target);
> > > > >> + mkv_write_tag_targets(s,
> MATROSKA_ID_TAGTARGETS_TRACKUID,
> > > > >> + track->uid, &tag_target);
> > > > >> pb = mkv->tags_bc;
> > > > >>
> > > > >> tag = start_ebml_master(pb, MATROSKA_ID_SIMPLETAG,
> 0);
> > > > >> @@ -1863,10 +1866,6 @@ static int
> mkv_write_header(AVFormatContext *s)
> > > > >> version = 4;
> > > > >> }
> > > > >>
> > > > >> - mkv->tracks = av_mallocz_array(s->nb_streams,
> > > sizeof(*mkv->tracks));
> > > > >> - if (!mkv->tracks) {
> > > > >> - return AVERROR(ENOMEM);
> > > > >> - }
> > > > >> ebml_header = start_ebml_master(pb, EBML_ID_HEADER,
> > > MAX_EBML_HEADER_SIZE);
> > > > >> put_ebml_uint (pb, EBML_ID_EBMLVERSION , 1);
> > > > >> put_ebml_uint (pb, EBML_ID_EBMLREADVERSION , 1);
> > > > >> @@ -2667,8 +2666,42 @@ static int webm_query_codec(enum AVCodecID
> > > codec_id, int std_compliance)
> > > > >> return 0;
> > > > >> }
> > > > >>
> > > > >> +static uint32_t mkv_get_uid(const mkv_track *tracks, int i,
> AVLFG *c)
> > > > >> +{
> > > > >> + uint32_t uid;
> > > > >> +
> > > > >> + for (int j = 0, k; j < 5; j++) {
> > > > >> + uid = av_lfg_get(c);
> > > > >> + if (!uid)
> > > > >> + continue;
> > > > >> + for (k = 0; k < i; k++) {
> > > > >> + if (tracks[k].uid == uid) {
> > > > >> + /* Was something wrong with our random seed? */
> > > > >> + av_lfg_init(c, av_get_random_seed());
> > > > >> + break;
> > > > >> + }
> > > > >> + }
> > > > >> + if (k == i)
> > > > >> + return uid;
> > > > >> + }
> > > > >> +
> > > > >> + /* Test the numbers from 1 to i. */
> > > > >> + for (int j = 1, k; j < i + 1; j++) {
> > > > >> + for (k = 0; k < i; k++) {
> > > > >> + if (tracks[k].uid == j)
> > > > >> + break;
> > > > >> + }
> > > > >> + if (k == i)
> > > > >> + return j;
> > > > >> + }
> > > > >> + /* Return i + 1. It can't coincide with another uid. */
> > > > >> + return i + 1;
> > > > >> +}
> > > > >> +
> > > > >> static int mkv_init(struct AVFormatContext *s)
> > > > >> {
> > > > >> + MatroskaMuxContext *mkv = s->priv_data;
> > > > >> + AVLFG c;
> > > > >> int i;
> > > > >>
> > > > >> if (s->nb_streams > MAX_TRACKS) {
> > > > >> @@ -2697,7 +2730,23 @@ static int mkv_init(struct AVFormatContext
> *s)
> > > > >> s->internal->avoid_negative_ts_use_pts = 1;
> > > > >> }
> > > > >>
> > > > >> + mkv->tracks = av_mallocz_array(s->nb_streams,
> > > sizeof(*mkv->tracks));
> > > > >> + if (!mkv->tracks) {
> > > > >> + return AVERROR(ENOMEM);
> > > > >> + }
> > > > >> +
> > > > >
> > > > >> + if (!(s->flags & AVFMT_FLAG_BITEXACT))
> > > > >> + av_lfg_init(&c, av_get_random_seed());
> > > > >
> > > > > would there be a disadvantage if the configuration of a stream /
> its
> > > content
> > > > > is used a seed ?
> > > > > That way it would be more deterministic
> > > > >
> > > > I see several disadvantages:
> > > > 1. If one duplicates a stream (i.e. muxes it twice in the same file),
> > > > they would get the same uid (unless one takes additional measures).
> > >
> > > If you took the configuration of all streams as a single seed that
> would
> > > not
> > > happen
> > >
> > >
> > > > 2. If the packet content is used, one can determine the uid only
> after
> > > > having received a packet. This would mean that one has to postpone
> > > > writing the Tracks element until one has a packet and the same goes
> > > > for the tags given that the uid is used to convey what they apply to.
> > > > One could solve this problem in the seekable, non-livestreaming case
> > > > by reserving size to be overwritten later, but this is nontrivial and
> > > > I'd like to avoid that.
> > > > 3. If the configuration record is used, then you won't get anything
> > > > remotely unique: Using the same encoder settings will likely produce
> > > > files with identical uids.
> > > >
> > > > Furthermore, this approach means that I can reuse the code for both
> > > > attachments as well as tracks. (See the next patch in this series.)
> > >
> > > fair enough, if its considered that avoiding some complexity is more
> > > important than deterministic output
> > >
> >
> > Just to be sure: The output will be deterministic if the bitexact flag is
> > set.
> > I don't see a reason for deterministic output if it is not set.
>
> the bitexact flag is used for multiple things
> 1. produce the same output between platforms and minor code changes
> 2. produce the same output when running the exact same binary with the
> exact
> same input
>
> This has a few problems
> code pathes disabled because of 1, cannot be tested easily because
> if bitexact is enabled the code to be tested is disabled
> if bitexact is disabled random values are put in the file making it not
> as easy to see if the code under test produces the same results each time
>
> Another problem is, obscuring problems, because
> if with a deterministic binary you get different output on 2 runs then you
> know you have a bug, a race condition or maybe a uninitialized variable or
> such.
> But with random values stored all over this is no longer vissible unless
> one uses bitexact which most people dont do by default
> so this could cause a decrease in chance detections of some bugs
>
> The Matroska muxer only behaves differently when in bitexact mode in two
ways: The UIDs are chosen deterministically (or in case of the SegmentUID:
not written at all) and certain strings that usually contain the
libavformat version no longer do so. All these things are in the file
header. There are no (and won't be) random values stored all over the
place. The code paths are very small and so I don't think that they will
obscure any bug.
I have also added a patch to store the UIDs that can be random on a fixed
size (namely eight, because the UIDs are now eight bytes long). This even
ensures that there is no offset/size difference between the output of two
files both produced with the same settings in non-bitexact mode (this can
currently happen with attachments if the upper byte of the random UID
vanishes). The differences between these files will be confined to the
header (the UIDs as well as the CRC-32 elements of the level 1-master
elements that contain the UIDs).
>
> [...]
> > > seed collisions are not at all impossible at 32bits
> > > also av_get_random_seed() can be expensive on some platforms so
> > > i think there should be a single place calling it as needed and not
> > > throwing the obtained entropy away.
> > >
> >
> > I didn't deem this important performance-wise, as this is only done a few
> > times during init.
> > But if you want to, I can reuse the AVLFG from mkv_init for the segment
> UID.
>
> get_generic_seed() is not really a fast function so when its used IIRC
> on some platforms its not really fast.
> And if in a usecase someone remuxes MANY small matroska files, i would
> suspect this could be noticable
>
> I changed the code so that av_get_random_seed() is only called once. (This
of course clashes with your "Either way if you need X bits of entropy in
the muxer there should be X/32 calls to av_get_random_seed().")
>
> [...]
> >
> >
> > > and, this
> > > + /* Test the numbers from 1 to i. */
> > > + for (int j = 1, k; j < i + 1; j++) {
> > > + for (k = 0; k < i; k++) {
> > > + if (tracks[k].uid == j)
> > > + break;
> > > + }
> > > + if (k == i)
> > > + return j;
> > > + }
> > >
> > > just smells "wrong"
> > >
> > > This code would only be executed if using AVLFG would repeatedly lead
> to
> > collisions;
>
> yes but this doesnt smell right
> if your random number generator produces no random numbers i dont think
> adding +1 is the solution.
> I mean if that really happens, theres some bug either in LFG the seed
> generation
> or in how it is used.
>
> Yes, this code was only intended to be run if the random number generator
is completely buggy. But it seems that this can unfortunately not ruled out
completely (
https://www.phoronix.com/scan.php?page=news_item&px=AMD-RdRand-Disable-15h-16h
).
Anyway, I have now changed the last part a bit so that it does not simply
count.
>
> > it is just there to guarantee that the UIDs are different, but actually
> it
> > is not intended to be run.
> >
> >
> > > if you want a simple non repeating UID, just count 0,1,2,3 for each
> > > and put that in the unused upper 32bit for example
> > >
> > > or you can even ditch LFG and use something that findamentgally
> generates
> > > non repeating output.
> > > UID[i] = seed + i
> > > comes to mind
> > > or if theres a reason why that is not good then you can also do
> something
> > > like
> > > UID[i] = (seed + i*(seed2|1)) & 0xFFFFFFFFFFFFFFFF;
> > >
> > > but maybe theres a reason behind the more complex UID generation that
> > > iam missing ?
> > >
> > > The reason I've chosen this is that this is what has already been used
> > for the UIDs of attachments, so I simply wanted to reuse this, while also
> > fixing its defects (namely that it does not ensure that the resulting
> UIDs
> > are actually distinct).
>
> sure it all should be using the same but i am not sure what is used is a
> good
> choice as it requires these extra duplicate checks
>
> I don't think that this code is either complicated or extensive.
I hope I have addressed all your concerns.
- Andreas
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