Re: [FFmpeg-devel] [PATCH 4/6] avcodec/mpegpicture: Don't copy unnecessarily, fix race

[Michael Niedermayer <michael@niedermayer.cc>]

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On Tue, Aug 16, 2022 at 10:38:55PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > On Sat, Aug 13, 2022 at 05:03:04PM +0200, Andreas Rheinhardt wrote:
> >> mpegvideo uses an array of Pictures and when it is done with using
> >> them, it only unreferences them incompletely: Some buffers are kept
> >> so that they can be reused lateron if the same slot in the Picture
> >> array is reused, making this a sort of a bufferpool.
> >> (Basically, a Picture is considered used if the AVFrame's buf is set.)
> >> Yet given that other pieces of the decoder may have a reference to
> >> these buffers, they need not be writable and are made writable using
> >> av_buffer_make_writable() when preparing a new Picture. This involves
> >> reading the buffer's data, although the old content of the buffer
> >> need not be retained.
> >>
> >> Worse, this read can be racy, because the buffer can be used by another
> >> thread at the same time. This happens for Real Video 3 and 4.
> >>
> >> This commit fixes this race by no longer copying the data;
> >> instead the old buffer is replaced by a new, zero-allocated buffer.
> >>
> >> (Here are the details of what happens with three or more decoding threads
> >> when decoding rv30.rm from the FATE-suite as happens in the rv30 test:
> >> The first decoding thread uses the first slot of its picture array
> >> to store its current pic; update_thread_context copies this for the
> >> second thread that decodes a P-frame. It uses the second slot in its
> >> Picture array to store its P-frame. This arrangement is then copied
> >> to the third decode thread, which decodes a B-frame. It uses the third
> >> slot in its Picture array for its current frame.
> >> update_thread_context copies this to the next thread. It unreferences
> >> the third slot containing the other B-frame and then it reuses this
> >> slot for its current frame. Because the pic array slots are only
> >> incompletely unreferenced, the buffers of the previous B-frame are
> >> still in there and they are not writable; in fact the previous
> >> thread is concurrently writing to them, causing races when making
> >> the buffer writable.)
> >>
> >> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> >> ---
> >>  libavcodec/mpegpicture.c | 16 +++++++++++++++-
> >>  1 file changed, 15 insertions(+), 1 deletion(-)
> > 
> > This causes a difference in some frames of: (i have not investigates why
> > just reporting as i noticed)
> > quite possibly thats just showing your bugfix in action
> > 
> > ./ffmpeg -y -bitexact -i fate/svq3/Vertical400kbit.sorenson3.mov -ps 50 -bf 1 -bitexact -an -qscale 5  -ss 40 -error_rate 4 -threads 1 /tmp/out4.avi 
> > ./ffmpeg -y -bitexact -v -1 -loglevel 0 -i /tmp/out4.avi -bitexact -vsync drop -f framecrc -
> > 
> > @@ -141,7 +141,7 @@
> >  0,         22,         22,        1,   115200, 0x4cc933e9
> >  0,         23,         23,        1,   115200, 0x693a40a9
> >  0,         24,         24,        1,   115200, 0x956e3b15
> > -0,         25,         25,        1,   115200, 0x61763ff4
> > +0,         25,         25,        1,   115200, 0xc9e53d1c
> >  0,         26,         26,        1,   115200, 0x5c5c3dfc
> >  0,         27,         27,        1,   115200, 0x7de641ea
> >  0,         28,         28,        1,   115200, 0xe6cc4136
> > @@ -187,7 +187,7 @@
> >  0,         68,         68,        1,   115200, 0x49dcbf4e
> >  0,         69,         69,        1,   115200, 0x1ea1c7d1
> >  0,         70,         70,        1,   115200, 0xdf77c67b
> > -0,         71,         71,        1,   115200, 0x7f6bd16d
> > +0,         71,         71,        1,   115200, 0x33d9d206
> >  0,         72,         72,        1,   115200, 0x5e37cb3a
> >  0,         73,         73,        1,   115200, 0x15abcda3
> >  0,         74,         74,        1,   115200, 0xbf4dcbd4
> > @@ -199,7 +199,7 @@
> >  0,         80,         80,        1,   115200, 0x17d1d667
> >  0,         81,         81,        1,   115200, 0x0c1fdf9c
> >  0,         82,         82,        1,   115200, 0x7eabde6b
> > -0,         83,         83,        1,   115200, 0xe623e7af
> > +0,         83,         83,        1,   115200, 0x3bf6e873
> >  0,         84,         84,        1,   115200, 0xf480dc82
> >  0,         85,         85,        1,   115200, 0x5fd6e098
> >  0,         86,         86,        1,   115200, 0xf520de95
> > @@ -211,7 +211,7 @@
> >  0,         92,         92,        1,   115200, 0x34cfe1c2
> >  0,         93,         93,        1,   115200, 0x1d94e1c3
> >  0,         94,         94,        1,   115200, 0x6d32e147
> > -0,         95,         95,        1,   115200, 0x7e40ee91
> > +0,         95,         95,        1,   115200, 0x09fbefd0
> >  0,         96,         96,        1,   115200, 0xa5f5eb43
> >  0,         97,         97,        1,   115200, 0x39b9ec3d
> >  0,         98,         98,        1,   115200, 0x3256ec18
> > 
> > [...]
> > 
> 
> Decoding this sample uses earlier values of mbskip_table. If I zero
> mbskip_table_buf in every ff_alloc_picture(), nothing changes due to
> this patch and (most importantly) the output of decoding does not depend
> upon the number of threads used (which it currently does -- with and
> without this patch).
> I don't know exactly where the bug is or whether there is a cheaper way
> to mitigate it than just to zero the buffer every time.
> I guess there are more such bugs affecting damaged samples than we are
> currently aware of.

Theres code in ff_er_frame_end() which cleans the mbskip_table
I dont remember it very much but it looks like that should not be optional
If this will not be run mbskip_table should maybe be zeroed on "alloc"

not sure this is the issue, iam just looking at the code 

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you fake or manipulate statistics in a paper in physics you will never
get a job again.
If you fake or manipulate statistics in a paper in medicin you will get
a job for life at the pharma industry.

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