[FFmpeg-devel] [PATCH 2/3] avcodec/fmvc: buffer size is stride based not 4*width

[Michael Niedermayer]

On Fri, Sep 02, 2022 at 06:48:57PM +0200, Paul B Mahol wrote:
> On Fri, Sep 2, 2022 at 6:32 PM Michael Niedermayer <michael at niedermayer.cc>
> wrote:
> 
> > On Mon, Jun 13, 2022 at 09:13:19PM +0200, Michael Niedermayer wrote:
> > > On Mon, Jun 13, 2022 at 12:10:44PM +0200, Paul B Mahol wrote:
> > > > On Mon, Jun 13, 2022 at 11:48 AM Anton Khirnov <anton at khirnov.net>
> > wrote:
> > > >
> > > > > Quoting Paul B Mahol (2022-06-13 11:34:44)
> > > > > > On Mon, Jun 13, 2022 at 11:10 AM Anton Khirnov <anton at khirnov.net>
> > > > > wrote:
> > > > > >
> > > > > > > Quoting Paul B Mahol (2022-06-13 10:04:04)
> > > > > > > > On Sat, Jun 11, 2022 at 4:55 PM Michael Niedermayer <
> > > > > > > michael at niedermayer.cc>
> > > > > > > > wrote:
> > > > > > > >
> > > > > > > > > On Sat, Jun 11, 2022 at 10:47:57AM +0200, Paul B Mahol wrote:
> > > > > > > > > > Have you actually tested this "change" ?
> > > > > > > > >
> > > > > > > > > On every file i found
> > > > > > > > > 6-methyl-5-hepten-2-one-CC-db_small.avi
> > > > > > > > > fmvcVirtualDub_small.avi
> > > > > > > > > skrzyzowanie4.avi
> > > > > > > > > fmvc-poc.avi
> > > > > > > > >
> > > > > > > > > are there any other files i should test it on ?
> > > > > > > > >
> > > > > > > >
> > > > > > > > Yes, the ones where stride != width.
> > > > > > >
> > > > > > > Give examples of such files then. And add more tests.
> > > > > > >
> > > > > > > You really should try to be more helpful if you care about this
> > code
> > > > > > > working.
> > > > > >
> > > > > >
> > > > > > Code works perfectly from start. There are always attempts to
> > break it.
> > > > > > Your attempts to belittle my work are futile.
> > > > >
> > > > > Perfect code should live in an external repository that is locked
> > > > > against modification.
> > > > >
> > > > > The ffmpeg repository is only for imperfect code that evolves with
> > time,
> > > > > and so requires changes.
> > > > >
> > > > >
> > > > I dunno what Michael attempts to fix. Decoder works fine with valid
> > files.
> > > > I doubt that encoder would encode random bytes or padding into valid
> > file
> > > > bitstream.
> > >
> > > the stride*4 / width*4 change was because of 2 things.
> > > first with AV_PIX_FMT_BGR24 the data stored is not width*4
> > >
> > > stride is in units of 4 bytes for some reason, so stride*4
> > > fixes this
> > > The 2nd issue is that the code addresses it by "s->stride * 4"
> > > so the buffer allocation should be stride*4 if we belive the
> > > other code is correct
> > >
> > >         src = s->buffer;
> > > ...
> > >         for (y = 0; y < avctx->height; y++) {
> > > ...
> > >             src += s->stride * 4;
> > >
> > > width*4 works because its bigger than stride*4 for BGR24 which is what
> > all
> > > samples i have use.
> > >
> > > also
> > >         ssrc = s->buffer;
> > >         ...
> > >         for (y = 0; y < avctx->height; y++) {
> > >             ...
> > >             ssrc += s->stride * 4;
> > > and
> > >         dst = (uint32_t *)s->buffer;
> > >
> > >         for (block = 0, y = 0; y < s->yb; y++) {
> > >             int block_h = s->blocks[block].h;
> > >             uint32_t *rect = dst;
> > >
> > >             for (x = 0; x < s->xb; x++) {
> > >                 int block_w = s->blocks[block].w;
> > >                 uint32_t *row = dst;
> > >
> > >                 block_h = s->blocks[block].h;
> > >                 if (s->blocks[block].xor) {
> > >                     for (k = 0; k < block_h; k++) {
> > >                         uint32_t *column = dst;
> > >                         for (l = 0; l < block_w; l++)
> > >                             *dst++ ^= *src++;
> > >                         dst = &column[s->stride];
> > >                     }
> > >                 }
> > >                 dst = &row[block_w];
> > >                 ++block;
> > >             }
> > >             dst = &rect[block_h * s->stride];
> > >         }
> > >
> > > Again, if you have fmvc files with more odd widths or other pixel formats
> > > these would be very welcome. I can just say the code as is in git is
> > wrong
> > > and the buffer size as is in git is wrong. I noticed this when i added
> > > a check to see if the buffer is only partly filled and realized its
> > > always partly filled even when the whole image is actually touched
> >
> > If there are no objections aka noone sees a bug in this then id like
> > to apply this
> >
> 
> Since when are partially filled buffers are bad thing?

- waste of memory
- breaks subsequent patch
- width and stride relate this way: 
  s->stride = (avctx->width * avctx->bits_per_coded_sample + 31) / 32;
  is width always bigger or equal ?
  If not we might be accessing outside the array because access uses
  stride, allocation width

Or one line awnser
Since when is it a good thing to mismatch allocation and access?

thx

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

Nations do behave wisely once they have exhausted all other alternatives. 
-- Abba Eban
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