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avcodec/mjpegdec: Use correct number of codes for VLC tables

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Commit 1249698e1b424cff8e77e6a83cfdbc9d11e01aa7 made
ff_mjpeg_decode_dht() call build_vlc() with a wrong (too hight)
number of codes. The reason it worked is that the lengths of the extraneous
entries is initialized to zero and ff_init_vlc_sparse() ignores codes
with a length of zero. But using a too high number of codes was
nevertheless bad, because a) the assert in build_vlc() could have been
triggered (namely if the real amount of codes is 256) and b) the loop in
build_vlc() uses initialized data (leading to Valgrind errors [1]).
Furthermore, the old code spend CPU cycles in said loop although the
result won't be used anyway.

[1]: http://fate.ffmpeg.org/report.cgi?slot=x86_64-archlinux-gcc-valgrind&time=20201008025137

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
This commit is contained in:
Andreas Rheinhardt 2020-10-08 19:22:35 +02:00
parent ccca62ef99
commit a2ccfc6bb1
1 changed files with 4 additions and 4 deletions
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8
libavcodec/mjpegdec.c
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@ -78,7 +78,7 @@ static int build_vlc(VLC *vlc, const uint8_t *bits_table,
build_huffman_codes(huff_size, huff_code, bits_table);
for (i = 0; i < 256; i++) {
for (i = 0; i < nb_codes; i++) {
huff_sym[i] = val_table[i] + 16 * is_ac;
if (is_ac && !val_table[i])
@ -295,15 +295,15 @@ int ff_mjpeg_decode_dht(MJpegDecodeContext *s)
/* build VLC and flush previous vlc if present */
ff_free_vlc(&s->vlcs[class][index]);
av_log(s->avctx, AV_LOG_DEBUG, "class=%d index=%d nb_codes=%d\n",
class, index, n + 1);
class, index, n);
if ((ret = build_vlc(&s->vlcs[class][index], bits_table, val_table,
n + 1, 0, class > 0)) < 0)
n, 0, class > 0)) < 0)
return ret;
if (class > 0) {
ff_free_vlc(&s->vlcs[2][index]);
if ((ret = build_vlc(&s->vlcs[2][index], bits_table, val_table,
n + 1, 0, 0)) < 0)
n, 0, 0)) < 0)
return ret;
}