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avcodec/wnv1: Use LE bitstream reader, avoid copying packet, fix memleak

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The Winnov WNV1 format is designed for a little-endian bitstream reader;
yet our decoder reversed every byte bitwise (in a buffer only
allocated for this purpose) to use a big-endian bitstream reader. This
commit stops this.

Two things needed to be done to achieve this: The codes in the table used
to initialize a VLC reader needed to be reversed bitwise (when
initializing a VLC in LE mode, it is expected that the first bit to be
read is in the least significant bit; with BE codes the first bit to be
read is the most significant bit of the code) and the following
expression needed to be adapted:

ff_reverse[get_bits(&w->gb, 8 - w->shift)]

But this is easy: When only the bits read are reversed, they coincide
with what a little-endian bitstream reader reads that reads the
original, not-reversed data. But ff_reverse always reverses the full
eight bits and this also performs a shift by (8 - (8 - w->shift)) on top
of reversing the bits read. So the above line needs to be changed to

get_bits(&w->gb, 8 - w->shift) << w->shift

and this also shows why the variable shift is named the way it is.

Finally, this also fixes a hypothetical memleak: For gigantic packets,
initializing a GetBitContext can fail and in this case, the buffer
containing the reversed data would leak.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
This commit is contained in:
Andreas Rheinhardt 2020-08-29 16:12:05 +02:00
parent 7c8cf40adc
commit 0166b1d1a6
1 changed files with 7 additions and 21 deletions
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28
libavcodec/wnv1.c
View File

@ -24,10 +24,10 @@
* Winnov WNV1 codec.
*/
#define BITSTREAM_READER_LE
#include "avcodec.h"
#include "get_bits.h"
#include "internal.h"
#include "mathops.h"
typedef struct WNV1Context {
@ -36,9 +36,9 @@ typedef struct WNV1Context {
} WNV1Context;
static const uint16_t code_tab[16][2] = {
{ 0x1FD, 9 }, { 0xFD, 8 }, { 0x7D, 7 }, { 0x3D, 6 }, { 0x1D, 5 }, { 0x0D, 4 }, { 0x005, 3 },
{ 0x17F, 9 }, { 0xBF, 8 }, { 0x5F, 7 }, { 0x2F, 6 }, { 0x17, 5 }, { 0x0B, 4 }, { 0x005, 3 },
{ 0x000, 1 },
{ 0x004, 3 }, { 0x0C, 4 }, { 0x1C, 5 }, { 0x3C, 6 }, { 0x7C, 7 }, { 0xFC, 8 }, { 0x1FC, 9 }, { 0xFF, 8 }
{ 0x01, 3 }, { 0x03, 4 }, { 0x07, 5 }, { 0x0F, 6 }, { 0x1F, 7 }, { 0x3F, 8 }, { 0x07F, 9 }, { 0xFF, 8 }
};
#define CODE_VLC_BITS 9
@ -50,7 +50,7 @@ static inline int wnv1_get_code(WNV1Context *w, int base_value)
int v = get_vlc2(&w->gb, code_vlc.table, CODE_VLC_BITS, 1);
if (v == 15)
return ff_reverse[get_bits(&w->gb, 8 - w->shift)];
return get_bits(&w->gb, 8 - w->shift) << w->shift;
else
return base_value + ((v - 7U) << w->shift);
}
@ -66,30 +66,17 @@ static int decode_frame(AVCodecContext *avctx,
unsigned char *Y,*U,*V;
int i, j, ret;
int prev_y = 0, prev_u = 0, prev_v = 0;
uint8_t *rbuf;
if (buf_size < 8 + avctx->height * (avctx->width/2)/8) {
av_log(avctx, AV_LOG_ERROR, "Packet size %d is too small\n", buf_size);
return AVERROR_INVALIDDATA;
}
rbuf = av_malloc(buf_size + AV_INPUT_BUFFER_PADDING_SIZE);
if (!rbuf) {
av_log(avctx, AV_LOG_ERROR, "Cannot allocate temporary buffer\n");
return AVERROR(ENOMEM);
}
memset(rbuf + buf_size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
if ((ret = ff_get_buffer(avctx, p, 0)) < 0) {
av_free(rbuf);
if ((ret = ff_get_buffer(avctx, p, 0)) < 0)
return ret;
}
p->key_frame = 1;
for (i = 8; i < buf_size; i++)
rbuf[i] = ff_reverse[buf[i]];
if ((ret = init_get_bits8(&l->gb, rbuf + 8, buf_size - 8)) < 0)
if ((ret = init_get_bits8(&l->gb, buf + 8, buf_size - 8)) < 0)
return ret;
if (buf[2] >> 4 == 6)
@ -127,7 +114,6 @@ static int decode_frame(AVCodecContext *avctx,
*got_frame = 1;
av_free(rbuf);
return buf_size;
}
@ -142,7 +128,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
code_vlc.table_allocated = 1 << CODE_VLC_BITS;
init_vlc(&code_vlc, CODE_VLC_BITS, 16,
&code_tab[0][1], 4, 2,
&code_tab[0][0], 4, 2, INIT_VLC_USE_NEW_STATIC);
&code_tab[0][0], 4, 2, INIT_VLC_USE_NEW_STATIC | INIT_VLC_LE);
return 0;
}