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avcodec/pgxdec: Fix issue with negative linesizes

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The PGX decoder accesses the lines via code like
(PIXEL*)frame->data[0] + i*frame->linesize[0]/sizeof(PIXEL)
where PIXEL is a macro parameter. This code has issues with negative
linesizes, because the type of sizeof(PIXEL) is size_t, so
that on common systems i*linesize/sizeof(PIXEL) will
always be an unsigned type that is very large in case linesize is
negative. This happens to work*, but it is undefined behaviour
and e.g. leads to "src/libavcodec/pgxdec.c:114:1: runtime error:
addition of unsigned offset to 0x7efe9c2b7040 overflowed to 0x7efe9c2b6040"
errors from UBSAN.
Fix this by using (PIXEL*)(frame->data[0] + i*frame->linesize[0]).
This is allowed because linesize has to be suitably aligned.

*: Converting a negative int to size_t works by adding SIZE_MAX + 1
to the number, so that the result is off by (SIZE_MAX + 1) /
sizeof(PIXEL). Converting the pointer arithmetic (performed on PIXELs)
back to ordinary pointers is tantamount to multiplying by sizeof(PIXEL),
so that the result is off by SIZE_MAX + 1; but SIZE_MAX + 1 == 0
for the underlying pointers.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
This commit is contained in:
Andreas Rheinhardt 2022-04-15 16:05:20 +02:00
parent 67f7960505
commit e4a26a64a8
1 changed files with 1 additions and 1 deletions
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2
libavcodec/pgxdec.c
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@ -97,7 +97,7 @@ error:
{ \
int i, j; \
for (i = 0; i < height; i++) { \
PIXEL *line = (PIXEL*)frame->data[0] + i*frame->linesize[0]/sizeof(PIXEL); \
PIXEL *line = (PIXEL*)(frame->data[0] + i * frame->linesize[0]); \
for (j = 0; j < width; j++) { \
unsigned val; \
if (sign) \