[FFmpeg-cvslog] avcodec/pgxdec: Fix issue with negative linesizes

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Fri Apr 15 16:05:20 2022 +0200| [e4a26a64a875d4376cd78f77905a58c4f0238d85] | committer: Andreas Rheinhardt

avcodec/pgxdec: Fix issue with negative linesizes

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 at gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=e4a26a64a875d4376cd78f77905a58c4f0238d85
---

 libavcodec/pgxdec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/pgxdec.c b/libavcodec/pgxdec.c
index c9ada5afb5..30895b51ee 100644
--- a/libavcodec/pgxdec.c
+++ b/libavcodec/pgxdec.c
@@ -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)                                                                   \