[FFmpeg-devel] [PATCH] Exception when frame is set NULL

[Andreas Rheinhardt]

Yu Yang:
> fftools/ffmpegc  When `ost->last_frame` is NULL, 'SEGV' occurs when accessing its pts.
>                  
> libavutil/framec `ost->last_frame` may be set NULL by av_frame_alloc(). In this situation，
>                  av_frame_unref() and av_frame_free() do nothing. Frame is not released.
> 
> ```c
> // in fftools/ffmpeg.c:1145
> 1145 static void do_video_out(OutputFile *of, ...)
> 
> 1148 {
>       ...
>              // `ost->last_frame` is NULL.
> 1272         av_log(NULL, AV_LOG_VERBOSE,
> 1273                "*** dropping frame %d from stream %d at ts %"PRId64"\n",
> 1274                ost->frame_number, ost->st->index, ost->last_frame->pts);
>       ...
> 1421     if (!ost->last_frame)
>              // `ost->last_frame` may be set NULL here.
> 1422         ost->last_frame = av_frame_alloc();
>       ...
> 
> 1433 }
> ```
> 
> coredump backtrace info:
> ==7192==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000088 (pc 0x0000005e87e2 bp 0x7fff84f0ffb0 sp 0x7fff84f0f020 T0)
> ==7192==The signal is caused by a READ memory access.
> ==7192==Hint: address points to the zero page.
>     #0 0x5e87e2 in do_video_out /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:1274:68
>     #1 0x5df341 in reap_filters /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:1548:25
>     #2 0x5d08b7 in transcode_from_filter /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:4644:15
>     #3 0x59e557 in transcode_step /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:4729:20
>     #4 0x593970 in transcode /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:4805:15
>     #5 0x58f7a4 in main /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:5010:9
>     #6 0x7f0fa9d900b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
>     #7 0x42033d in _start (/home/r1/ffmpeg/ffmpeg_4.4.1+0x42033d)
> 
> Reported-by: TOTE Robot <oslab at tsinghua.edu.cn>
> Signed-off-by: Yu Yang <young_chelsea at 163.com>
> ---
>  fftools/ffmpeg.c  | 7 ++++---
>  libavutil/frame.c | 9 ++++-----
>  2 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
> index cfb04d5eff..cade05f762 100644
> --- a/fftools/ffmpeg.c
> +++ b/fftools/ffmpeg.c
> @@ -1265,9 +1265,10 @@ static void do_video_out(OutputFile *of,
>  
>      if (nb0_frames == 0 && ost->last_dropped) {
>          nb_frames_drop++;
> -        av_log(NULL, AV_LOG_VERBOSE,
> -               "*** dropping frame %d from stream %d at ts %"PRId64"\n",
> -               ost->frame_number, ost->st->index, ost->last_frame->pts);
> +        if (ost->last_frame)
> +            av_log(NULL, AV_LOG_VERBOSE,
> +                   "*** dropping frame %d from stream %d at ts %"PRId64"\n",
> +                   ost->frame_number, ost->st->index, ost->last_frame->pts);
>      }
>      if (nb_frames > (nb0_frames && ost->last_dropped) + (nb_frames > nb0_frames)) {
>          if (nb_frames > dts_error_threshold * 30) {
> diff --git a/libavutil/frame.c b/libavutil/frame.c
> index d4d3ad6988..9c866320a7 100644
> --- a/libavutil/frame.c
> +++ b/libavutil/frame.c
> @@ -111,11 +111,10 @@ AVFrame *av_frame_alloc(void)
>  
>  void av_frame_free(AVFrame **frame)
>  {
> -    if (!frame || !*frame)
> -        return;
> -
> -    av_frame_unref(*frame);
> -    av_freep(frame);
> +    if (*frame)
> +        av_frame_unref(*frame);
> +    if (frame)
> +        av_freep(frame);
>  }
>  
>  static int get_video_buffer(AVFrame *frame, int align)
> 

This change to frame.c is also completely wrong; this frame should
probably not be constantly allocated and freed and the code at lines
1422-1428 should actually error out in case of allocation error.

- Andreas