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

[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)
-- 
2.33.1