[FFmpeg-devel] [PATCH] libavcodec/avpacketc packet release exception

[Yu Yang]

'pkt' and '*pkt' should be judged separately for release. 
SEGV by a READ memory access (address points to the zero page) 

```c
// in fftools/ffmpeg.c:515
 515 static void ffmpeg_cleanup(int ret)
 516 {
       ...

 626     for (i = 0; i < nb_input_files; i++) {
 627         avformat_close_input(&input_files[i]->ctx);
             // `input_files[0] == NULL` but `&input_files[0]->pkt == 0x68`, see below;
 628         av_packet_free(&input_files[i]->pkt);
 629         av_freep(&input_files[i]);
 630     }

       ...
 674 }
```
```c
// in libavcodec/avpacket.c:75
 75 void av_packet_free(AVPacket **pkt)
 76 {
       // pkt == 0x68, `*pkt` cause `SEGV`.
 77     if (!pkt || !*pkt)
 78         return;
 79 
 80     av_packet_unref(*pkt);
 81     av_freep(pkt);
 82 }
```

coredump backtrace info:
==4536==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000068 (pc 0x000002a794d0 bp 0x7ffdf587a910 sp 0x7ffdf587a8e0 T0)
==4536==The signal is caused by a READ memory access.
==4536==Hint: address points to the zero page.
    #0 0x2a794d0 in av_packet_free /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/libavcodec/avpacket.c:77:18
    #1 0x592107 in ffmpeg_cleanup /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:628:9
    #2 0x55fe0e in exit_program /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/cmdutils.c:136:9
    #3 0x4cfcd4 in open_input_file /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg_opt.c:1268:9  exit_program
    #4 0x4c9dc0 in open_files /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg_opt.c:3338:15     
    #5 0x4c9295 in ffmpeg_parse_options /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg_opt.c:3378:11 open_file
    #6 0x58f241 in main /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:4988:11
    #7 0x7f122a83d0b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #8 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>
---
 libavcodec/avpacket.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/libavcodec/avpacket.c b/libavcodec/avpacket.c
index d8d8fef3b9..8348bec581 100644
--- a/libavcodec/avpacket.c
+++ b/libavcodec/avpacket.c
@@ -74,11 +74,10 @@ AVPacket *av_packet_alloc(void)
 
 void av_packet_free(AVPacket **pkt)
 {
-    if (!pkt || !*pkt)
-        return;
-
-    av_packet_unref(*pkt);
-    av_freep(pkt);
+    if (*pkt)
+        av_packet_unref(*pkt);
+    if (pkt)
+        av_freep(pkt);
 }
 
 static int packet_alloc(AVBufferRef **buf, int size)
-- 
2.33.1