[FFmpeg-devel] fftools/ffmpeg_optc AVDictionary **opts, If memory allocation fails,

[Andreas Rheinhardt]

Yy:
> 
> 
>> 2021年12月3日 下午8:06，Yy <young_chelsea at 163.com> 写道：
>>
>>
>>
>>> 2021年12月3日 下午5:42，Andreas Rheinhardt <andreas.rheinhardt at outlook.com <mailto:andreas.rheinhardt at outlook.com>> 写道：
>>>
>>> Yu Yang:
>>>> Opts is assigned by setup_find_stream_info_opts(). Opts may be NULL.
>>>> This situation is compatible in avformat_find_stream_info(). 
>>>> Before av_dict_free(), the necessary checks were ignored.
>>>>
>>>> // in fftools/ffmpeg_opt.c:1266
>>>> 1067 static int open_input_file(OptionsContext *o, const char *filename)
>>>> 1068 {
>>>>     ...
>>>> 1191         AVDictionary **opts = setup_find_stream_info_opts(ic, o->g->codec_opts);
>>>>     ...
>>>> 1196         ret = avformat_find_stream_info(ic, opts);
>>>> 1197 
>>>> 1198         for (i = 0; i < orig_nb_streams; i++)
>>>> 1199             av_dict_free(&opts[i]);
>>>>     ...
>>>> 1342 }
>>>> ```
>>>> ```c
>>>> // in libavutil/dict.c:203
>>>> 203 void av_dict_free(AVDictionary **pm)
>>>> 204 {
>>>> 205     AVDictionary *m = *pm;
>>>>     ...
>>>> 215 }
>>>>
>>>> coredump backtrace info:
>>>> ==6235==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000006ba9c2f bp 0x7ffc3d5baa30 sp 0x7ffc3d5ba9a0 T0)
>>>> ==6235==The signal is caused by a READ memory access.
>>>> ==6235==Hint: address points to the zero page.
>>>>   #0 0x6ba9c2f in av_dict_free /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/libavutil/dict.c:205:23
>>>>   #1 0x4ce5ac in open_input_file /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg_opt.c:1199:13
>>>>   #2 0x4c9dc0 in open_files /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg_opt.c:3338:15
>>>>   #3 0x4c9295 in ffmpeg_parse_options /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg_opt.c:3378:11
>>>>   #4 0x58f241 in main /home/r1/ffmpeg/ffmpeg-4.4.1/build/src/fftools/ffmpeg.c:4988:11
>>>>   #5 0x7fe35197f0b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
>>>>   #6 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_opt.c | 9 +++++----
>>>> 1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
>>>> index a27263b879..a9fc54d948 100644
>>>> --- a/fftools/ffmpeg_opt.c
>>>> +++ b/fftools/ffmpeg_opt.c
>>>> @@ -1197,10 +1197,11 @@ static int open_input_file(OptionsContext *o, const char *filename)
>>>>        /* If not enough info to get the stream parameters, we decode the
>>>>           first frames to get it. (used in mpeg case for example) */
>>>>        ret = avformat_find_stream_info(ic, opts);
>>>> -
>>>> -        for (i = 0; i < orig_nb_streams; i++)
>>>> -            av_dict_free(&opts[i]);
>>>> -        av_freep(&opts);
>>>> +        if (opts){
>>>> +            for (i = 0; i < orig_nb_streams; i++)
>>>> +                av_dict_free(&opts[i]);
>>>> +            av_freep(&opts);
>>>> +        }
>>>>
>>>>        if (ret < 0) {
>>>>            av_log(NULL, AV_LOG_FATAL, "%s: could not find codec parameters\n", filename);
>>>>
>>>
>>> You should instead check setup_find_stream_info_opts() (either only call
>>> it if orig_nb_streams is > 0 or modify it to return an error code given
>>> that it can currently return NULL even on nonerror).
>> Thanks for your advice, i will try to fix it again.
>>> - Andreas
> Hi, I try to fix this problem lastday and it not work. Today, I refer to your suggestion 
> above to modify. But I think may be you misunderstood this bug. Because of no alloc memory for stream opts, ‘&opts[I]’ free caused crash.  And if orig_nb_streams == 0, 
> crash won’t happen. So I think it is unnecessary to fix it call setup_find_stream_info_opts()
> If orig_nb_streams is > 0. Opts == NULL when  orig_nb_streams == 0 or no alloc memory. 
> I don’t think it is error. Because avformat_find_stream_info() allow opts == NULL. 
> But we need to check if opts == NULL before releasing. May be this fix is right?
> How do you think?   
> Thx.
> -Yu Yang

We don't need to check for opts == NULL before releasing if we actually
checked that this array has been properly allocated in case it needs to
be allocated; in case it could not be allocated, we should not call
avformat_find_stream_info() at all.

- Andreas