[FFmpeg-devel] [PATCH 1/2] avformat/utils: free existing extradata before trying to allocate a new one

James Almer jamrial at gmail.com
Wed Mar 7 04:31:00 EET 2018


On 3/6/2018 11:29 PM, Michael Niedermayer wrote:
> On Tue, Mar 06, 2018 at 11:03:55PM -0300, James Almer wrote:
>> On 3/6/2018 10:47 PM, Michael Niedermayer wrote:
>>> On Tue, Mar 06, 2018 at 01:42:36AM -0300, James Almer wrote:
>>>> This prevents leaks in the rare cases the function is called when extradata
>>>> already exists.
>>>>
>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>> ---
>>>>  libavformat/utils.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/libavformat/utils.c b/libavformat/utils.c
>>>> index 72531d4185..31340a484b 100644
>>>> --- a/libavformat/utils.c
>>>> +++ b/libavformat/utils.c
>>>> @@ -3245,6 +3245,7 @@ int ff_alloc_extradata(AVCodecParameters *par, int size)
>>>>  {
>>>>      int ret;
>>>>  
>>>> +    av_freep(&par->extradata);
>>>>      if (size < 0 || size >= INT32_MAX - AV_INPUT_BUFFER_PADDING_SIZE) {
>>>>          par->extradata = NULL;
>>>>          par->extradata_size = 0;
>>>
>>> This causes memory corruption
>>> ...
>>> [mpegts @ 0x7f8c74000a80] PES packet size mismatch
>>> *** Error in `./ffplay': double free or corruption (fasttop): 0x00007f8c7402d9c0 ***
>>> Aborted (core dumped)
>>
>> Is something freeing extradata and leaving a dangling pointer before
>> eventually calling ff_alloc_extradata()?
>> At least the two calls in mpegts.c don't seem to do that.
>>
>>>
>>> I think this should not have been applied so quickly, i tested it as soon as i
>>> had time and saw it but it was applied already
>>
>> Fate passed when i tested it, and i got a positive review from the
>> author of the function in question, that's why i pushed it. Sorry.
>>
>>>
>>> If it helps i can debug the cases i see to find out which calls cause them but
>>> someone will still have to review all call sites probably for this change to
>>> be safe
>>
>> Yes, help would be welcome. Crashes like this probably hint at frees
>> leaving dangling pointers across the codebase.
> 
> heres some valgrind output
> 
> ==8248== Thread 15:peated 5 times
> ==8248== Invalid free() / delete / delete[] / realloc()
> ==8248==    at 0x4C2B5D9: free (vg_replace_malloc.c:446)
> ==8248==    by 0x75B0FE: ff_get_extradata (in ffmpeg/ffplay_g)
> ==8248==    by 0x68F1E1: ff_mp4_read_dec_config_descr (in ffmpeg/ffplay_g)

Figures. The patch i sent a few minutes ago should fix it then.

> ==8248==    by 0x6D7A2C: m4sl_cb (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D6539: handle_packet (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D69B3: handle_packets (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D6A53: mpegts_read_packet (in ffmpeg/ffplay_g)
> ==8248==    by 0x754AA1: ff_read_packet (in ffmpeg/ffplay_g)
> ==8248==    by 0x75E32F: avformat_find_stream_info (in ffmpeg/ffplay_g)
> ==8248==    by 0x4B86D8: read_thread (in ffmpeg/ffplay_g)
> ==8248==    by 0x74742EC: SDL_RunThread (SDL_thread.c:282)
> ==8248==    by 0x74D21A8: RunThread (SDL_systhread.c:75)
> ==8248==  Address 0x3085a000 is 0 bytes inside a block of size 88 free'd
> ==8248==    at 0x4C2B5D9: free (vg_replace_malloc.c:446)
> ==8248==    by 0x68F1CD: ff_mp4_read_dec_config_descr (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D7A2C: m4sl_cb (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D6539: handle_packet (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D69B3: handle_packets (in ffmpeg/ffplay_g)
> ==8248==    by 0x6D6A53: mpegts_read_packet (in ffmpeg/ffplay_g)
> ==8248==    by 0x754AA1: ff_read_packet (in ffmpeg/ffplay_g)
> ==8248==    by 0x75E32F: avformat_find_stream_info (in ffmpeg/ffplay_g)
> ==8248==    by 0x4B86D8: read_thread (in ffmpeg/ffplay_g)
> ==8248==    by 0x74742EC: SDL_RunThread (SDL_thread.c:282)
> ==8248==    by 0x74D21A8: RunThread (SDL_systhread.c:75)
> ==8248==    by 0xB8A2183: start_thread (pthread_create.c:312)
> ==8248== 
>     Last message repeated 5 times
> 
> i can provide a better debug output tomorrow, need to go afk now
>     
>     
> [...]
> 
> 
> 
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