Re: [FFmpeg-devel] [PATCH 1/4] avcodec/ass: Faster ff_ass_add_rect()

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Michael Niedermayer:
> On Fri, Dec 17, 2021 at 11:15:06PM +0100, Andreas Rheinhardt wrote:
>> Michael Niedermayer:
>>> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
>>> ---
>>>  libavcodec/ass.c | 32 ++++++++++++++++++++++++++------
>>>  libavcodec/ass.h |  7 +++++++
>>>  2 files changed, 33 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/libavcodec/ass.c b/libavcodec/ass.c
>>> index 725e4d42ba1..d0a4d678bb4 100644
>>> --- a/libavcodec/ass.c
>>> +++ b/libavcodec/ass.c
>>> @@ -114,17 +114,30 @@ char *ff_ass_get_dialog(int readorder, int layer, const char *style,
>>>                         speaker ? speaker : "", text);
>>>  }
>>>  
>>> -int ff_ass_add_rect(AVSubtitle *sub, const char *dialog,
>>> +int ff_ass_add_rect2(AVSubtitle *sub, const char *dialog,
>>>                      int readorder, int layer, const char *style,
>>> -                    const char *speaker)
>>> +                    const char *speaker, size_t *nb_rect_allocated)
>>>  {
>>> -    AVSubtitleRect **rects, *rect;
>>> +    AVSubtitleRect **rects = sub->rects, *rect;
>>>      char *ass_str;
>>> +    uint64_t new_nb = 0;
>>>  
>>> -    rects = av_realloc_array(sub->rects, sub->num_rects+1, sizeof(*sub->rects));
>>> -    if (!rects)
>>> +    if (nb_rect_allocated && *nb_rect_allocated <= sub->num_rects) {
>>> +        new_nb = sub->num_rects + sub->num_rects/16LL + 1;
>>> +    } else if (!nb_rect_allocated)
>>> +        new_nb = sub->num_rects + 1LL;
>>> +    if (new_nb > SIZE_MAX)
>>>          return AVERROR(ENOMEM);
>>
>> AVSubtitle.num_rects is unsigned, so this number should always be
>> bounded by UINT_MAX (and nb_rect_allocated can be a pointer to unsigned,
>> too).
> 
> i had that initially but then wanted to move to better types but missed
> this, changed it back locally
> 
Simply changing it back is wrong, because it might happen that the old
num_rects is < UINT_MAX with new_nb > UINT_MAX. In this case one should
clip new_nb to UINT_MAX.
(This case can't happen currently: The loop in ccaption_dec can lead to
at most INT_MAX/3 rects; the other two ff_ass_add_rect() can bring this
to at most INT_MAX/3 + 2.)

- Andreas
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