Re: [FFmpeg-devel] [PATCH v2 2/2] avformat/mov: Don't use entry[-1] in pointer arithmetic

[James Almer <jamrial@gmail.com>]

On 2/26/2024 7:15 PM, Andreas Rheinhardt wrote:
> It is undefined behaviour.
> Fixes many failed tests with UBSan and GCC 13 like
> "src/libavformat/mov.c:4229:44: runtime error: store to address
> 0x5572abe20f80 with insufficient space for an object of type 'struct
> MOVIndexRange'"
> (The line number does not refer to the line where &entry[-1]
> is assigned.)
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
> Now switching to next_index_range in order to avoid
> having to duplicate the current_index_range == NULL check.

I prefer the first version. mov_fix_index() is called once per track 
during init (and then it may not be called depending on user options), 
so it's hardly a bottleneck, and next_index_range[-1] is much less clear 
and intuitive at a quick glance than current_index_range.

> 
>   libavformat/mov.c | 17 ++++++++---------
>   1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 71e8f7ae8f..d7da19998b 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -4022,7 +4022,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
>       int num_discarded_begin = 0;
>       int first_non_zero_audio_edit = -1;
>       int packet_skip_samples = 0;
> -    MOVIndexRange *current_index_range;
> +    MOVIndexRange *next_index_range;
>       int found_keyframe_after_edit = 0;
>       int found_non_empty_edit = 0;
>   
> @@ -4038,7 +4038,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
>           return;
>       }
>       msc->current_index_range = msc->index_ranges;
> -    current_index_range = msc->index_ranges - 1;
> +    next_index_range = msc->index_ranges;
>   
>       // Clean AVStream from traces of old index
>       sti->index_entries = NULL;
> @@ -4225,11 +4225,11 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
>               }
>   
>               // Update the index ranges array
> -            if (current_index_range < msc->index_ranges || index != current_index_range->end) {
> -                current_index_range++;
> -                current_index_range->start = index;
> +            if (next_index_range == msc->index_ranges || index != next_index_range[-1].end) {
> +                next_index_range->start = index;
> +                next_index_range++;
>               }
> -            current_index_range->end = index + 1;
> +            next_index_range[-1].end = index + 1;
>   
>               // Only start incrementing DTS in frame_duration amounts, when we encounter a frame in edit list.
>               if (edit_list_start_encountered > 0) {
> @@ -4289,9 +4289,8 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
>       av_freep(&frame_duration_buffer);
>   
>       // Null terminate the index ranges array
> -    current_index_range++;
> -    current_index_range->start = 0;
> -    current_index_range->end = 0;
> +    next_index_range->start = 0;
> +    next_index_range->end   = 0;
>       msc->current_index = msc->index_ranges[0].start;
>   }
>   
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