[FFmpeg-devel] Re: [PATCH] avformat/rtpenc_h264_hevc: Check space for nal_length_size in ff_rtp_send_h264_hevc() (PR #20746)

[Kieran Kunhya via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On Fri, 24 Oct 2025, 14:52 michaelni via ffmpeg-devel, <
ffmpeg-devel@ffmpeg.org> wrote:

> PR #20746 opened by michaelni
> URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20746
> Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20746.patch
>
> Fixes: memcpy with negative size
> Fixes: momo_trip-poc/input
>
> Reported-by: Momoko Shiraishi <shiraishi@os.is.s.u-tokyo.ac.jp>
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
>
>
> >From 3924caed9dd6345bcfa5ce09e9dbc8d5403a7525 Mon Sep 17 00:00:00 2001
> From: Michael Niedermayer <michael@niedermayer.cc>
> Date: Fri, 24 Oct 2025 20:29:23 +0200
> Subject: [PATCH] avformat/rtpenc_h264_hevc: Check space for
> nal_length_size in
>  ff_rtp_send_h264_hevc()
>
> Fixes: memcpy with negative size
> Fixes: momo_trip-poc/input
>
> Reported-by: Momoko Shiraishi <shiraishi@os.is.s.u-tokyo.ac.jp>
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavformat/rtpenc_h264_hevc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/libavformat/rtpenc_h264_hevc.c
> b/libavformat/rtpenc_h264_hevc.c
> index 4d222dca75..ea19cb0627 100644
> --- a/libavformat/rtpenc_h264_hevc.c
> +++ b/libavformat/rtpenc_h264_hevc.c
> @@ -196,6 +196,8 @@ void ff_rtp_send_h264_hevc(AVFormatContext *s1, const
> uint8_t *buf1, int size)
>              r1 = ff_nal_mp4_find_startcode(r, end, s->nal_length_size);
>              if (!r1)
>                  r1 = end;
> +            if (r1 - r < s->nal_length_size)
> +                break;
>              r += s->nal_length_size;
>          } else {
>              while (!*(r++));
> --
> 2.49.1
>

Is this not a bug in ff_nal_mp4_find_startcode?

If not, please add a comment as to the reason this condition happens.

Kieran

>
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