[FFmpeg-devel] [PR] avformat/dashdec: fix calc_min_seg_no underflow (PR #21480)

[zotan via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21480 opened by zotan
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21480
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21480.patch

Fixes an integer underflow in calc_min_seg_no in cases where availability_start_time - time_shift_buffer_depth is negative (i.e. the stream started within one time_shift_buffer_depth window).


>From 3c45e3a49b279e2abc2060acc4a3e52cbb9ccd1a Mon Sep 17 00:00:00 2001
From: Laura Hausmann <laura@hausmann.dev>
Date: Thu, 15 Jan 2026 22:40:37 +0100
Subject: [PATCH] avformat/dashdec: fix calc_min_seg_no underflow

Fixes an integer underflow in calc_min_seg_no in cases where availability_start_time - time_shift_buffer_depth is negative (i.e. the stream started within one time_shift_buffer_depth window).
---
 libavformat/dashdec.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libavformat/dashdec.c b/libavformat/dashdec.c
index 500d8ca518..4a0b368851 100644
--- a/libavformat/dashdec.c
+++ b/libavformat/dashdec.c
@@ -1438,8 +1438,15 @@ static int64_t calc_min_seg_no(AVFormatContext *s, struct representation *pls)
     int64_t num = 0;
 
     if (c->is_live && pls->fragment_duration) {
+        uint64_t current_time_in_sec = get_current_time_in_sec();
         av_log(s, AV_LOG_TRACE, "in live mode\n");
-        num = pls->first_seq_no + (((get_current_time_in_sec() - c->availability_start_time) - c->time_shift_buffer_depth) * pls->fragment_timescale) / pls->fragment_duration;
+        if (current_time_in_sec - c->availability_start_time < c->time_shift_buffer_depth) {
+            av_log(s, AV_LOG_TRACE, "timeShiftBufferDepth not reached yet\n");
+            num = pls->first_seq_no;
+        }
+        else {
+            num = pls->first_seq_no + (((current_time_in_sec - c->availability_start_time) - c->time_shift_buffer_depth) * pls->fragment_timescale) / pls->fragment_duration;
+        }
     } else {
         num = pls->first_seq_no;
     }
-- 
2.52.0

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