[PR] [release/9.0] avformat/movenc: fix crash when flushing a fragment with no data (PR #24295)

PR #24295 opened by ffmpeg-devel
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24295
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24295.patch

**Backport:** https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24183

# Summary
`mov_flush_fragment()` dereferences a NULL mov->mdat_buf when an MP4 trailer is written before any packet was muxed. This crashes every fragmented mode that defers the initial moov to the first flush (including hybrid_fragmented).

Regression in >= 8.1 from [627da1111c9d](https://code.ffmpeg.org/FFmpeg/FFmpeg/commit/627da1111c9d8ebf8a3a190a164cd502b99a09cb).

Reproduction: this command segfaults
`ffmpeg -f lavfi -i color=s=64x64 -frames:v 0 -movflags +frag_keyframe out.mp4`

# Tests
Ran on macOS / arm64, tested on booth 8.1 and the newly 9. versions.
- fate passes
- clean exit 0 (no segfaults) from previous reproduction command after the patch
- Regular non-empty outputs are byte-identical.


>From 0e07fd112d4fcb56777ea1d734dbf8e739d476b9 Mon Sep 17 00:00:00 2001
From: Ackanir <[email protected]>
Date: Mon, 17 Aug 2026 14:36:53 +0200
Subject: [PATCH] avformat/movenc: fix crash when flushing a fragment with no
 data

mov_flush_fragment() calls ffio_reset_dyn_buf() on mov->mdat_buf when
writing the initial moov. That buffer is allocated lazily on the first
packet, so it is still NULL when the trailer is written before anything
was muxed, and ffio_reset_dyn_buf() dereferences it.

627da1111c9d replaced ffio_free_dyn_buf(), which tolerates NULL, with
ffio_reset_dyn_buf(), which does not, turning this case into a crash.

This affects every fragmented mode that defers the initial moov to the
first flush.

Skip the block entirely when the buffer was never opened. Nothing
references the empty mdat it would otherwise write, as no samples were
muxed.

Reproduced with:
ffmpeg -f lavfi -i color=s=64x64 -frames:v 0 -movflags +frag_keyframe out.mp4

Fixes: 627da1111c9d ("libavformat/movenc: Uses dynamic buffers for fragmented chunks")
Signed-off-by: Ackanir <[email protected]>
(cherry picked from commit 9f35e220ffbba21c88356eb2bbfa3679ede93793)
---
 libavformat/movenc.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index aacde4ebe7..6f93eb3e36 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -6771,11 +6771,13 @@ static int mov_flush_fragment(AVFormatContext *s, int force)
             return 0;
         }
 
-        buf_size = avio_get_dyn_buf(mov->mdat_buf, &buf);
-        avio_wb32(s->pb, buf_size + 8);
-        ffio_wfourcc(s->pb, "mdat");
-        avio_write(s->pb, buf, buf_size);
-        ffio_reset_dyn_buf(mov->mdat_buf);
+        if (mov->mdat_buf) {
+            buf_size = avio_get_dyn_buf(mov->mdat_buf, &buf);
+            avio_wb32(s->pb, buf_size + 8);
+            ffio_wfourcc(s->pb, "mdat");
+            avio_write(s->pb, buf, buf_size);
+            ffio_reset_dyn_buf(mov->mdat_buf);
+        }
 
         if (mov->flags & FF_MOV_FLAG_GLOBAL_SIDX)
             mov->reserved_header_pos = avio_tell(s->pb);
-- 
2.52.0

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