[FFmpeg-devel] [PR] avcodec/elbg: error out from try_shift_candidate() when clipping occurred (PR #21785)

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

PR #21785 opened by michaelni
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21785
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21785.patch

The code assumes that (a+b)+c = a+(b+c) but this does not hold
true when clipping occurred.

A better fix is to use int64_t or ensure clipping always cancels

Fixes: error becoming negative
Fixes: 472729757/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CINEPAK_fuzzer-5712538546536448

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>


>From cecb39fe619391d0960ceec6cfe9ed29b26706d3 Mon Sep 17 00:00:00 2001
From: Michael Niedermayer <michael@niedermayer.cc>
Date: Mon, 16 Feb 2026 03:34:25 +0100
Subject: [PATCH] avcodec/elbg: error out from try_shift_candidate() when
 clipping occurred

The code assumes that (a+b)+c = a+(b+c) but this does not hold
true when clipping occurred.

A better fix is to use int64_t or ensure clipping always cancels

Fixes: error becoming negative
Fixes: 472729757/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CINEPAK_fuzzer-5712538546536448

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
---
 libavcodec/elbg.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libavcodec/elbg.c b/libavcodec/elbg.c
index 84043af4fd..37bc955bc2 100644
--- a/libavcodec/elbg.c
+++ b/libavcodec/elbg.c
@@ -302,6 +302,9 @@ static void try_shift_candidate(ELBGContext *elbg, int idx[3])
     for (j=0; j<3; j++)
         olderror += elbg->utility[idx[j]];
 
+    if (olderror >= INT_MAX)
+        return;
+
     memset(newcentroid[2], 0, elbg->dim*sizeof(int));
 
     for (k=0; k<2; k++)
-- 
2.52.0

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