From f7bbcb91b77c77c0842ae3ac72309aa6625cc69a Mon Sep 17 00:00:00 2001
From: Bogdan Lisman <bogdan@pydevsolutions.com>
Date: Mon, 15 Jun 2026 03:24:14 +0300
Subject: [PATCH] avcodec/snowenc: fix SIGFPE in get_dc() when a block lies
 outside the plane

get_dc() divides the accumulated, OBMC-weighted DC by aa, the sum of the
squared OBMC weights taken over the in-plane pixels.  When an OBMC block
falls entirely outside the plane - e.g. a tiny chroma plane after mcdeint
splits a frame into fields - no pixel contributes, aa stays 0 and the
ROUNDED_DIV() divides by zero (SIGFPE).  ab is 0 in exactly the same case,
so the result degenerates to 0; return it directly.

Reproducible with the GPL mcdeint filter in slow/extra_slow mode, e.g.

    ffmpeg -f lavfi -i testsrc=s=128x2 -vf mcdeint=mode=slow -f null -

Add a self-contained lavfi-based FATE regression test for the slow mode,
which previously crashed and is therefore not covered by the existing
sample-based fast/medium tests.

Fixes trac ticket #7779.

Signed-off-by: Bogdan Lisman <bogdan@pydevsolutions.com>
(cherry picked from commit a62d99692780431aeb8cc96665434d5ec3210985)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
---
 libavcodec/snowenc.c               | 3 +++
 tests/fate/filter-video.mak        | 3 +++
 tests/ref/fate/filter-mcdeint-slow | 8 ++++++++
 3 files changed, 14 insertions(+)
 create mode 100644 tests/ref/fate/filter-mcdeint-slow

diff --git a/libavcodec/snowenc.c b/libavcodec/snowenc.c
index 94a592f69b..e92f082579 100644
--- a/libavcodec/snowenc.c
+++ b/libavcodec/snowenc.c
@@ -725,6 +725,9 @@ static int get_dc(SnowEncContext *enc, int mb_x, int mb_y, int plane_index)
     }
     *b= backup;
 
+    if (!aa)
+        return 0;
+
     return av_clip_uint8( ROUNDED_DIV((int64_t)ab<<LOG2_OBMC_MAX, aa) ); //FIXME we should not need clipping
 }
 
diff --git a/tests/fate/filter-video.mak b/tests/fate/filter-video.mak
index cfb9b55462..2ec4c5f49f 100644
--- a/tests/fate/filter-video.mak
+++ b/tests/fate/filter-video.mak
@@ -54,6 +54,9 @@ FATE_FILTER_SAMPLES-$(call FILTERDEMDEC, MCDEINT, MPEGTS, MPEG2VIDEO, SNOW_ENCOD
 FATE_FILTER-$(call FILTERFRAMECRC, MCDEINT TESTSRC, SNOW_ENCODER) += fate-filter-mcdeint-slow-edge
 fate-filter-mcdeint-slow-edge: CMD = framecrc -auto_conversion_filters -flags bitexact -lavfi testsrc=s=5x32:r=25:d=1,mcdeint=mode=slow -frames:v 3
 
+FATE_FILTER-$(call FILTERFRAMECRC, MCDEINT TESTSRC, SNOW_ENCODER) += fate-filter-mcdeint-slow
+fate-filter-mcdeint-slow: CMD = framecrc -auto_conversion_filters -flags bitexact -lavfi testsrc=s=128x2:r=25:d=1,mcdeint=mode=slow -frames:v 3
+
 FATE_FILTER_SAMPLES-$(call FILTERDEMDEC, CODECVIEW, RM, RV40) += fate-filter-codecview-mvs
 fate-filter-codecview-mvs: CMD = framecrc -flags2 +export_mvs -i $(TARGET_SAMPLES)/real/spygames-2MB.rmvb -vf codecview=mv=pf+bf+bb -frames:v 60 -an
 
diff --git a/tests/ref/fate/filter-mcdeint-slow b/tests/ref/fate/filter-mcdeint-slow
new file mode 100644
index 0000000000..0888d44805
--- /dev/null
+++ b/tests/ref/fate/filter-mcdeint-slow
@@ -0,0 +1,8 @@
+#tb 0: 1/25
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 128x2
+#sar 0: 1/1
+0,          0,          0,        1,      768, 0x6d637c68
+0,          1,          1,        1,      768, 0x07867cb0
+0,          2,          2,        1,      768, 0xa1ef7d04