[FFmpeg-devel] [PATCH] avformat/mov: (v3) fix get_eia608_packet

* [FFmpeg-devel] [PATCH] avformat/mov: (v3) fix get_eia608_packet
@ 2025-02-13 21:01 Pavel Koshevoy
  2025-02-13 21:24 ` Pavel Koshevoy
  0 siblings, 1 reply; 2+ messages in thread
From: Pavel Koshevoy @ 2025-02-13 21:01 UTC (permalink / raw)
  To: ffmpeg-devel; +Cc: Pavel Koshevoy

The problem is reproducible with "Test for Quicktime 608 CC file.mov"
from https://samples.ffmpeg.org/MPEG2/subcc/

ffmpeg -i "Test for Quicktime 608 CC file.mov" -map 0 -c copy -y remuxed.mov

Prior to the fix QuickTime Player playback of remuxed.mov would
render garbage text for "English CC" subtitles.
---
 libavformat/mov.c | 68 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 58 insertions(+), 10 deletions(-)

diff --git a/libavformat/mov.c b/libavformat/mov.c
index 85aef33b19..84165735f7 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -10790,23 +10790,71 @@ static int mov_change_extradata(AVStream *st, AVPacket *pkt)
 
 static int get_eia608_packet(AVIOContext *pb, AVPacket *pkt, int size)
 {
-    int new_size, ret;
+    /* We can't make assumptions about the structure of the payload,
+       because it may include multiple cdat and cdt2 samples. */
+    const uint32_t cdat = AV_RB32("cdat");
+    const uint32_t cdt2 = AV_RB32("cdt2");
+    int ret, out_size = 0;
+
+    /* a valid payload must have size, 4cc, and at least 1 byte pair: */
+    if (size <= 10 || size % 2 != 0)
+        return AVERROR_INVALIDDATA;
 
-    if (size <= 8)
+    /* avoid an int overflow: */
+    if ((size - 8) / 2 >= INT_MAX / 3)
         return AVERROR_INVALIDDATA;
-    new_size = ((size - 8) / 2) * 3;
-    ret = av_new_packet(pkt, new_size);
+
+    ret = av_new_packet(pkt, ((size - 8) / 2) * 3);
     if (ret < 0)
         return ret;
 
-    avio_skip(pb, 8);
-    for (int j = 0; j < new_size; j += 3) {
-        pkt->data[j] = 0xFC;
-        pkt->data[j+1] = avio_r8(pb);
-        pkt->data[j+2] = avio_r8(pb);
+    /* parse and re-format the c608 payload in one pass. */
+    while (size >= 10) {
+        const uint32_t atom_size = avio_rb32(pb);
+        const uint32_t atom_type = avio_rb32(pb);
+        const int32_t data_size = atom_size - 8;
+        const uint8_t cc_field =
+            atom_type == cdat ? 1 :
+            atom_type == cdt2 ? 2 :
+            0;
+
+        /* account for bytes consumed for atom size and type. */
+        size -= 8;
+
+        /* make sure the atom size stays within the buffer boundaries. */
+        if (data_size < 2 || data_size > size) {
+            ret = AVERROR_INVALIDDATA;
+            break;
+        }
+
+        /* make sure the payload size is consistent with N byte pairs. */
+        if (atom_size % 2 != 0) {
+            ret = AVERROR_INVALIDDATA;
+            break;
+        }
+
+        if (!cc_field) {
+            /* neither cdat or cdt2 ... skip it */
+            avio_skip(pb, data_size);
+            size -= data_size;
+            continue;
+        }
+
+        for (int32_t i = 0; i < data_size; i += 2) {
+            pkt->data[out_size] = (0x1F << 3) | (1 << 2) | (cc_field - 1);
+            pkt->data[out_size + 1] = avio_r8(pb);
+            pkt->data[out_size + 2] = avio_r8(pb);
+            out_size += 3;
+            size -= 2;
+        }
     }
 
-    return 0;
+    if (size > 0)
+        /* skip any remaining unread portion of the input payload */
+        avio_skip(pb, size);
+
+    av_shrink_packet(pkt, out_size);
+    return ret;
 }
 
 static int mov_finalize_packet(AVFormatContext *s, AVStream *st, AVIndexEntry *sample,
-- 
2.43.0

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