[FFmpeg-devel] [PATCH] lavc/avpacket: fill padding area on side data split.

[Clément Bœsch]

On Sat, Jun 01, 2013 at 02:09:47PM +0200, Clément Bœsch wrote:
> The padding data is assumed to be 0 in several places, notably in
> subtitles. This problem was not detected with fate-sub-srt test because
> the first element of the side data (x1) is 0 in the test, so the
> trailing side data present in the packet wasn't read by the decoder. The
> issue can be observed with a large enough x1.
> 
> It is also noted in FF_INPUT_BUFFER_PADDING_SIZE doxy that MPEG
> bitstreams require that padding with 0, so it might fix other issues.
> ---
>  libavcodec/avpacket.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/libavcodec/avpacket.c b/libavcodec/avpacket.c
> index 198365e..2bfb501 100644
> --- a/libavcodec/avpacket.c
> +++ b/libavcodec/avpacket.c
> @@ -389,6 +389,7 @@ int av_packet_split_side_data(AVPacket *pkt){
>              p-= size+5;
>          }
>          pkt->size -= 8;
> +        memset(pkt->data + pkt->size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
>          pkt->side_data_elems = i+1;
>          return 1;
>      }

New version after Michael's comment on IRC.

-- 
Clément B.
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From e05a63d6a48f06639e2002a5c615528a7a7fb8b6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20B=C5=93sch?= <ubitux at gmail.com>
Date: Sat, 1 Jun 2013 14:07:24 +0200
Subject: [PATCH] lavc/avpacket: fill padding area on side data split.

The padding data is assumed to be 0 in several places, notably in
subtitles. This problem was not detected with fate-sub-srt test because
the first element of the side data (x1) is 0 in the test, so the
trailing side data present in the packet wasn't read by the decoder. The
issue can be observed with a large enough x1.

It is also noted in FF_INPUT_BUFFER_PADDING_SIZE doxy that MPEG
bitstreams require that padding with 0, so it might fix other issues.
---
 libavcodec/avpacket.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libavcodec/avpacket.c b/libavcodec/avpacket.c
index 198365e..e276c60 100644
--- a/libavcodec/avpacket.c
+++ b/libavcodec/avpacket.c
@@ -356,7 +356,7 @@ int av_packet_merge_side_data(AVPacket *pkt){
 int av_packet_split_side_data(AVPacket *pkt){
     if (!pkt->side_data_elems && pkt->size >12 && AV_RB64(pkt->data + pkt->size - 8) == FF_MERGE_MARKER){
         int i;
-        unsigned int size;
+        unsigned int size, orig_pktsize = pkt->size;
         uint8_t *p;
 
         p = pkt->data + pkt->size - 8 - 5;
@@ -389,6 +389,13 @@ int av_packet_split_side_data(AVPacket *pkt){
             p-= size+5;
         }
         pkt->size -= 8;
+        /* FFMIN() prevents overflow in case the packet wasn't allocated with
+         * proper padding.
+         * If the side data is smaller than the buffer padding size, the
+         * remaining bytes should have already been filled with zeros by the
+         * original packet allocation anyway. */
+        memset(pkt->data + pkt->size, 0,
+               FFMIN(orig_pktsize - pkt->size, FF_INPUT_BUFFER_PADDING_SIZE));
         pkt->side_data_elems = i+1;
         return 1;
     }
-- 
1.8.3

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