[FFmpeg-devel] [PATCH 05/14] avcodec/ffv1dec: Fix data races emanating from copying whole context

[Andreas Rheinhardt]

When using frame threading, the FFV1 decoder's update_thread_context()
function copies the whole context and afterwards restores some allocated
fields with backups made earlier. Among these fields are the
ThreadFrames and the source context's ThreadFrames can change
concurrently without any synchronization, leading to data races which
are undefined behaviour even if they don't lead to problems in
practice (as the destination's own ThreadFrames are restored directly
thereafter).

Fix this by only copying the actually needed fields.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
 libavcodec/ffv1dec.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/libavcodec/ffv1dec.c b/libavcodec/ffv1dec.c
index c08ec5c1b7..9a9ee10a4c 100644
--- a/libavcodec/ffv1dec.c
+++ b/libavcodec/ffv1dec.c
@@ -1031,18 +1031,12 @@ static int update_thread_context(AVCodecContext *dst, const AVCodecContext *src)
     if (dst == src)
         return 0;
 
-    {
-        ThreadFrame picture = fdst->picture, last_picture = fdst->last_picture;
-        uint8_t (*initial_states[MAX_QUANT_TABLES])[32];
-        struct FFV1Context *slice_context[MAX_SLICES];
-        memcpy(initial_states, fdst->initial_states, sizeof(fdst->initial_states));
-        memcpy(slice_context,  fdst->slice_context , sizeof(fdst->slice_context));
-
-        memcpy(fdst, fsrc, sizeof(*fdst));
-        memcpy(fdst->initial_states, initial_states, sizeof(fdst->initial_states));
-        memcpy(fdst->slice_context,  slice_context , sizeof(fdst->slice_context));
-        fdst->picture      = picture;
-        fdst->last_picture = last_picture;
+    copy_fields(fdst, fsrc, fsrc);
+    fdst->use32bit     = fsrc->use32bit;
+    memcpy(fdst->state_transition, fsrc->state_transition,
+           sizeof(fdst->state_transition));
+    memcpy(fdst->quant_table, fsrc->quant_table, sizeof(fsrc->quant_table));
+
         for (i = 0; i<fdst->num_h_slices * fdst->num_v_slices; i++) {
             FFV1Context *fssrc = fsrc->slice_context[i];
             FFV1Context *fsdst = fdst->slice_context[i];
@@ -1050,7 +1044,6 @@ static int update_thread_context(AVCodecContext *dst, const AVCodecContext *src)
         }
         av_assert0(!fdst->plane[0].state);
         av_assert0(!fdst->sample_buffer);
-    }
 
     av_assert1(fdst->max_slice_count == fsrc->max_slice_count);
 
-- 
2.27.0