[FFmpeg-devel] [PATCH 1/2] avcodec/mpeg12dec: Use saturated addition when combining, error_count

* [FFmpeg-devel] [PATCH 1/2] avcodec/mpeg12dec: Use saturated addition when combining, error_count
@ 2025-03-14 19:32 Andreas Rheinhardt
  2025-03-16  3:51 ` Andreas Rheinhardt
  0 siblings, 1 reply; 2+ messages in thread
From: Andreas Rheinhardt @ 2025-03-14 19:32 UTC (permalink / raw)
  To: FFmpeg development discussions and patches

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Patches attached.

- Andreas

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From e79b422c0ed3732ec15bd25bdf2ad87f45dbddfd Mon Sep 17 00:00:00 2001
From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Date: Fri, 14 Mar 2025 19:10:57 +0100
Subject: [PATCH 1/2] avcodec/mpeg12dec: Use saturated addition when combining
 error_count

Fixes undefined integer overflows. The overflows could always
happen, yet before 4d8b706b1d33e75eb30b289c152280d4535c40e6
it was not undefined because the code implicitly used atomic
types, for which signed integer overflow is defined.

Reported-by: Kacper Michajlow <kasper93@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavcodec/mpeg12dec.c | 62 ++++++++++++++++++++++--------------------
 1 file changed, 33 insertions(+), 29 deletions(-)

diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
index ffe0710470..efb66ef956 100644
--- a/libavcodec/mpeg12dec.c
+++ b/libavcodec/mpeg12dec.c
@@ -2233,6 +2233,36 @@ static int mpeg_decode_gop(AVCodecContext *avctx,
     return 0;
 }
 
+static void mpeg12_execute_slice_threads(AVCodecContext *avctx,
+                                         Mpeg1Context *const s)
+{
+    if (HAVE_THREADS && (avctx->active_thread_type & FF_THREAD_SLICE) &&
+        !avctx->hwaccel) {
+        MpegEncContext *const s2 = &s->mpeg_enc_ctx;
+        int error_count = 0;
+        av_assert0(avctx->thread_count > 1);
+
+        avctx->execute(avctx, slice_decode_thread,
+                       s2->thread_context, NULL,
+                       s->slice_count, sizeof(void *));
+
+        for (int i = 0; i < s->slice_count; i++) {
+            MpegEncContext *const slice = s2->thread_context[i];
+            int slice_err = atomic_load_explicit(&slice->er.error_count,
+                                                 memory_order_relaxed);
+            // error_count can get set to INT_MAX on serious errors.
+            // So use saturated addition.
+            if ((unsigned)slice_err > INT_MAX - error_count) {
+                error_count = INT_MAX;
+                break;
+            }
+            error_count += slice_err;
+        }
+        atomic_store_explicit(&s2->er.error_count, error_count,
+                              memory_order_relaxed);
+    }
+}
+
 static int decode_chunks(AVCodecContext *avctx, AVFrame *picture,
                          int *got_output, const uint8_t *buf, int buf_size)
 {
@@ -2250,22 +2280,7 @@ static int decode_chunks(AVCodecContext *avctx, AVFrame *picture,
         buf_ptr = avpriv_find_start_code(buf_ptr, buf_end, &start_code);
         if (start_code > 0x1ff) {
             if (!skip_frame) {
-                if (HAVE_THREADS &&
-                    (avctx->active_thread_type & FF_THREAD_SLICE) &&
-                    !avctx->hwaccel) {
-                    int error_count = 0;
-                    int i;
-                    av_assert0(avctx->thread_count > 1);
-
-                    avctx->execute(avctx, slice_decode_thread,
-                                   &s2->thread_context[0], NULL,
-                                   s->slice_count, sizeof(void *));
-                    for (i = 0; i < s->slice_count; i++)
-                        error_count += atomic_load_explicit(&s2->thread_context[i]->er.error_count,
-                                                            memory_order_relaxed);
-                    atomic_store_explicit(&s2->er.error_count, error_count,
-                                          memory_order_relaxed);
-                }
+                mpeg12_execute_slice_threads(avctx, s);
 
                 ret = slice_end(avctx, picture, got_output);
                 if (ret < 0)
@@ -2324,19 +2339,8 @@ static int decode_chunks(AVCodecContext *avctx, AVFrame *picture,
                 s2->intra_dc_precision= 3;
                 s2->intra_matrix[0]= 1;
             }
-            if (HAVE_THREADS && (avctx->active_thread_type & FF_THREAD_SLICE) &&
-                !avctx->hwaccel && s->slice_count) {
-                int error_count = 0;
-                int i;
-
-                avctx->execute(avctx, slice_decode_thread,
-                               s2->thread_context, NULL,
-                               s->slice_count, sizeof(void *));
-                for (i = 0; i < s->slice_count; i++)
-                    error_count += atomic_load_explicit(&s2->thread_context[i]->er.error_count,
-                                                        memory_order_relaxed);
-                atomic_store_explicit(&s2->er.error_count, error_count,
-                                      memory_order_relaxed);
+            if (s->slice_count) {
+                mpeg12_execute_slice_threads(avctx, s);
                 s->slice_count = 0;
             }
             if (last_code == 0 || last_code == SLICE_MIN_START_CODE) {
-- 
2.45.2


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From 12d3af209ed16a136d917f8b12e8547213fef52f Mon Sep 17 00:00:00 2001
From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Date: Fri, 14 Mar 2025 20:18:37 +0100
Subject: [PATCH 2/2] avcodec/mpeg12dec: Don't assert on thread_count

Nothing in this decoder would break if the generic code were to be
changed to allow slice "threading" with only one thread.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavcodec/mpeg12dec.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
index efb66ef956..c36471a8b3 100644
--- a/libavcodec/mpeg12dec.c
+++ b/libavcodec/mpeg12dec.c
@@ -2240,7 +2240,6 @@ static void mpeg12_execute_slice_threads(AVCodecContext *avctx,
         !avctx->hwaccel) {
         MpegEncContext *const s2 = &s->mpeg_enc_ctx;
         int error_count = 0;
-        av_assert0(avctx->thread_count > 1);
 
         avctx->execute(avctx, slice_decode_thread,
                        s2->thread_context, NULL,
@@ -2538,7 +2537,6 @@ static int decode_chunks(AVCodecContext *avctx, AVFrame *picture,
                     int threshold = (s2->mb_height * s->slice_count +
                                      s2->slice_context_count / 2) /
                                     s2->slice_context_count;
-                    av_assert0(avctx->thread_count > 1);
                     if (threshold <= mb_y) {
                         MpegEncContext *thread_context = s2->thread_context[s->slice_count];
 
-- 
2.45.2


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