avcodec/012v: redesign main loop

Fixes out of array accesses
Fixes: ffmpeg_012v_crash.ts
Found-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Reviewed-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/012v.c

@@ -38,7 +38,7 @@ static av_cold int zero12v_decode_init(AVCodecContext *avctx)
 static int zero12v_decode_frame(AVCodecContext *avctx, void *data,
                                 int *got_frame, AVPacket *avpkt)
 {
-    int line = 0, ret;
+    int line, ret;
     const int width = avctx->width;
     AVFrame *pic = data;
     uint16_t *y, *u, *v;
@@ -67,45 +67,45 @@ static int zero12v_decode_frame(AVCodecContext *avctx, void *data,
     pic->pict_type = AV_PICTURE_TYPE_I;
     pic->key_frame = 1;
 
-    y = (uint16_t *)pic->data[0];
-    u = (uint16_t *)pic->data[1];
-    v = (uint16_t *)pic->data[2];
     line_end = avpkt->data + stride;
+    for (line = 0; line < avctx->height; line++) {
+        uint16_t y_temp[6] = {0x8000, 0x8000, 0x8000, 0x8000, 0x8000, 0x8000};
+        uint16_t u_temp[3] = {0x8000, 0x8000, 0x8000};
+        uint16_t v_temp[3] = {0x8000, 0x8000, 0x8000};
+        int x;
+        y = (uint16_t *)(pic->data[0] + line * pic->linesize[0]);
+        u = (uint16_t *)(pic->data[1] + line * pic->linesize[1]);
+        v = (uint16_t *)(pic->data[2] + line * pic->linesize[2]);
 
-    while (line++ < avctx->height) {
-        while (1) {
-            uint32_t t = AV_RL32(src);
+        for (x = 0; x < width; x += 6) {
+            uint32_t t;
+
+            if (width - x < 6 || line_end - src < 16) {
+                y = y_temp;
+                u = u_temp;
+                v = v_temp;
+            }
+
+            if (line_end - src < 4)
+                break;
+
+            t = AV_RL32(src);
             src += 4;
             *u++ = t <<  6 & 0xFFC0;
             *y++ = t >>  4 & 0xFFC0;
             *v++ = t >> 14 & 0xFFC0;
 
-            if (src >= line_end - 1) {
-                *y = 0x80;
-                src++;
-                line_end += stride;
-                y = (uint16_t *)(pic->data[0] + line * pic->linesize[0]);
-                u = (uint16_t *)(pic->data[1] + line * pic->linesize[1]);
-                v = (uint16_t *)(pic->data[2] + line * pic->linesize[2]);
+            if (line_end - src < 4)
                 break;
-            }
 
             t = AV_RL32(src);
             src += 4;
             *y++ = t <<  6 & 0xFFC0;
             *u++ = t >>  4 & 0xFFC0;
             *y++ = t >> 14 & 0xFFC0;
-            if (src >= line_end - 2) {
-                if (!(width & 1)) {
-                    *y = 0x80;
-                    src += 2;
-                }
-                line_end += stride;
-                y = (uint16_t *)(pic->data[0] + line * pic->linesize[0]);
-                u = (uint16_t *)(pic->data[1] + line * pic->linesize[1]);
-                v = (uint16_t *)(pic->data[2] + line * pic->linesize[2]);
+
+            if (line_end - src < 4)
                 break;
-            }
 
             t = AV_RL32(src);
             src += 4;
@@ -113,15 +113,8 @@ static int zero12v_decode_frame(AVCodecContext *avctx, void *data,
             *y++ = t >>  4 & 0xFFC0;
             *u++ = t >> 14 & 0xFFC0;
 
-            if (src >= line_end - 1) {
-                *y = 0x80;
-                src++;
-                line_end += stride;
-                y = (uint16_t *)(pic->data[0] + line * pic->linesize[0]);
-                u = (uint16_t *)(pic->data[1] + line * pic->linesize[1]);
-                v = (uint16_t *)(pic->data[2] + line * pic->linesize[2]);
+            if (line_end - src < 4)
                 break;
-            }
 
             t = AV_RL32(src);
             src += 4;
@@ -129,18 +122,21 @@ static int zero12v_decode_frame(AVCodecContext *avctx, void *data,
             *v++ = t >>  4 & 0xFFC0;
             *y++ = t >> 14 & 0xFFC0;
 
-            if (src >= line_end - 2) {
-                if (width & 1) {
-                    *y = 0x80;
-                    src += 2;
-                }
-                line_end += stride;
-                y = (uint16_t *)(pic->data[0] + line * pic->linesize[0]);
-                u = (uint16_t *)(pic->data[1] + line * pic->linesize[1]);
-                v = (uint16_t *)(pic->data[2] + line * pic->linesize[2]);
+            if (width - x < 6)
                 break;
         }
+
+        if (x < width) {
+            y = x   + (uint16_t *)(pic->data[0] + line * pic->linesize[0]);
+            u = x/2 + (uint16_t *)(pic->data[1] + line * pic->linesize[1]);
+            v = x/2 + (uint16_t *)(pic->data[2] + line * pic->linesize[2]);
+            memcpy(y, y_temp, sizeof(*y) * (width - x));
+            memcpy(u, u_temp, sizeof(*u) * (width - x + 1) / 2);
+            memcpy(v, v_temp, sizeof(*v) * (width - x + 1) / 2);
         }
+
+        line_end += stride;
+        src = line_end - stride;
     }
 
     *got_frame = 1;