zephyr: userspace: sof_dma: allow circular SG lists

Allow a non-null pointer at the end of the DMA transfer block list,
if and only if it points to the first entry in the block list.

The SOF DAI module sets the DMA transfers blocks like this and
this change is required to use DAI module from user-space.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

zephyr/syscall/sof_dma.c

@@ -109,19 +109,28 @@ static inline void z_vrfy_sof_dma_release_channel(struct sof_dma *dma,
  */
 static inline struct dma_block_config *deep_copy_dma_blk_cfg_list(struct dma_config *cfg)
 {
-	struct dma_block_config *kern_cfg =
-		rmalloc(0, sizeof(*kern_cfg) * cfg->block_count);
+	struct dma_block_config *kern_cfg;
 	struct dma_block_config *kern_prev = NULL, *kern_next, *user_next;
 	int i = 0;
 
+	if (!cfg->block_count)
+		return NULL;
+
+	kern_cfg = rmalloc(0, sizeof(*kern_cfg) * cfg->block_count);
 	if (!kern_cfg)
 		return NULL;
 
 	for (user_next = cfg->head_block, kern_next = kern_cfg;
 	     user_next;
-	     user_next = user_next->next_block, kern_next++) {
-		if (++i > cfg->block_count)
-			goto err;
+	     user_next = user_next->next_block, kern_next++, i++) {
+		if (i == cfg->block_count) {
+			/* last block can point to first one */
+			if (user_next != cfg->head_block)
+				goto err;
+
+			kern_prev->next_block = kern_cfg;
+			break;
+		}
 
 		if (k_usermode_from_copy(kern_next, user_next, sizeof(*kern_next)))
 			goto err;