ALSA: mixart: Reduce size of mixart_timer_notify

The mixart_timer_notify structure was larger than could be represented
by the mixart_msg_data array storage. Adjust the size to as large as
possible to fix the warning seen with -Warray-bounds builds:

sound/pci/mixart/mixart_core.c: In function 'snd_mixart_threaded_irq':
sound/pci/mixart/mixart_core.c:447:50: error: array subscript 'struct mixart_timer_notify[0]' is partly outside array bounds of 'u32[128]' {aka 'unsigned int[128]'} [-Werror=array-bounds]
  447 |                                 for(i=0; i<notify->stream_count; i++) {
      |                                                  ^~
sound/pci/mixart/mixart_core.c:328:12: note: while referencing 'mixart_msg_data'
  328 | static u32 mixart_msg_data[MSG_DEFAULT_SIZE / 4];
      |            ^~~~~~~~~~~~~~~
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211207062941.2413679-1-keescook@chromium.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/mixart/mixart_core.c

@@ -23,8 +23,6 @@
 #define MSG_DESCRIPTOR_SIZE         0x24
 #define MSG_HEADER_SIZE             (MSG_DESCRIPTOR_SIZE + 4)
 
-#define MSG_DEFAULT_SIZE            512
-
 #define MSG_TYPE_MASK               0x00000003    /* mask for following types */
 #define MSG_TYPE_NOTIFY             0             /* embedded -> driver (only notification, do not get_msg() !) */
 #define MSG_TYPE_COMMAND            1             /* driver <-> embedded (a command has no answer) */
@@ -444,6 +442,7 @@ irqreturn_t snd_mixart_threaded_irq(int irq, void *dev_id)
 				struct mixart_timer_notify *notify;
 				notify = (struct mixart_timer_notify *)mixart_msg_data;
 
+				BUILD_BUG_ON(sizeof(notify) > sizeof(mixart_msg_data));
 				for(i=0; i<notify->stream_count; i++) {
 
 					u32 buffer_id = notify->streams[i].buffer_id;

sound/pci/mixart/mixart_core.h

@@ -49,6 +49,7 @@ enum mixart_message_id {
 	MSG_CLOCK_SET_PROPERTIES             = 0x200002,
 };
 
+#define MSG_DEFAULT_SIZE            512
 
 struct mixart_msg
 {
@@ -251,10 +252,17 @@ struct mixart_sample_pos
 	u32   sample_pos_low_part;
 } __attribute__((packed));
 
+/*
+ * This structure is limited by the size of MSG_DEFAULT_SIZE. Instead of
+ * having MIXART_MAX_STREAM_PER_CARD * MIXART_MAX_CARDS many streams,
+ * this is capped to have a total size below MSG_DEFAULT_SIZE.
+ */
+#define MIXART_MAX_TIMER_NOTIFY_STREAMS				\
+	((MSG_DEFAULT_SIZE - sizeof(u32)) / sizeof(struct mixart_sample_pos))
 struct mixart_timer_notify
 {
 	u32                  stream_count;
-	struct mixart_sample_pos  streams[MIXART_MAX_STREAM_PER_CARD * MIXART_MAX_CARDS];
+	struct mixart_sample_pos  streams[MIXART_MAX_TIMER_NOTIFY_STREAMS];
 } __attribute__((packed));