ALSA: mixart: range checking proc file

The original code doesn't take into consideration that the value of
MIXART_BA0_SIZE - pos can be less than zero which would lead to a large
unsigned value for "count".

Also I moved the check that read size is a multiple of 4 bytes below
the code that adjusts "count".
Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: <stable@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/mixart/mixart.c

@@ -1161,13 +1161,15 @@ static long snd_mixart_BA0_read(struct snd_info_entry *entry, void *file_private
 				unsigned long count, unsigned long pos)
 {
 	struct mixart_mgr *mgr = entry->private_data;
+	unsigned long maxsize;
 
-	count = count & ~3; /* make sure the read size is a multiple of 4 bytes */
-	if(count <= 0)
+	if (pos >= MIXART_BA0_SIZE)
 		return 0;
-	if(pos + count > MIXART_BA0_SIZE)
-		count = (long)(MIXART_BA0_SIZE - pos);
-	if(copy_to_user_fromio(buf, MIXART_MEM( mgr, pos ), count))
+	maxsize = MIXART_BA0_SIZE - pos;
+	if (count > maxsize)
+		count = maxsize;
+	count = count & ~3; /* make sure the read size is a multiple of 4 bytes */
+	if (copy_to_user_fromio(buf, MIXART_MEM(mgr, pos), count))
 		return -EFAULT;
 	return count;
 }
@@ -1180,13 +1182,15 @@ static long snd_mixart_BA1_read(struct snd_info_entry *entry, void *file_private
 				unsigned long count, unsigned long pos)
 {
 	struct mixart_mgr *mgr = entry->private_data;
+	unsigned long maxsize;
 
-	count = count & ~3; /* make sure the read size is a multiple of 4 bytes */
-	if(count <= 0)
+	if (pos > MIXART_BA1_SIZE)
 		return 0;
-	if(pos + count > MIXART_BA1_SIZE)
-		count = (long)(MIXART_BA1_SIZE - pos);
-	if(copy_to_user_fromio(buf, MIXART_REG( mgr, pos ), count))
+	maxsize = MIXART_BA1_SIZE - pos;
+	if (count > maxsize)
+		count = maxsize;
+	count = count & ~3; /* make sure the read size is a multiple of 4 bytes */
+	if (copy_to_user_fromio(buf, MIXART_REG(mgr, pos), count))
 		return -EFAULT;
 	return count;
 }