ASoC: Use __counted_by() annotation for snd_soc_pcm_runtime

The struct snd_soc_pcm_runtime has a flex array of snd_soc_component
objects at its end, and the size is kept in num_components field.
We can add __counted_by() annotation for compiler's assistance to
catch array overflows.

A slight additional change is the assignment of rtd->components[];
the array counter has to be incremented at first for avoiding
false-positive reports from compilers.

Also, the allocation size of snd_soc_pcm_runtime is cleaned up with
the standard struct_size() helper, too.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20240726155237.21961-1-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/soc.h

@@ -1209,8 +1209,9 @@ struct snd_soc_pcm_runtime {
 
 	bool initialized;
 
+	/* CPU/Codec/Platform */
 	int num_components;
-	struct snd_soc_component *components[]; /* CPU/Codec/Platform */
+	struct snd_soc_component *components[] __counted_by(num_components);
 };
 
 /* see soc_new_pcm_runtime()  */

sound/soc/soc-core.c

@@ -326,8 +326,8 @@ static int snd_soc_rtd_add_component(struct snd_soc_pcm_runtime *rtd,
 	}
 
 	/* see for_each_rtd_components */
-	rtd->components[rtd->num_components] = component;
-	rtd->num_components++;
+	rtd->num_components++; // increment flex array count at first
+	rtd->components[rtd->num_components - 1] = component;
 
 	return 0;
 }
@@ -494,7 +494,6 @@ static struct snd_soc_pcm_runtime *soc_new_pcm_runtime(
 	struct snd_soc_card *card, struct snd_soc_dai_link *dai_link)
 {
 	struct snd_soc_pcm_runtime *rtd;
-	struct snd_soc_component *component;
 	struct device *dev;
 	int ret;
 	int stream;
@@ -521,10 +520,10 @@ static struct snd_soc_pcm_runtime *soc_new_pcm_runtime(
 	 * for rtd
 	 */
 	rtd = devm_kzalloc(dev,
-			   sizeof(*rtd) +
-			   sizeof(component) * (dai_link->num_cpus +
-						 dai_link->num_codecs +
-						 dai_link->num_platforms),
+			   struct_size(rtd, components,
+				       dai_link->num_cpus +
+				       dai_link->num_codecs +
+				       dai_link->num_platforms),
 			   GFP_KERNEL);
 	if (!rtd) {
 		device_unregister(dev);