ASoC: sigmadsp: Add __counted_by for struct sigmadsp_data and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() and size_sub() helpers, instead of the
open-coded version, to calculate the size for the allocation of the
whole flexible structure, including of course, the flexible-array
member.

This code was found with the help of Coccinelle, and audited and
fixed manually.
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZSRvh1j2MVVhuOUv@workSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/sigmadsp.c

@@ -43,7 +43,7 @@ struct sigmadsp_data {
 	uint32_t samplerates;
 	unsigned int addr;
 	unsigned int length;
-	uint8_t data[];
+	uint8_t data[] __counted_by(length);
 };
 
 struct sigma_fw_chunk {
@@ -270,7 +270,7 @@ static int sigma_fw_load_data(struct sigmadsp *sigmadsp,
 
 	length -= sizeof(*data_chunk);
 
-	data = kzalloc(sizeof(*data) + length, GFP_KERNEL);
+	data = kzalloc(struct_size(data, data, length), GFP_KERNEL);
 	if (!data)
 		return -ENOMEM;
 
@@ -413,7 +413,8 @@ static int process_sigma_action(struct sigmadsp *sigmadsp,
 		if (len < 3)
 			return -EINVAL;
 
-		data = kzalloc(sizeof(*data) + len - 2, GFP_KERNEL);
+		data = kzalloc(struct_size(data, data, size_sub(len, 2)),
+			       GFP_KERNEL);
 		if (!data)
 			return -ENOMEM;