ASoC: soc-utils: Check string pointer validity in snd_soc_dlc_is_dummy()

In the recently added snd_soc_dlc_is_dummy(), the helper uses the .name
and .dai_name fields without checking their validity.

For .name, this field is NULL if the component is matched by .of_node
instead. In fact, only one of these fields may be set. This caused a
NULL pointer dereference on MediaTek MT8195 and MT8188 platforms with
the subsequent conversion to snd_soc_dlc_is_dummy() in their machine
drivers. The codecs are all matches through the device tree, so their
.name fields are empty.

For .dai_name, there are cases where this field is empty, such as for
the following component definitions:

	#define COMP_EMPTY()                    { }
	#define COMP_PLATFORM(_name)            { .name = _name }
	#define COMP_AUX(_name)                 { .name = _name }
	#define COMP_CODEC_CONF(_name)          { .name = _name }

Or the single link CPU DAI case in the simple audio card family, as
covered by simple_util_canonicalize_cpu(), in which the .dai_name
field is explicitly cleared.

To fix this, check the validity of the fields before using them in
string comparison.

Fixes: 3e021f3 ("ASoC: soc-utils: add snd_soc_dlc_is_dummy()")
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://patch.msgid.link/20250516050549.407133-1-wenst@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: wens

sound/soc/soc-utils.c

@@ -267,8 +267,8 @@ int snd_soc_dlc_is_dummy(struct snd_soc_dai_link_component *dlc)
 	if (dlc == &snd_soc_dummy_dlc)
 		return true;
 
-	if (strcmp(dlc->name,	  snd_soc_dummy_dlc.name)	== 0 ||
-	    strcmp(dlc->dai_name, snd_soc_dummy_dlc.dai_name)	== 0)
+	if ((dlc->name     && strcmp(dlc->name,     snd_soc_dummy_dlc.name)     == 0) ||
+	    (dlc->dai_name && strcmp(dlc->dai_name, snd_soc_dummy_dlc.dai_name) == 0))
 		return true;
 
 	return false;