ALSA: control: add ioctl to retrieve full card components

The fixed-size components field in SNDRV_CTL_IOCTL_CARD_INFO can be too
small on systems with many audio devices.

Keep the existing struct snd_ctl_card_info ABI intact and add a new
ioctl SNDRV_CTL_IOCTL_CARD_BYTES that carries a variable-length payload
selected by a type discriminator. The first defined type
SND_CTL_CARD_BTYPE_COMPONENTS returns the full components string. The
ioctl is designed to be reused for other variable-length card payloads
in the future.

The user-space caller may set data_allocated == 0 (or data == NULL) to
query the required length; otherwise the kernel copies the payload into
the user buffer and writes back the actual length in data_len.

When the legacy components field in struct snd_ctl_card_info is
truncated, '>' is written just before the NUL terminator to signal to
user-space that the full string is available via the new ioctl.

card->components is now dynamically allocated and grown in 32 byte
increments via krealloc(), capped at 512 bytes.

Link: https://github.com/alsa-project/alsa-lib/pull/494
Suggested-by: Jaroslav Kysela <perex@perex.cz>
Suggested-by: Takashi Iwai <tiwai@suse.com>
Signed-off-by: Maciej Strozek <mstrozek@opensource.cirrus.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260720103505.1860399-2-mstrozek@opensource.cirrus.com

3 files changed, 94 insertions, 7 deletions

diff --git a/sound/core/control.c b/sound/core/control.c
index 1116a40d11ae..78ce7bc936d2 100644
--- a/sound/core/control.c
+++ b/sound/core/control.c
@@ -38,7 +38,7 @@ struct snd_kctl_ioctl {
 	snd_kctl_ioctl_func_t fioctl;
 };
 
-static DECLARE_RWSEM(snd_ioctl_rwsem);
+DECLARE_RWSEM(snd_ioctl_rwsem);
 static DECLARE_RWSEM(snd_ctl_layer_rwsem);
 static LIST_HEAD(snd_control_ioctls);
 #ifdef CONFIG_COMPAT
@@ -872,23 +872,81 @@ static int snd_ctl_card_info(struct snd_card *card, struct snd_ctl_file * ctl,
 {
 	struct snd_ctl_card_info *info __free(kfree) =
 		kzalloc(sizeof(*info), GFP_KERNEL);
+	ssize_t n;
 
 	if (! info)
 		return -ENOMEM;
+
+	static_assert(sizeof(info->components) >= 2);
+
 	scoped_guard(rwsem_read, &snd_ioctl_rwsem) {
+		const char *components = card->components;
+
+		if (!components)
+			components = "";
+
 		info->card = card->number;
 		strscpy(info->id, card->id, sizeof(info->id));
 		strscpy(info->driver, card->driver, sizeof(info->driver));
 		strscpy(info->name, card->shortname, sizeof(info->name));
 		strscpy(info->longname, card->longname, sizeof(info->longname));
 		strscpy(info->mixername, card->mixername, sizeof(info->mixername));
-		strscpy(info->components, card->components, sizeof(info->components));
+		n = strscpy(info->components, components, sizeof(info->components));
+		if (n < 0) // mark the truncation with '>' before NULL terminator
+			info->components[sizeof(info->components) - 2] = '>';
 	}
 	if (copy_to_user(arg, info, sizeof(struct snd_ctl_card_info)))
 		return -EFAULT;
 	return 0;
 }
 
+static int snd_ctl_card_bytes(struct snd_card *card,
+			      struct snd_ctl_card_bytes *info,
+			      unsigned int __user *data_len_out)
+{
+	unsigned int data_len;
+
+	switch (info->type) {
+	case SND_CTL_CARD_BTYPE_COMPONENTS:
+		scoped_guard(rwsem_read, &snd_ioctl_rwsem) {
+			const char *components = card->components;
+
+			if (!components)
+				components = "";
+
+			data_len = strlen(components) + 1;
+
+			if (!info->data || info->data_allocated == 0)
+				break;
+
+			if (info->data_allocated < data_len)
+				return -ENOMEM;
+
+			if (copy_to_user(u64_to_user_ptr(info->data), components, data_len))
+				return -EFAULT;
+		}
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	if (put_user(data_len, data_len_out))
+		return -EFAULT;
+
+	return 0;
+}
+
+static int snd_ctl_card_bytes_user(struct snd_card *card,
+				   struct snd_ctl_card_bytes __user *_info)
+{
+	struct snd_ctl_card_bytes info;
+
+	if (copy_from_user(&info, _info, sizeof(info)))
+		return -EFAULT;
+
+	return snd_ctl_card_bytes(card, &info, &_info->data_len);
+}
+
 static int snd_ctl_elem_list(struct snd_card *card,
 			     struct snd_ctl_elem_list *list)
 {
@@ -1986,6 +2044,8 @@ static long snd_ctl_ioctl(struct file *file, unsigned int cmd, unsigned long arg
 		return put_user(SNDRV_CTL_VERSION, ip) ? -EFAULT : 0;
 	case SNDRV_CTL_IOCTL_CARD_INFO:
 		return snd_ctl_card_info(card, ctl, cmd, argp);
+	case SNDRV_CTL_IOCTL_CARD_BYTES:
+		return snd_ctl_card_bytes_user(card, argp);
 	case SNDRV_CTL_IOCTL_ELEM_LIST:
 		return snd_ctl_elem_list_user(card, argp);
 	case SNDRV_CTL_IOCTL_ELEM_INFO:
diff --git a/sound/core/control_compat.c b/sound/core/control_compat.c
index 4ad571087ff5..f14d9f5e94be 100644
--- a/sound/core/control_compat.c
+++ b/sound/core/control_compat.c
@@ -446,6 +446,7 @@ static inline long snd_ctl_ioctl_compat(struct file *file, unsigned int cmd, uns
 	switch (cmd) {
 	case SNDRV_CTL_IOCTL_PVERSION:
 	case SNDRV_CTL_IOCTL_CARD_INFO:
+	case SNDRV_CTL_IOCTL_CARD_BYTES:
 	case SNDRV_CTL_IOCTL_SUBSCRIBE_EVENTS:
 	case SNDRV_CTL_IOCTL_POWER:
 	case SNDRV_CTL_IOCTL_POWER_STATE:
diff --git a/sound/core/init.c b/sound/core/init.c
index 0372756048cd..19ec68db561b 100644
--- a/sound/core/init.c
+++ b/sound/core/init.c
@@ -589,6 +589,9 @@ static int snd_card_do_free(struct snd_card *card)
 		snd_mixer_oss_notify_callback(card, SND_MIXER_OSS_NOTIFY_FREE);
 #endif
 	snd_device_free_all(card);
+	kfree(card->components);
+	card->components = NULL;
+	card->components_alloc_size = 0;
 	if (card->private_free)
 		card->private_free(card);
 #ifdef CONFIG_SND_CTL_DEBUG
@@ -1035,16 +1038,39 @@ int snd_component_add(struct snd_card *card, const char *component)
 {
 	char *ptr;
 	int len = strlen(component);
+	unsigned int cur_len, need_len;
 
-	ptr = strstr(card->components, component);
-	if (ptr != NULL) {
-		if (ptr[len] == '\0' || ptr[len] == ' ')	/* already there */
-			return 1;
+	guard(rwsem_write)(&snd_ioctl_rwsem);
+
+	if (card->components) {
+		ptr = strstr(card->components, component);
+		if (ptr) {
+			if (ptr[len] == '\0' || ptr[len] == ' ')	/* already there */
+				return 1;
+		}
+		cur_len = strlen(card->components) + 1;
+	} else {
+		cur_len = 0;
 	}
-	if (strlen(card->components) + 1 + len + 1 > sizeof(card->components)) {
+
+	need_len = cur_len + len + 1;
+	if (need_len > 512) {
 		snd_BUG();
 		return -ENOMEM;
 	}
+
+	if (need_len > card->components_alloc_size) {
+		unsigned int new_alloc = roundup(need_len, 32);
+
+		ptr = krealloc(card->components, new_alloc, GFP_KERNEL);
+		if (!ptr)
+			return -ENOMEM;
+		if (!card->components)
+			ptr[0] = '\0';
+		card->components = ptr;
+		card->components_alloc_size = new_alloc;
+	}
+
 	if (card->components[0] != '\0')
 		strcat(card->components, " ");
 	strcat(card->components, component);