[PATCH v3 2/2] ALSA: control: add ioctl to retrieve full card components

Maciej Strozek
2026-03-03T15:02:33Z

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
to retrieve the full components string.

When the legacy components field is truncated, append '>' to indicate
that the full string is available via the new ioctl.

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>
---
Changes for v3:
- change components field to a dynamic array resizable in 32 byte increments
- removed SNDRV_CTL_COMPONENTS_LEN define
- sanity check if 'components' requests more than 512 bytes
- added a commit to clean up trailing whitespaces
- alsa-utils link no longer needed
Changes for v2:
- do not modify existing card->components field
- add a new ioctl and struct to keep the full components string
- handle the split/trim in snd_ctl_card_info()
---
include/sound/core.h | 4 ++--
include/uapi/sound/asound.h | 14 ++++++++++++-
sound/core/control.c | 35 ++++++++++++++++++++++++++++++++-
sound/core/control_compat.c | 2 ++
sound/core/init.c | 39 +++++++++++++++++++++++++++++--------
5 files changed, 82 insertions(+), 12 deletions(-)

diff --git a/include/sound/core.h b/include/sound/core.h
index 4093ec82a0a1..2b58f79b524d 100644
--- a/include/sound/core.h
+++ b/include/sound/core.h
@@ -87,8 +87,8 @@ struct snd_card {
char longname[80]; /* name of this soundcard */
char irq_descr[32]; /* Interrupt description */
char mixername[80]; /* mixer name */
- char components[128]; /* card components delimited with
- space */
+ char *components_ptr;
+ unsigned int components_ptr_alloc_size; // current memory allocation components_ptr.
struct module *module; /* top-level module */

void *private_data; /* private data for soundcard */
diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index d3ce75ba938a..422b0b07613d 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -1058,7 +1058,7 @@ struct snd_timer_tread {
* *
****************************************************************************/

-#define SNDRV_CTL_VERSION SNDRV_PROTOCOL_VERSION(2, 0, 9)
+#define SNDRV_CTL_VERSION SNDRV_PROTOCOL_VERSION(2, 0, 10)

struct snd_ctl_card_info {
int card; /* card number */
@@ -1072,6 +1072,17 @@ struct snd_ctl_card_info {
unsigned char components[128]; /* card components / fine identification, delimited with one space (AC97 etc..) */
};

+/*
+ * Card components can exceed the fixed 128 bytes in snd_ctl_card_info.
+ * Use SNDRV_CTL_IOCTL_CARD_COMPONENTS to retrieve the full string.
+ *
+ */
+struct snd_ctl_card_components {
+ int card;
+ unsigned int length;
+ unsigned char *components;
+};
+
typedef int __bitwise snd_ctl_elem_type_t;
#define SNDRV_CTL_ELEM_TYPE_NONE ((__force snd_ctl_elem_type_t) 0) /* invalid */
#define SNDRV_CTL_ELEM_TYPE_BOOLEAN ((__force snd_ctl_elem_type_t) 1) /* boolean type */
@@ -1198,6 +1209,7 @@ struct snd_ctl_tlv {

#define SNDRV_CTL_IOCTL_PVERSION _IOR('U', 0x00, int)
#define SNDRV_CTL_IOCTL_CARD_INFO _IOR('U', 0x01, struct snd_ctl_card_info)
+#define SNDRV_CTL_IOCTL_CARD_COMPONENTS _IOWR('U', 0x02, struct snd_ctl_card_components)
#define SNDRV_CTL_IOCTL_ELEM_LIST _IOWR('U', 0x10, struct snd_ctl_elem_list)
#define SNDRV_CTL_IOCTL_ELEM_INFO _IOWR('U', 0x11, struct snd_ctl_elem_info)
#define SNDRV_CTL_IOCTL_ELEM_READ _IOWR('U', 0x12, struct snd_ctl_elem_value)
diff --git a/sound/core/control.c b/sound/core/control.c
index 374e703d15a9..d793dbf85d15 100644
--- a/sound/core/control.c
+++ b/sound/core/control.c
@@ -876,9 +876,13 @@ 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) {
info->card = card->number;
strscpy(info->id, card->id, sizeof(info->id));
@@ -886,13 +890,40 @@ static int snd_ctl_card_info(struct snd_card *card, struct snd_ctl_file * ctl,
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, card->components_ptr, 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_components(struct snd_card *card,
+ struct snd_ctl_card_components __user *_info)
+{
+ struct snd_ctl_card_components info;
+ unsigned int copy_len;
+
+ if (copy_from_user(&info, _info, sizeof(info)))
+ return -EFAULT;
+ if (!info.components)
+ return -EINVAL;
+
+ scoped_guard(rwsem_read, &snd_ioctl_rwsem) {
+ copy_len = strlen(card->components_ptr) + 1;
+ info.card = card->number;
+ info.length = copy_len;
+ if (copy_to_user(info.components, card->components_ptr, copy_len))
+ return -EFAULT;
+ }
+
+ if (copy_to_user(_info, &info, sizeof(info)))
+ return -EFAULT;
+
+ return 0;
+}
+
static int snd_ctl_elem_list(struct snd_card *card,
struct snd_ctl_elem_list *list)
{
@@ -1988,6 +2019,8 @@ 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_COMPONENTS:
+ return snd_ctl_card_components(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..d325f9d0b8a1 100644
--- a/sound/core/control_compat.c
+++ b/sound/core/control_compat.c
@@ -456,6 +456,8 @@ static inline long snd_ctl_ioctl_compat(struct file *file, unsigned int cmd, uns
case SNDRV_CTL_IOCTL_TLV_WRITE:
case SNDRV_CTL_IOCTL_TLV_COMMAND:
return snd_ctl_ioctl(file, cmd, (unsigned long)argp);
+ case SNDRV_CTL_IOCTL_CARD_COMPONENTS:
+ return snd_ctl_card_components(ctl->card, argp);
case SNDRV_CTL_IOCTL_ELEM_LIST32:
return snd_ctl_elem_list_compat(ctl->card, argp);
case SNDRV_CTL_IOCTL_ELEM_INFO32:
diff --git a/sound/core/init.c b/sound/core/init.c
index 593c05895e11..426ed8916aa9 100644
--- a/sound/core/init.c
+++ b/sound/core/init.c
@@ -590,6 +590,8 @@ 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_ptr);
+ card->components_ptr_alloc_size = 0;
if (card->private_free)
card->private_free(card);
#ifdef CONFIG_SND_CTL_DEBUG
@@ -1036,19 +1038,40 @@ 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;
+ if (card->components_ptr) {
+ ptr = strstr(card->components_ptr, component);
+ if (ptr) {
+ if (ptr[len] == '\0' || ptr[len] == ' ') /* already there */
+ return 1;
+ }
+ cur_len = strlen(card->components_ptr) + 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 (card->components[0] != '\0')
- strcat(card->components, " ");
- strcat(card->components, component);
+
+ if (need_len > card->components_ptr_alloc_size) {
+ unsigned int new_alloc = roundup(need_len, 32);
+
+ ptr = krealloc(card->components_ptr, new_alloc, GFP_KERNEL);
+ if (!ptr)
+ return -ENOMEM;
+ if (!card->components_ptr)
+ ptr[0] = '\0';
+ card->components_ptr = ptr;
+ card->components_ptr_alloc_size = new_alloc;
+ }
+
+ if (card->components_ptr[0] != '\0')
+ strcat(card->components_ptr, " ");
+ strcat(card->components_ptr, component);
return 0;
}
EXPORT_SYMBOL(snd_component_add);
--
2.48.1