[PATCH] ALSA: timer: Handle userspace-driven timer ioctls from 32-bit tasks

[Danish Khateeb <danishkhateeb03@gmail.com>]

32-bit tasks on a 64-bit kernel can't use userspace-driven timers. The
compat handler for /dev/snd/timer doesn't list SNDRV_TIMER_IOCTL_CREATE,
so creating a timer fails with -ENOTTY. The file returned for a timer
has no .compat_ioctl either, so SNDRV_TIMER_IOCTL_TRIGGER fails with
-ENOTTY too.

struct snd_timer_uinfo has the same layout for 32-bit and 64-bit tasks,
and SNDRV_TIMER_IOCTL_TRIGGER takes no argument, so both commands can
be passed through unchanged.

The utimer selftest doesn't catch this when built with -m32: it takes
-ENOTTY from SNDRV_TIMER_IOCTL_CREATE to mean CONFIG_SND_UTIMER=n and
skips.

Fixes: 37745918e0e7 ("ALSA: timer: Introduce virtual userspace-driven timers")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
---

Notes:
    Tested on x86_64 in QEMU (sound/for-linus plus this patch, KASAN and lockdep):
    - A 32-bit program that creates a utimer, starts it on a second
      /dev/snd/timer fd and triggers it 10 times gets -ENOTTY from CREATE
      before. After, it reads back 10 ticks, the same as the 64-bit build.
    - The -m32 utimer selftest skips before ("CONFIG_SND_UTIMER not
      enabled") and runs after. Its tick-count case fails before and after,
      in 32-bit and 64-bit builds alike: the global-timer helper segfaults
      on this host once the timer ticks.
    - No splats. Built with W=1 for arm64 (COMPAT=y, LLVM=1) and for x86_64
      with SND_UTIMER=n.

 sound/core/timer.c        | 1 +
 sound/core/timer_compat.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/sound/core/timer.c b/sound/core/timer.c
index 679b26435670..69a59b25fe0a 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -2176,6 +2176,7 @@ static const struct file_operations snd_utimer_fops = {
 	.llseek		=	noop_llseek,
 	.release	=	snd_utimer_release,
 	.unlocked_ioctl	=	snd_utimer_ioctl,
+	.compat_ioctl	=	snd_utimer_ioctl,
 };
 
 static int snd_utimer_start(struct snd_timer *t)
diff --git a/sound/core/timer_compat.c b/sound/core/timer_compat.c
index 25ee81c1668b..5bab00466de7 100644
--- a/sound/core/timer_compat.c
+++ b/sound/core/timer_compat.c
@@ -99,6 +99,7 @@ static long __snd_timer_user_ioctl_compat(struct file *file, unsigned int cmd,
 	case SNDRV_TIMER_IOCTL_PAUSE:
 	case SNDRV_TIMER_IOCTL_PAUSE_OLD:
 	case SNDRV_TIMER_IOCTL_NEXT_DEVICE:
+	case SNDRV_TIMER_IOCTL_CREATE:
 		return __snd_timer_user_ioctl(file, cmd, (unsigned long)argp, true);
 	case SNDRV_TIMER_IOCTL_GPARAMS32:
 		return snd_timer_user_gparams_compat(file, argp);

base-commit: f090e7ff26fe08be4cd57e0950872ede4af5351a
-- 
2.55.0