ALSA: pcm: Return -EBUSY for OSS ioctls changing busy streams

OSS PCM stream management isn't modal but it allows ioctls issued at
any time for changing the parameters.  In the previous hardening
patch ("ALSA: pcm: Avoid potential races between OSS ioctls and
read/write"), we covered these races and prevent the corruption by
protecting the concurrent accesses via params_lock mutex.  However,
this means that some ioctls that try to change the stream parameter
(e.g. channels or format) would be blocked until the read/write
finishes, and it may take really long.

Basically changing the parameter while reading/writing is an invalid
operation, hence it's even more user-friendly from the API POV if it
returns -EBUSY in such a situation.

This patch adds such checks in the relevant ioctls with the addition
of read/write access refcount.

Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>

include/sound/pcm_oss.h

@@ -57,6 +57,7 @@ struct snd_pcm_oss_runtime {
 	char *buffer;				/* vmallocated period */
 	size_t buffer_used;			/* used length from period buffer */
 	struct mutex params_lock;
+	atomic_t rw_ref;		/* concurrent read/write accesses */
 #ifdef CONFIG_SND_PCM_OSS_PLUGINS
 	struct snd_pcm_plugin *plugin_first;
 	struct snd_pcm_plugin *plugin_last;