Re: [PATCH 09/10] ALSA: pcm: Add snd_pcm_ops for snd_pcm_link()
From: Timo Wischer
Date: Wed Mar 27 2019 - 05:27:12 EST
On 3/27/19 10:11, Takashi Iwai wrote:
On Wed, 27 Mar 2019 09:34:40 +0100,
Timo Wischer wrote:
On 3/26/19 17:00, Takashi Iwai wrote:
On Tue, 26 Mar 2019 16:16:54 +0100,
Timo Wischer wrote:
On 3/26/19 15:23, Takashi Iwai wrote:
On Tue, 26 Mar 2019 12:25:37 +0100,
Timo Wischer wrote:
On 3/26/19 09:35, Takashi Iwai wrote:
On Tue, 26 Mar 2019 08:49:33 +0100,
<twischer@xxxxxxxxxxxxxx> wrote:
From: Timo Wischer <twischer@xxxxxxxxxxxxxx>
snd_pcm_link() can be called by the user as long
as the device is not
yet started. Therefore currently a driver which wants to iterate over
the linked substreams has to do this at the start trigger. But the start
trigger should not block for a long time. Therefore there is no callback
which can be used to iterate over the linked substreams without delaying
the start trigger.
This patch introduces a new callback function which will be called after
the linked substream list was updated by snd_pcm_link(). This callback
function is allowed to block for a longer time without interfering the
synchronized start up of linked substreams.
Signed-off-by: Timo Wischer
<twischer@xxxxxxxxxxxxxx>
Well, the idea appears interesting, but I'm afraid
that the
implementation is still racy. The place you're calling the new
callback isn't protected, hence the stream can be triggered while
calling it. That is, even during operating your loopback link_changed
callback, another thread is able to start the stream.
Hi Takashi,
As far as I got you mean the following scenario:
* snd_pcm_link() is called for a HW sound card
+ loopback_snd_timer_link_changed()
The start may happen at this point.
In this case the last link status will be used and aloop will print a
warning "Another sound timer was requested but at least one device is
already running...".
Without this patch set a similar issue already exists. When calling
snd_pcm_start() before snd_pcm_link() was done the additional device
linked by the snd_pcm_link() will not be started.
Therefore the application has already to take care about the order of
the calls.
Yes, but it doesn't matter for now, just because other drivers do care
the PCM links only for trigger callback. Now you're trying to add
something new but in an incomplete manner.
+ loopback_snd_timer_open()
+ spin_lock_irqsave(&dpcm->cable->lock, flags);
* snd_pcm_start() called for aloop sound card
+ loopback_trigger()
+ spin_lock(&cable->lock) -> has to wait till loopback_snd_timer_open()
calls spin_unlock_irqrestore()
So far snd_pcm_start() has to wait for loopback_snd_timer_open().
* loopback_snd_timer_open() will continue with
+ dpcm->cable->snd_timer.instance = NULL;
+ spin_unlock_irqrestore()
* loopback_trigger() can enter the lock
+ loopback_snd_timer_start() will fail with -EINVAL due to
(loopback_trigger == NULL)
At least this will not result into memory corruption due to race or any other
wired behavior.
I don't expect the memory corruption, but my point is that dealing
with linked streams is still tricky. It was considered for the
lightweight coupled start/stop operation, and something intensively
depending on the linked status was out of the original design...
But my expectation is that snd_pcm_link(hw, aloop) or snd_pcm_link(aloop, hw)
is only called by the application calling snd_pcm_start(aloop)
because the same aloop device cannot be opened by multiple applications at the
same time.
Do you see an use case where one application would call snd_pcm_start() in
parallel with snd_pcm_link() (somehow configuring the device)?
It's not about the actual application usages but rather against the
malicious attacks. Especially aloop is a virtual device that is
available allover the places, it may be deployed / attacked easily.
The attack we are identifying here can only be done by the application
opening the aloop device.
An application allowed to open the aloop device is anyway able to
manipulate the audio streaming.
Right, and if it such a racy access may lead to a driver misbehavior,
it's a big concern. The proposed callback usage is racy, so some
other implementation might be broken easily in future.
Or do you see an attack which would influence any other device/stream
not opened by this application?
May be we should add an additional synchronization mechanism in pcm_native.c
to avoid call of snd_pcm_link() in parallel with snd_pcm_start().
If it really matters... Honestly speaking, I'm not fully convinced
whether we want to deal with this using the PCM link mechanism.
What's the motivation for using the linked streams at the first place?
That's one of the biggest missing piece in the whole picture.
In general when the user uses snd_pcm_link() it expects that the
linked devices are somehow synchronized.
Any applications already using snd_pcm_link() do not need to be
adapted to use the new feature of aloop (for example JACK or ALSA
multi plugin)
But when linking a HW sound card and aloop without this patch set,
both devices will be started in sync but
the snd_pcm_period_eleapsed() calls of the different devices will
drift. To avoid this the aloop plugin can automatically use the right
timer.
If this feature is not implemented the user has to use snd_pcm_link()
to trigger snd_pcm_start() in sync but also has to configure the aloop
plugin to use the right sound timer.
May be the linked cards can change during runtime of the
system. Without this feature the aloop kernel driver has to be loaded
with different kernel parameters
to select the right timer.
ALSA controls cannot be used easily. Selecting the sound timer by the
card number could be error prone because the card ID could change
between system starts.
Therefore an ALSA control supporting the card name should be
used. This could be for example done via an ALSA enum control. But in
this case the names of all sound cards of the system has to be
available
on aloop probe() call. But at this point in time the sound cards
probed after aloop are not available. Therefore only the sound timers
of the sound cards probed before aloop are selectable.
Hm. For me this patch series looks very hackish. As mentioned, the
PCM link usage is rather just a synchronous trigger start/stop for
multiple streams belonging to the same hardware; in that sense, it'd
be possible to adapt some mechanism for aloop, but at most it should
be much less intrusive change, e.g. just doing the multiple
loopback_timer_start() in a single loop.
What do you mean by "just doing the multiple loopback_timer_start() in
a single loop."?
The snd_pcm_link() call information are mainly used to select the
right sound timer. Starting the timer in sync is a nice add-on.
Therefore by simple starting all timers in a for loop I would not
select the right sound timer.
(May be I misunderstood your example)
My example is only about the system timer.
I could call the link_changed() callback inside
snd_pcm_stream_lock_irq() same lock also hold by snd_pcm_start().
But an application could still call snd_pcm_link() and snd_pcm_start()
in parallel e.g.:
snd_pcm_common_ioctl(IOCTL_LINK)
task switch
snd_pcm_common_ioctl(IOCTL_START)
snd_pcm_start()
done
continue previous task
The same could for example happen with snd_pcm_start() and snd_hw_free():
snd_pcm_common_ioctl(IOCTL_START)
task switch
snd_pcm_common_ioctl(IOCTL_HW_FREE)
snd_hw_free()
Done
continue previous task
snd_pcm_start() will fail with an error code
Therefore I do not think we have to synchronize the IOCTL calls (this
can only be done in the user application or ALSAlib). We only have to
return an proper error code in case of misusage.
As far as I understand ALSA was not designed for multithread usage. So
the user (or ALSAlib) has to be aware of calling the functions in the
right order.
Otherwise the user could expect returned error codes.
Well, your assumption is plain wrong. ALSA is designed for
multithread usage. And the kernel works for multithread.
It's not about the right usage. It's about the robustness, about
whether any malicious code may lead to a serious defunct. The opened
race in calls may easily introduce such a failure.
In case of my patch set snd_pcm_start() will always fail with a proper
error code as long as no sound timer is running. This could be the
case if snd_pcm_start() is called before the first snd_pcm_link()
call.
This is also the case if snd_pcm_start() is called when the
snd_pcm_link() call has closed the old timer but not yet opened the
new one.
In case of snd_pcm_link() is called after snd_pcm_start() aloop will
write a proper warning (that the snd_pcm_link() call will have no
effect because the stream is already running).
All member variables used in loopback_snd_timer_open(),
loopback_snd_timer_source_update() and loopback_snd_timer_start() are
always protected by cable->lock. Therefore no one can read an invalid
state of these variables and the misusage can always be detected.
Therefore I am not sure what I should change. May be you could
reference to a source code line which looks hackish for you?
In principle, the PCM ops is supposed to be safe for operating a
certain stuff. If a state change may happen during the operation,
this should be called inside PCM stream lock. So the design of the
new callback itself is fragile.
then, it comes to a point to re-setup the timer at the link change.
The idea is interesting, but it's a wrong usage of PCM link feature,
to be honest.
That said, I find the changes up to patch 8 are acceptable (with some
fixes expected), but the link feature isn't.
Thanks for clarification. Do you think there is a chance to get an
acceptable snd_pcm_link() feature when using PCM stream lock for
snd_pcm_link()?
In this case I think snd_timer_open() has to be adapted to use
non-blocking calls only (do not use mutexs).
Or do I have to use another API to configure the sound timer at runtime
(e.g. ALSA controls or RW kernel module parameters)?
Best regards
Timo