[PATCH] pcm: Fix shm initialization race-condition

[PATCH] pcm: Fix shm initialization race-condition

[Ismael Luceno]

Easily seen when two threads try at the same time, one of them will fail.

The bug was identified by using apulse with Skype.

Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
Fixes: https://github.com/i-rinat/apulse/issues/38
Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
---
 src/pcm/pcm_direct.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
index c3925cc20fd3..643498375b34 100644
--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -96,11 +96,12 @@ int snd_pcm_direct_shm_create_or_connect(snd_pcm_direct_t *dmix)
 retryget:
 	dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
 			     dmix->ipc_perm);
-	if (dmix->shmid < 0) {
-		if (errno == ENOENT)
+	if (dmix->shmid < 0 && errno == ENOENT) {
 		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
 					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
 			first_instance = 1;
+		else if (errno == EEXIST)
+			goto retryget;
 	}
 	err = -errno;
 	if (dmix->shmid < 0) {
-- 
2.9.2

Re: [PATCH] pcm: Fix shm initialization race-condition

[Ismael Luceno]

On 22/Ago/2016 13:04, Ismael Luceno wrote:
> Easily seen when two threads try at the same time, one of them will fail.
> 
> The bug was identified by using apulse with Skype.
> 
> Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> Fixes: https://github.com/i-rinat/apulse/issues/38
> Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
<...>

Oops, subject should be: [PATCH v2]...

Re: [PATCH] pcm: Fix shm initialization race-condition

[Takashi Iwai]

On Mon, 22 Aug 2016 18:04:33 +0200,
Ismael Luceno wrote:
> 
> Easily seen when two threads try at the same time, one of them will fail.
> 
> The bug was identified by using apulse with Skype.
> 
> Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> Fixes: https://github.com/i-rinat/apulse/issues/38
> Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>

OK, I'll apply this one, as this change alone is fine.

But in general, this implies that the application hitting this is
buggy.  You can't reliably call snd_pcm_open() for the same PCM stream
concurrently via multi threads.  You have to introduce the proper
protection in the caller side.


thanks,

Takashi


> ---
>  src/pcm/pcm_direct.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> index c3925cc20fd3..643498375b34 100644
> --- a/src/pcm/pcm_direct.c
> +++ b/src/pcm/pcm_direct.c
> @@ -96,11 +96,12 @@ int snd_pcm_direct_shm_create_or_connect(snd_pcm_direct_t *dmix)
>  retryget:
>  	dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>  			     dmix->ipc_perm);
> -	if (dmix->shmid < 0) {
> -		if (errno == ENOENT)
> +	if (dmix->shmid < 0 && errno == ENOENT) {
>  		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>  					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
>  			first_instance = 1;
> +		else if (errno == EEXIST)
> +			goto retryget;
>  	}
>  	err = -errno;
>  	if (dmix->shmid < 0) {
> -- 
> 2.9.2
> 

Re: [PATCH] pcm: Fix shm initialization race-condition

[Ismael Luceno]

On 22/Ago/2016 17:11, Takashi Iwai wrote:
> On Mon, 22 Aug 2016 17:02:41 +0200,
> Ismael Luceno wrote:
> > 
> > On 22/Ago/2016 11:26, Takashi Iwai wrote:
> > > On Sun, 14 Aug 2016 02:28:52 +0200,
> > > Ismael Luceno wrote:
> > > > 
> > > > Easily seen when two threads try at the same time, one of them will fail.
> > > > 
> > > > The bug was identified by using apulse with Skype.
> > > > 
> > > > Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> > > > Fixes: https://github.com/i-rinat/apulse/issues/38
> > > > Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
> > > > ---
> > > >  src/pcm/pcm_direct.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> > > > index c3925cc20fd3..b5215ba35406 100644
> > > > --- a/src/pcm/pcm_direct.c
> > > > +++ b/src/pcm/pcm_direct.c
> > > > @@ -101,6 +101,8 @@ retryget:
> > > >  		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
> > > >  					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
> > > >  			first_instance = 1;
> > > > +		if (dmix->shmid < 0 && errno == EEXIST)
> > > > +			goto retryget;
> > > 
> > > Hrm, but this would result in an endless loop if the shm was already
> > > taken persistently.
> > 
> > If so, shouldn't the first call to shmget succeed?
> > 
> > To me it seems very unlikely that both calls continuosly fail.
> 
> Well, you are inserting a loop at the code below:
> 
> retryget:
>         dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>                              dmix->ipc_perm);
>         if (dmix->shmid < 0) {
>                 if (errno == ENOENT)
>                 if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>                                              IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
>                         first_instance = 1;
> ==>		if (dmix->shmid < 0 && errno == EEXIST)
> ==>			goto retryget;
>         }
> 
> 
> It's in the if block when the first shmget() fails.  If there was
> already a shm with the given id that had been assigned by another (not
> necessarily by alsa-lib but by whatever program), the first shmget
> returns an error with EEXIST.  Then it goes back again in a loop; and
> it can be endless if another program doesn't release the shm.

I will fix my patch, I see a problem now, but the spirit of the
idea should be right: if the first call fails with ENOENT, and the
second with EEXIST, it should retry, and getting EEXIST again should
be very unlikely.

> > > Also, which call does give a negative shmid, actually?  It's from the
> > > first shmget() or the second shmget()?
> > 
> > What happens is that both threads go down that path but, of course,
> > only one succeeds in the second shmget call.
> 
> This should have been protected by a sempahore beforehand.
> And if it's about threads, the application itself has to take care of
> the race.  alsa-lib is no thread-safe, after all.
> 
> So, did you see the issue with multiple processes, or is it about the
> multi-threads?

Threads in this case, but could happen with processes as well.

Re: [PATCH] pcm: Fix shm initialization race-condition

[Takashi Iwai]

On Mon, 22 Aug 2016 17:02:41 +0200,
Ismael Luceno wrote:
> 
> On 22/Ago/2016 11:26, Takashi Iwai wrote:
> > On Sun, 14 Aug 2016 02:28:52 +0200,
> > Ismael Luceno wrote:
> > > 
> > > Easily seen when two threads try at the same time, one of them will fail.
> > > 
> > > The bug was identified by using apulse with Skype.
> > > 
> > > Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> > > Fixes: https://github.com/i-rinat/apulse/issues/38
> > > Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
> > > ---
> > >  src/pcm/pcm_direct.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> > > index c3925cc20fd3..b5215ba35406 100644
> > > --- a/src/pcm/pcm_direct.c
> > > +++ b/src/pcm/pcm_direct.c
> > > @@ -101,6 +101,8 @@ retryget:
> > >  		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
> > >  					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
> > >  			first_instance = 1;
> > > +		if (dmix->shmid < 0 && errno == EEXIST)
> > > +			goto retryget;
> > 
> > Hrm, but this would result in an endless loop if the shm was already
> > taken persistently.
> 
> If so, shouldn't the first call to shmget succeed?
> 
> To me it seems very unlikely that both calls continuosly fail.

Well, you are inserting a loop at the code below:

retryget:
        dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
                             dmix->ipc_perm);
        if (dmix->shmid < 0) {
                if (errno == ENOENT)
                if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
                                             IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
                        first_instance = 1;
==>		if (dmix->shmid < 0 && errno == EEXIST)
==>			goto retryget;
        }


It's in the if block when the first shmget() fails.  If there was
already a shm with the given id that had been assigned by another (not
necessarily by alsa-lib but by whatever program), the first shmget
returns an error with EEXIST.  Then it goes back again in a loop; and
it can be endless if another program doesn't release the shm.

> > Also, which call does give a negative shmid, actually?  It's from the
> > first shmget() or the second shmget()?
> 
> What happens is that both threads go down that path but, of course,
> only one succeeds in the second shmget call.

This should have been protected by a sempahore beforehand.
And if it's about threads, the application itself has to take care of
the race.  alsa-lib is no thread-safe, after all.

So, did you see the issue with multiple processes, or is it about the
multi-threads?


Takashi

Re: [PATCH] pcm: Fix shm initialization race-condition

[Ismael Luceno]

On 22/Ago/2016 11:26, Takashi Iwai wrote:
> On Sun, 14 Aug 2016 02:28:52 +0200,
> Ismael Luceno wrote:
> > 
> > Easily seen when two threads try at the same time, one of them will fail.
> > 
> > The bug was identified by using apulse with Skype.
> > 
> > Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> > Fixes: https://github.com/i-rinat/apulse/issues/38
> > Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
> > ---
> >  src/pcm/pcm_direct.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> > index c3925cc20fd3..b5215ba35406 100644
> > --- a/src/pcm/pcm_direct.c
> > +++ b/src/pcm/pcm_direct.c
> > @@ -101,6 +101,8 @@ retryget:
> >  		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
> >  					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
> >  			first_instance = 1;
> > +		if (dmix->shmid < 0 && errno == EEXIST)
> > +			goto retryget;
> 
> Hrm, but this would result in an endless loop if the shm was already
> taken persistently.

If so, shouldn't the first call to shmget succeed?

To me it seems very unlikely that both calls continuosly fail.

> Also, which call does give a negative shmid, actually?  It's from the
> first shmget() or the second shmget()?

What happens is that both threads go down that path but, of course,
only one succeeds in the second shmget call.

Re: [PATCH] pcm: Fix shm initialization race-condition

[Takashi Iwai]

On Sun, 14 Aug 2016 02:28:52 +0200,
Ismael Luceno wrote:
> 
> Easily seen when two threads try at the same time, one of them will fail.
> 
> The bug was identified by using apulse with Skype.
> 
> Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> Fixes: https://github.com/i-rinat/apulse/issues/38
> Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
> ---
>  src/pcm/pcm_direct.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> index c3925cc20fd3..b5215ba35406 100644
> --- a/src/pcm/pcm_direct.c
> +++ b/src/pcm/pcm_direct.c
> @@ -101,6 +101,8 @@ retryget:
>  		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>  					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
>  			first_instance = 1;
> +		if (dmix->shmid < 0 && errno == EEXIST)
> +			goto retryget;

Hrm, but this would result in an endless loop if the shm was already
taken persistently.

Also, which call does give a negative shmid, actually?  It's from the
first shmget() or the second shmget()?
(The existing code is ugly and we need to fix the indentation
 there...)


thanks,

Takashi