linux-kernel - RE: [PATCH v2 8/8] ALSA: aloop: Support runtime change of snd

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Message-ID: <2dc6e7841e97441aa3b91fca8e5629e9@svr-ies-mbx-02.mgc.mentorg.com>
Date:   Thu, 7 Nov 2019 10:40:18 +0000
From:   "Gabbasov, Andrew" <Andrew_Gabbasov@...tor.com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jaroslav Kysela" <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        Timo Wischer <twischer@...adit-jv.com>
Subject: RE: [PATCH v2 8/8] ALSA: aloop: Support runtime change of snd_timer
 via info interface

Hello Takashi,

Thank you very much for your feedback!

> -----Original Message-----
> From: Takashi Iwai [mailto:tiwai@...e.de]
> Sent: Thursday, November 07, 2019 11:06 AM
> To: Gabbasov, Andrew
> Cc: alsa-devel@...a-project.org; linux-kernel@...r.kernel.org; Jaroslav
> Kysela; Takashi Iwai; Timo Wischer
> Subject: Re: [PATCH v2 8/8] ALSA: aloop: Support runtime change of
> snd_timer via info interface
> 
> On Tue, 05 Nov 2019 15:32:18 +0100,
> Andrew Gabbasov wrote:
> >
> > Show and change sound card timer source with read-write info
> > file in proc filesystem. Initial string can still be set as
> > module parameter.
> >
> > The timer source string value can be changed at any time,
> > but it is latched by PCM substream open callback (the first one
> > for a particular cable). At this point it is actually used, that
> > is the string is parsed, and the timer is looked up and opened.
> >
> > The timer source is set for a loopback card (the same as initial
> > setting by module parameter), but every cable uses the value,
> > current at the moment of open.
> >
> > Setting the value to empty string switches the timer to jiffies.
> >
> > Signed-off-by: Andrew Gabbasov <andrew_gabbasov@...tor.com>
> 
> Unfortunately the whole code here are racy.  It may lead to a crash or
> use-after-free easily.  Some locking is needed definitely.

You are right, using and changing of loopback->timer_source should be protected.
I'll add locking with loopback->cable_lock to the bodies of print_timer_source_info()
and change_timer_source_info() (like in the example diff below), similarly to other
/proc files and mixer controls. All other uses of loopback->timer_source are already
covered by loopback->cable_lock, except for loopback_set_timer_source() call from
loopback_probe(), that is done at the very early stage and doesn't conflict with other
uses. I think, in order to avoid racing problems, this will be enough, won't it?

Thanks.

Best regards,
Andrew

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index 415128a97774..ca9307dd780e 100644
--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -1684,8 +1684,10 @@ static void print_timer_source_info(struct snd_info_entry *entry,
 {
        struct loopback *loopback = entry->private_data;

+       mutex_lock(&loopback->cable_lock);
        snd_iprintf(buffer, "%s\n",
                    loopback->timer_source ? loopback->timer_source : "");
+       mutex_unlock(&loopback->cable_lock);
 }

 static void change_timer_source_info(struct snd_info_entry *entry,
@@ -1694,8 +1696,10 @@ static void change_timer_source_info(struct snd_info_entry *entry,
        struct loopback *loopback = entry->private_data;
        char line[64];

+       mutex_lock(&loopback->cable_lock);
        if (!snd_info_get_line(buffer, line, sizeof(line)))
                loopback_set_timer_source(loopback, strim(line));
+       mutex_unlock(&loopback->cable_lock);
 }

 static int loopback_timer_source_proc_new(struct loopback *loopback)