linux-kernel - Re: [PATCH] ALSA: control led: fix memory leak in snd_ctl_led

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Date:   Mon, 31 May 2021 10:03:38 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Dongliang Mu <mudongliangabcd@...il.com>
Cc:     perex@...ex.cz, tiwai@...e.com, alsa-devel@...a-project.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzbot+08a7d8b51ea048a74ffb@...kaller.appspotmail.com
Subject: Re: [PATCH] ALSA: control led: fix memory leak in
 snd_ctl_led_register

On Mon, May 31, 2021 at 02:20:37PM +0800, Dongliang Mu wrote:
> On Mon, May 31, 2021 at 12:40 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> >
> > On Mon, May 31, 2021 at 11:03:36AM +0800, Dongliang Mu wrote:
> > > On Sat, May 29, 2021 at 5:35 AM 慕冬亮 <mudongliangabcd@...il.com> wrote:
> > > >
> > > >
> > > >
> > > > > On May 28, 2021, at 10:05 PM, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > > > >
> > > > > On Fri, May 28, 2021 at 09:50:49PM +0800, Dongliang Mu wrote:
> > > > >>
> > > > >> Can you please give some advise on how to fix this WARN issue?
> > > > >
> > > > > But it feels like it spoils the fun if I write the commit...  Anyway:
> > > >
> > > > It’s fine. I am still in the learning process. It’s also good to learn experience by comparing your patch and my patch.
> > > >
> > > > >
> > > > > regards,
> > > > > dan carpenter
> > > > >
> > > > > diff --git a/sound/core/control_led.c b/sound/core/control_led.c
> > > > > index 25f57c14f294..dd357abc1b58 100644
> > > > > --- a/sound/core/control_led.c
> > > > > +++ b/sound/core/control_led.c
> > > > > @@ -740,6 +740,7 @@ static int __init snd_ctl_led_init(void)
> > > > >                       for (; group > 0; group--) {
> > > > >                               led = &snd_ctl_leds[group - 1];
> > > > >                               device_del(&led->dev);
> > > > > +                             device_put(&led->dev);
> > > > >                       }
> > > > >                       device_del(&snd_ctl_led_dev);
> > > > >                       return -ENOMEM;
> > > > > @@ -768,6 +769,7 @@ static void __exit snd_ctl_led_exit(void)
> > > > >       for (group = 0; group < MAX_LED; group++) {
> > > > >               led = &snd_ctl_leds[group];
> > > > >               device_del(&led->dev);
> > > > > +             device_put(&led->dev);
> > > > >       }
> > > > >       device_del(&snd_ctl_led_dev);
> > > > >       snd_ctl_led_clean(NULL);
> > >
> > > Hi Dan,
> > >
> > > I tried this patch, and it still triggers the memleak.
> >
> > Did your patch fix the leak?  Because my patch should have been
> > equivalent except for it fixes an additional leak in the snd_ctl_led_init()
> > error path.
> 
> The syzbot link is [1]. I have tested my patch in the syzbot dashboard
> and my local workspace.
> 
> I think the reason why your patch did not work should be
> led_card(struct snd_ctl_led_card) is already freed before returning in
> snd_ctl_led_sysfs_remove, rather than led(struct snd_ctl_led). See the
> implementation of snd_ctl_led_sysfs_remove for some details. Please
> correct me if I make any mistakes.
> 
> static void snd_ctl_led_sysfs_remove(struct snd_card *card)
> {
>         unsigned int group;
>         struct snd_ctl_led_card *led_card;
>         struct snd_ctl_led *led;
>         char link_name[32];
> 
>         for (group = 0; group < MAX_LED; group++) {
>                 led = &snd_ctl_leds[group];
>                 led_card = led->cards[card->number];
>                 if (!led_card)
>                         continue;
>                 snprintf(link_name, sizeof(link_name), "led-%s", led->name);
>                 sysfs_remove_link(&card->ctl_dev.kobj, link_name);
>                 sysfs_remove_link(&led_card->dev.kobj, "card");
>                 device_del(&led_card->dev);
>                 put_device(&led_card->dev);
>                 kfree(led_card);
>                 led->cards[card->number] = NULL;
>         }
> }

This is frustrating to look at because it's not a diff so it doesn't
show what you changed.  I think you are saying that you added the
put_device(&led_card->dev);.  That's true.  There are some other leaks
as well.  We should just fix them all.  Use device_unregister() because
it's cleaner.

If both device_initialize() and device_add() succeed then call
device_unregister() to unwind.

diff --git a/sound/core/control_led.c b/sound/core/control_led.c
index 25f57c14f294..561fe45e4449 100644
--- a/sound/core/control_led.c
+++ b/sound/core/control_led.c
@@ -700,7 +700,7 @@ static void snd_ctl_led_sysfs_remove(struct snd_card *card)
 		snprintf(link_name, sizeof(link_name), "led-%s", led->name);
 		sysfs_remove_link(&card->ctl_dev.kobj, link_name);
 		sysfs_remove_link(&led_card->dev.kobj, "card");
-		device_del(&led_card->dev);
+		device_unregister(&led_card->dev);
 		kfree(led_card);
 		led->cards[card->number] = NULL;
 	}
@@ -739,9 +739,9 @@ static int __init snd_ctl_led_init(void)
 			put_device(&led->dev);
 			for (; group > 0; group--) {
 				led = &snd_ctl_leds[group - 1];
-				device_del(&led->dev);
+				device_unregister(&led->dev);
 			}
-			device_del(&snd_ctl_led_dev);
+			device_unregister(&snd_ctl_led_dev);
 			return -ENOMEM;
 		}
 	}
@@ -767,9 +767,9 @@ static void __exit snd_ctl_led_exit(void)
 	}
 	for (group = 0; group < MAX_LED; group++) {
 		led = &snd_ctl_leds[group];
-		device_del(&led->dev);
+		device_unregister(&led->dev);
 	}
-	device_del(&snd_ctl_led_dev);
+	device_unregister(&snd_ctl_led_dev);
 	snd_ctl_led_clean(NULL);
 }