Re: [PATCH] ALSA: control led: fix memory leak in snd_ctl_led_register

From: Dongliang Mu
Date: Mon May 31 2021 - 04:08:44 EST

On Mon, May 31, 2021 at 3:34 PM Dongliang Mu <mudongliangabcd@xxxxxxxxx> wrote:
>
> On Mon, May 31, 2021 at 3:03 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxx> 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@xxxxxxxxx> wrote:
> > > > > >
> > > > > >
> > > > > >
> > > > > > > On May 28, 2021, at 10:05 PM, Dan Carpenter <dan.carpenter@xxxxxxxxxx> 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.
>
> Oh, I see your point. Yeah, we should fix these memory leaks all. I
> agree with device_unregister.
>
> >
> > If both device_initialize() and device_add() succeed then call
> > device_unregister() to unwind.
>
> BTW, have you tested this new patch on two memory leaks?
>

Please keep in mind that if we don't have any release method for
struct snd_ctl_led_card, it will trigger a WARN[1] in the
device_release function. That's why I have to add one dummy release
function.

if (dev->release)
dev->release(dev);
else if (dev->type && dev->type->release)
dev->type->release(dev);
else if (dev->class && dev->class->dev_release)
dev->class->dev_release(dev);
else
WARN(1, KERN_ERR "Device '%s' does not have a release()
function, it is broken and must be fixed. See
Documentation/core-api/kobject.rst.\n",
dev_name(dev));

[1] https://elixir.bootlin.com/linux/latest/source/drivers/base/core.c#L2110

> >
> > 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);
> > }
> >