Re: INFO: REPRODUCED: memory leak in gpio device in 6.2-rc6

[Mirsad Goran Todorovac]

On 21.2.2023. 14:52, Mirsad Goran Todorovac wrote:
> On 20. 02. 2023. 14:43, Andy Shevchenko wrote:
> > On Mon, Feb 20, 2023 at 02:10:00PM +0100, Mirsad Todorovac wrote:
> > > On 2/16/23 15:16, Bartosz Golaszewski wrote:
> >
> > ...
> >
> > > As Mr. McKenney once said, a bunch of monkeys with keyboard could
> > > have done it in a considerable number of trials and errors ;-)
> > >
> > > But here I have something that could potentially leak as well. I could not devise a
> > > reproducer due to the leak being lightly triggered only in extreme memory contention.
> > >
> > > See it for yourself:
> > >
> > > drivers/gpio/gpio-sim.c:
> > >  301 static int gpio_sim_setup_sysfs(struct gpio_sim_chip *chip)
> > >  302 {
> > >  303         struct device_attribute *val_dev_attr, *pull_dev_attr;
> > >  304         struct gpio_sim_attribute *val_attr, *pull_attr;
> > >  305         unsigned int num_lines = chip->gc.ngpio;
> > >  306         struct device *dev = chip->gc.parent;
> > >  307         struct attribute_group *attr_group;
> > >  308         struct attribute **attrs;
> > >  309         int i, ret;
> > >  310
> > >  311         chip->attr_groups = devm_kcalloc(dev, sizeof(*chip->attr_groups),
> > >  312                                          num_lines + 1, GFP_KERNEL);
> > >  313         if (!chip->attr_groups)
> > >  314                 return -ENOMEM;
> > >  315
> > >  316         for (i = 0; i < num_lines; i++) {
> > >  317                 attr_group = devm_kzalloc(dev, sizeof(*attr_group), GFP_KERNEL);
> > >  318                 attrs = devm_kcalloc(dev, GPIO_SIM_NUM_ATTRS, sizeof(*attrs),
> > >  319                                      GFP_KERNEL);
> > >  320                 val_attr = devm_kzalloc(dev, sizeof(*val_attr), GFP_KERNEL);
> > >  321                 pull_attr = devm_kzalloc(dev, sizeof(*pull_attr), GFP_KERNEL);
> > >  322                 if (!attr_group || !attrs || !val_attr || !pull_attr)
> > >  323                         return -ENOMEM;
> > >  324
> > >  325                 attr_group->name = devm_kasprintf(dev, GFP_KERNEL,
> > >  326                                                   "sim_gpio%u", i);
> > >  327                 if (!attr_group->name)
> > >  328                         return -ENOMEM;
> > >
> > > Apparently, if the memory allocation only partially succeeds, in the theoretical case
> > > that the system is close to its kernel memory exhaustion, `return -ENOMEM` would not
> > > free the partially succeeded allocs, would it?
> > >
> > > To explain it better, I tried a version that is not yet full doing "all or nothing"
> > > memory allocation for the gpio-sim driver, because I am not that familiar with the
> > > driver internals.
> >
> > devm_*() mean that the resource allocation is made in a managed manner, so when
> > it's done, it will be freed automatically.
>
> Didn't see that one coming ... :-/ "buzzing though the bush ..."
>
> > The question is: is the lifetime of the attr_groups should be lesser or the
> > same as chip->gc.parent? Maybe it's incorrect to call devm_*() in the first place?
>
> Bona fide said, I hope that automatic deallocation does things in the right order.
> I've realised that devm_kzalloc() calls devm_kmalloc() that registers allocations on
> a per driver list. But I am not sure how chip->gc was allocated?
>
> Here is said it is allocated in drivers/gpio/gpio-sim.c:386 in gpio_sim_add_bank(),
> as a part of
>
>      struct gpio_sim_chip *chip;
>      struct gpio_chip *gc;
>
>      gc = &chip->gc;
>
> and gc->parent is set to
>
>      gc->parent = dev;
>
> in line 420, which appears called before gpio_sim_setup_sysfs() and the lines above.

P.S.

(I am sorry, but my Thunderbird made unreadable mess from the previous reply.)

The exact line is:

    chip  =  devm_kzalloc (dev,  sizeof(*chip),  GFP_KERNEL);

so I guess it is reasonable to assume that chip will also be deallocated after attr_groups.

chip->gc.parent appears to be a mere pointer to dev parameter in

    static  int  gpio_sim_add_bank(struct  fwnode_handle *swnode, struct device *dev)

This is OTOH called from:

    static  int  gpio_sim_probe (struct  platform_device *pdev)
    {
        struct  device          *dev = &pdev->dev;
        struct  fwnode_handle   *swnode;
        int  ret;

        device_for_each_child_node(dev,  swnode)  {
        	ret  =  gpio_sim_add_bank(swnode, dev);

Which means dev passed to chip->gc.parent is initialised with &pdev->dev from
pdev parm of gpio_sim_probe().

This is OTOH referenced from the very:

    static struct platform_driver gpio_sim_driver = {
	.driver = {
		.name = "gpio-sim",
		.of_match_table = gpio_sim_of_match,
	    },
	.probe = gpio_sim_probe
    };

Hope this helps.

There's more to this than meets the eye, but this is really an idiot's attempt to
analyse a Linux kernel driver. 😄

> If I understood well, automatic deallocation on unloading the driver goes
> in the reverse order, so lifetime of chip appears to be longer than attr_groups,
> but I am really not that good at this ...
>
> > Or maybe the chip->gc.parent should be changed to something else (actual GPIO
> > device, but then it's unclear how to provide the attributes in non-racy way
> Really, dunno. I have to repeat that my learning curve cannot adapt so quickly.
>
> I merely gave the report of KMEMLEAK, otherwise I am not a Linux kernel
> device expert nor would be appropriate to try the craft not earned ;-)
>
> Regards,
> Mirsad

--
Mirsad Todorovac
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb
Republic of Croatia, the European Union

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