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

Mirsad Goran Todorovac
2023-02-21T13:52:50Z

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.

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 Goran Todorovac
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia
The European Union