Re: [PATCH v2 4/6] devres: handle zero size in devm_kmalloc()

[Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>]

Hi Bartosz,

On Mon, Jun 29, 2020 at 1:56 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
> From: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
>
> Make devm_kmalloc() behave similarly to non-managed kmalloc(): return
> ZERO_SIZE_PTR when requested size is 0. Update devm_kfree() to handle
> this case.

This is wrong if you consider devm_krealloc API that you added. The
premise of devm_krealloc() is that it does not disturb devres "stack",
however in this case there is no entry in the stack. Consider:

	ptr = devm_kzalloc(dev, 0, GFP_KERNEL);
	...
	more devm API calls
	...

	/* This allocation will be on top of devm stack, not bottom ! */
	ptr = devm_krealloc(dev, ptr, 16, GFP_KERNEL);

And also:

	ptr = devm_kzalloc(dev, 16, GFP_KERNEL);
	...
	more devm API calls
	...
	/* Here we lose out position */
	ptr = devm_krealloc(dev, ptr, 0, GFP_KERNEL);
	...
	/* and now our memory allocation will be released first */
	ptr = devm_krealloc(dev, ptr, 16, GFP_KERNEL);


IMO special-casing 0-size allocations for managed memory allocations
should not be done.

Thanks.

-- 
Dmitry