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

[Bartosz Golaszewski]

On Sun, Apr 11, 2021 at 5:21 AM Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
>
> 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

You're right about the ordering being lost. At the same time
allocating 0 bytes is quite a special case and should result in
returning ZERO_SIZE_PTR as the fault dump resulting from its
dereference will indicate what the bug is.

I need to give it a thought because I'm not yet sure what the right
solution would be. Let me get back to you.

Bartosz