On Sun, Apr 11, 2021 at 5:21 AM Dmitry Torokhov
[off-list ref] wrote:

Hi Bartosz,

On Mon, Jun 29, 2020 at 1:56 PM Bartosz Golaszewski [off-list ref] wrote:

quoted

From: Bartosz Golaszewski <redacted>

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