linux-kernel - Re: [PATCH v2 4/6] devres: handle zero size in devm

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Message-ID: <CAMRc=MfU8QFPWu_e6oQgtq0cBA34kCSi9=9-1iaFW4gW+yXN6A@mail.gmail.com>
Date:   Mon, 12 Apr 2021 21:23:06 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jean Delvare <jdelvare@...e.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, linux-hwmon@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v2 4/6] devres: handle zero size in devm_kmalloc()

On Sun, Apr 11, 2021 at 5:21 AM Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
>
> Hi Bartosz,
>
> On Mon, Jun 29, 2020 at 1:56 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> >
> > From: Bartosz Golaszewski <bgolaszewski@...libre.com>
> >
> > 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