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

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Date:   Fri, 10 Jul 2020 18:03:36 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Jon Hunter <jonathanh@...dia.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 <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <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 Fri, Jul 10, 2020 at 3:46 PM Jon Hunter <jonathanh@...dia.com> wrote:
>
> Hi Bartosz,
>
> On 29/06/2020 07:50, Bartosz Golaszewski 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.
> >
> > Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>
> > ---
> >  drivers/base/devres.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> > index 1df1fb10b2d9..ed615d3b9cf1 100644
> > --- a/drivers/base/devres.c
> > +++ b/drivers/base/devres.c
> > @@ -819,6 +819,9 @@ void *devm_kmalloc(struct device *dev, size_t size, gfp_t gfp)
> >  {
> >       struct devres *dr;
> >
> > +     if (unlikely(!size))
> > +             return ZERO_SIZE_PTR;
> > +
> >       /* use raw alloc_dr for kmalloc caller tracing */
> >       dr = alloc_dr(devm_kmalloc_release, size, gfp, dev_to_node(dev));
> >       if (unlikely(!dr))
> > @@ -950,10 +953,10 @@ void devm_kfree(struct device *dev, const void *p)
> >       int rc;
> >
> >       /*
> > -      * Special case: pointer to a string in .rodata returned by
> > -      * devm_kstrdup_const().
> > +      * Special cases: pointer to a string in .rodata returned by
> > +      * devm_kstrdup_const() or NULL/ZERO ptr.
> >        */
> > -     if (unlikely(is_kernel_rodata((unsigned long)p)))
> > +     if (unlikely(is_kernel_rodata((unsigned long)p) || ZERO_OR_NULL_PTR(p)))
> >               return;
> >
> >       rc = devres_destroy(dev, devm_kmalloc_release,
>
>
> This change caught a bug in one of our Tegra drivers, which I am in the
> process of fixing. Once I bisected to this commit it was easy to track
> down, but I am wondering if there is any reason why we don't add a
> WARN_ON() if size is 0 in devm_kmalloc? It was essentially what I ended
> up doing to find the bug.
>
> Jon
>
> --
> nvpublic

Hi Jon,

this is in line with what the regular kmalloc() does. If size is zero,
it returns ZERO_SIZE_PTR. It's not an error condition. Actually in
user-space malloc() does a similar thing: for size == 0 it allocates
one-byte and returns a pointer to it (at least in glibc).

Bartosz