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

Bartosz Golaszewski
2020-07-10T16:03:51Z

On Fri, Jul 10, 2020 at 3:46 PM Jon Hunter <jonathanh@nvidia.com> wrote:
>
> Hi Bartosz,
>
> On 29/06/2020 07:50, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bgolaszewski@baylibre.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@baylibre.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