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

[Bartosz Golaszewski]

On Fri, Jul 10, 2020 at 6:11 PM Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
>
>
> On 10/07/2020 17:03, Bartosz Golaszewski wrote:
> > On Fri, Jul 10, 2020 at 3:46 PM Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
> >>
> >> Hi Bartosz,
> >>
> >> On 29/06/2020 07:50, Bartosz Golaszewski 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.
> >>>
> >>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
> >>> ---
> >>>  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).
>
>
> Yes that's fine, I was just wondering if there is any reason not to WARN
> as well?
>
> Cheers
> Jon
>

Why? Nothing bad happens. Regular kmalloc() doesn't warn, why should
devm_kmalloc() do?

Bartosz