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

Jon Hunter
2020-07-10T13:46:33Z

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

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