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Subject: Re: [PATCH v2 4/6] devres: handle zero size in devm_kmalloc()
To:     Bartosz Golaszewski <brgl@bgdev.pl>,
        Jonathan Corbet <corbet@lwn.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
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        Jean Delvare <jdelvare@suse.com>
CC:     <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-hwmon@vger.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>
References: <20200629065008.27620-1-brgl@bgdev.pl>
 <20200629065008.27620-5-brgl@bgdev.pl>
From:   Jon Hunter <jonathanh@nvidia.com>
Message-ID: <5c2e7514-b6d0-1331-37b0-d17a0cdb9693@nvidia.com>
Date:   Fri, 10 Jul 2020 14:46:22 +0100
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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