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Date:   Mon, 28 Feb 2022 12:51:58 +0200
From:   Abel Vesa <abel.vesa@nxp.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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Subject: Re: [PATCH v3 01/11] driver: platform: Add helper for safer setting
 of driver_override
Message-ID: <YhypTr5754yK9WGi@abelvesa>
References: <20220227135214.145599-1-krzysztof.kozlowski@canonical.com>
 <20220227135214.145599-2-krzysztof.kozlowski@canonical.com>
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On 22-02-27 14:52:04, Krzysztof Kozlowski wrote:
> Several core drivers and buses expect that driver_override is a
> dynamically allocated memory thus later they can kfree() it.
> 
> However such assumption is not documented, there were in the past and
> there are already users setting it to a string literal. This leads to
> kfree() of static memory during device release (e.g. in error paths or
> during unbind):
> 
>     kernel BUG at ../mm/slub.c:3960!
>     Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
>     ...
>     (kfree) from [<c058da50>] (platform_device_release+0x88/0xb4)
>     (platform_device_release) from [<c0585be0>] (device_release+0x2c/0x90)
>     (device_release) from [<c0a69050>] (kobject_put+0xec/0x20c)
>     (kobject_put) from [<c0f2f120>] (exynos5_clk_probe+0x154/0x18c)
>     (exynos5_clk_probe) from [<c058de70>] (platform_drv_probe+0x6c/0xa4)
>     (platform_drv_probe) from [<c058b7ac>] (really_probe+0x280/0x414)
>     (really_probe) from [<c058baf4>] (driver_probe_device+0x78/0x1c4)
>     (driver_probe_device) from [<c0589854>] (bus_for_each_drv+0x74/0xb8)
>     (bus_for_each_drv) from [<c058b48c>] (__device_attach+0xd4/0x16c)
>     (__device_attach) from [<c058a638>] (bus_probe_device+0x88/0x90)
>     (bus_probe_device) from [<c05871fc>] (device_add+0x3dc/0x62c)
>     (device_add) from [<c075ff10>] (of_platform_device_create_pdata+0x94/0xbc)
>     (of_platform_device_create_pdata) from [<c07600ec>] (of_platform_bus_create+0x1a8/0x4fc)
>     (of_platform_bus_create) from [<c0760150>] (of_platform_bus_create+0x20c/0x4fc)
>     (of_platform_bus_create) from [<c07605f0>] (of_platform_populate+0x84/0x118)
>     (of_platform_populate) from [<c0f3c964>] (of_platform_default_populate_init+0xa0/0xb8)
>     (of_platform_default_populate_init) from [<c01031f8>] (do_one_initcall+0x8c/0x404)
>     (do_one_initcall) from [<c0f012c0>] (kernel_init_freeable+0x3d0/0x4d8)
>     (kernel_init_freeable) from [<c0a7def0>] (kernel_init+0x8/0x114)
>     (kernel_init) from [<c01010b4>] (ret_from_fork+0x14/0x20)
> 
> Provide a helper which clearly documents the usage of driver_override.
> This will allow later to reuse the helper and reduce amount of
> duplicated code.
> 
> Convert the platform driver to use new helper and make the
> driver_override field const char (it is not modified by the core).
> 
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
> ---
>  drivers/base/driver.c           | 51 +++++++++++++++++++++++++++++++++
>  drivers/base/platform.c         | 28 +++---------------
>  include/linux/device/driver.h   |  2 ++
>  include/linux/platform_device.h |  7 ++++-
>  4 files changed, 63 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/base/driver.c b/drivers/base/driver.c
> index 8c0d33e182fd..353750b0bbc5 100644
> --- a/drivers/base/driver.c
> +++ b/drivers/base/driver.c
> @@ -30,6 +30,57 @@ static struct device *next_device(struct klist_iter *i)
>  	return dev;
>  }
>  
> +/**
> + * driver_set_override() - Helper to set or clear driver override.
> + * @dev: Device to change
> + * @override: Address of string to change (e.g. &device->driver_override);
> + *            The contents will be freed and hold newly allocated override.
> + * @s: NUL terminated string, new driver name to force a match, pass empty
> + *     string to clear it
> + * @len: length of @s
> + *
> + * Helper to set or clear driver override in a device, intended for the cases
> + * when the driver_override field is allocated by driver/bus code.
> + *
> + * Returns: 0 on success or a negative error code on failure.
> + */
> +int driver_set_override(struct device *dev, const char **override,
> +			const char *s, size_t len)

TBH, I think it would make more sense to have this generic
driver_set_override receive only the dev and the string. And then,
each bus type will have their own implementation that handle things
their own way. This would allow all the drivers that will use this to
do something like this:

	ret = driver_set_override(&pdev->dev, "override_string");

I think it would look more cleaner.

> +{
> +	const char *new, *old;
> +	char *cp;
> +
> +	if (!dev || !override || !s)
> +		return -EINVAL;
> +
> +	/* We need to keep extra room for a newline */
> +	if (len >= (PAGE_SIZE - 1))
> +		return -EINVAL;
> +
> +	new = kstrndup(s, len, GFP_KERNEL);
> +	if (!new)
> +		return -ENOMEM;
> +
> +	cp = strchr(new, '\n');
> +	if (cp)
> +		*cp = '\0';
> +
> +	device_lock(dev);
> +	old = *override;
> +	if (cp != new) {
> +		*override = new;
> +	} else {
> +		kfree(new);
> +		*override = NULL;
> +	}
> +	device_unlock(dev);
> +
> +	kfree(old);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(driver_set_override);
> +
>  /**
>   * driver_for_each_device - Iterator for devices bound to a driver.
>   * @drv: Driver we're iterating.
> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> index 6cb04ac48bf0..8dd87f44bd74 100644
> --- a/drivers/base/platform.c
> +++ b/drivers/base/platform.c
> @@ -1275,31 +1275,11 @@ static ssize_t driver_override_store(struct device *dev,
>  				     const char *buf, size_t count)
>  {
>  	struct platform_device *pdev = to_platform_device(dev);
> -	char *driver_override, *old, *cp;
> -
> -	/* We need to keep extra room for a newline */
> -	if (count >= (PAGE_SIZE - 1))
> -		return -EINVAL;
> -
> -	driver_override = kstrndup(buf, count, GFP_KERNEL);
> -	if (!driver_override)
> -		return -ENOMEM;
> -
> -	cp = strchr(driver_override, '\n');
> -	if (cp)
> -		*cp = '\0';
> -
> -	device_lock(dev);
> -	old = pdev->driver_override;
> -	if (strlen(driver_override)) {
> -		pdev->driver_override = driver_override;
> -	} else {
> -		kfree(driver_override);
> -		pdev->driver_override = NULL;
> -	}
> -	device_unlock(dev);
> +	int ret;
>  
> -	kfree(old);
> +	ret = driver_set_override(dev, &pdev->driver_override, buf, count);
> +	if (ret)
> +		return ret;
>  
>  	return count;
>  }
> diff --git a/include/linux/device/driver.h b/include/linux/device/driver.h
> index 15e7c5e15d62..700453017e1c 100644
> --- a/include/linux/device/driver.h
> +++ b/include/linux/device/driver.h
> @@ -151,6 +151,8 @@ extern int __must_check driver_create_file(struct device_driver *driver,
>  extern void driver_remove_file(struct device_driver *driver,
>  			       const struct driver_attribute *attr);
>  
> +int driver_set_override(struct device *dev, const char **override,
> +			const char *s, size_t len);
>  extern int __must_check driver_for_each_device(struct device_driver *drv,
>  					       struct device *start,
>  					       void *data,
> diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
> index 7c96f169d274..e39963889aa3 100644
> --- a/include/linux/platform_device.h
> +++ b/include/linux/platform_device.h
> @@ -31,7 +31,12 @@ struct platform_device {
>  	struct resource	*resource;
>  
>  	const struct platform_device_id	*id_entry;
> -	char *driver_override; /* Driver name to force a match */
> +	/*
> +	 * Driver name to force a match.
> +	 * Do not set directly, because core frees it.
> +	 * Use driver_set_override() to set or clear it.
> +	 */
> +	const char *driver_override;
>  
>  	/* MFD cell pointer */
>  	struct mfd_cell *mfd_cell;
> -- 
> 2.32.0
>