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Date:   Tue, 25 Feb 2020 14:51:41 +0200
From:   Mika Westerberg <mika.westerberg@linux.intel.com>
To:     Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Subject: Re: [PATCH v1 1/3] nvmem: Add support for write-only instances
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On Mon, Feb 24, 2020 at 05:42:33PM +0000, Nicholas Johnson wrote:
> Mika Westerberg requires write-only nvmem for the Thunderbolt driver.
> Refer to 03cd45d2e219 ("thunderbolt: Prevent crash if non-active NVMem
> file is read"). Hence, there is at least one real-world use for
> write-only nvmem instances.

Well, I don't require anything ;-) It is the thunderbolt driver that has
one nvmem device that is write-only and it may take advantage of this.

> Add support for write-only nvmem instances by changing the nvmem attrs
> to 0222 if the .reg_read callback is not populated.
> 
> Add a WARN_ON in case a driver populates neither .reg_read nor
> .reg_write because this behaviour should clearly never occur.
> 
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
> ---
>  drivers/nvmem/nvmem-sysfs.c | 77 +++++++++++++++++++++++++++++++++----
>  1 file changed, 70 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/nvmem/nvmem-sysfs.c b/drivers/nvmem/nvmem-sysfs.c
> index 9e0c429cd..be3b94f0b 100644
> --- a/drivers/nvmem/nvmem-sysfs.c
> +++ b/drivers/nvmem/nvmem-sysfs.c
> @@ -147,6 +147,30 @@ static const struct attribute_group *nvmem_ro_dev_groups[] = {
>  	NULL,
>  };
>  
> +/* write only permission */
> +static struct bin_attribute bin_attr_wo_nvmem = {
> +	.attr	= {
> +		.name	= "nvmem",
> +		.mode	= 0222,

I would say no sysfs attribute should ever be writable by the world.

Actually I think maybe we make this one only writeable by root, in other
words it would always require ->root_only to be set.

> +	},
> +	.write	= bin_attr_nvmem_write,
> +};
> +
> +static struct bin_attribute *nvmem_bin_wo_attributes[] = {
> +	&bin_attr_wo_nvmem,
> +	NULL,
> +};
> +
> +static const struct attribute_group nvmem_bin_wo_group = {
> +	.bin_attrs	= nvmem_bin_wo_attributes,
> +	.attrs		= nvmem_attrs,
> +};
> +
> +static const struct attribute_group *nvmem_wo_dev_groups[] = {
> +	&nvmem_bin_wo_group,
> +	NULL,
> +};
> +
>  /* default read/write permissions, root only */
>  static struct bin_attribute bin_attr_rw_root_nvmem = {
>  	.attr	= {
> @@ -196,16 +220,50 @@ static const struct attribute_group *nvmem_ro_root_dev_groups[] = {
>  	NULL,
>  };
>  
> +/* write only permission, root only */
> +static struct bin_attribute bin_attr_wo_root_nvmem = {
> +	.attr	= {
> +		.name	= "nvmem",
> +		.mode	= 0200,
> +	},
> +	.write	= bin_attr_nvmem_write,
> +};
> +
> +static struct bin_attribute *nvmem_bin_wo_root_attributes[] = {
> +	&bin_attr_wo_root_nvmem,
> +	NULL,
> +};
> +
> +static const struct attribute_group nvmem_bin_wo_root_group = {
> +	.bin_attrs	= nvmem_bin_wo_root_attributes,
> +	.attrs		= nvmem_attrs,
> +};
> +
> +static const struct attribute_group *nvmem_wo_root_dev_groups[] = {
> +	&nvmem_bin_wo_root_group,
> +	NULL,
> +};
> +
>  const struct attribute_group **nvmem_sysfs_get_groups(
>  					struct nvmem_device *nvmem,
>  					const struct nvmem_config *config)
>  {
> -	if (config->root_only)
> -		return nvmem->read_only ?
> -			nvmem_ro_root_dev_groups :
> -			nvmem_rw_root_dev_groups;
> -
> -	return nvmem->read_only ? nvmem_ro_dev_groups : nvmem_rw_dev_groups;
> +	/*
> +	 * If neither reg_read nor reg_write are provided, we cannot use this
> +	 * nvmem entry, as any operation will cause kernel mode NULL reference.
> +	 */
> +	WARN_ON(!nvmem->reg_read && !nvmem->reg_write);

This should also be documented in kernel-doc of struct nvmem_config.

> +
> +	if (nvmem->reg_read && nvmem->reg_write)
> +		return config->root_only ?
> +			nvmem_rw_root_dev_groups : nvmem_rw_dev_groups;
> +
> +	if (nvmem->reg_read && !nvmem->reg_write)
> +		return config->root_only ?
> +			nvmem_ro_root_dev_groups : nvmem_ro_dev_groups;
> +
> +	return config->root_only ?
> +		nvmem_wo_root_dev_groups : nvmem_wo_dev_groups;
>  }
>  
>  /*
> @@ -224,11 +282,16 @@ int nvmem_sysfs_setup_compat(struct nvmem_device *nvmem,
>  	if (!config->base_dev)
>  		return -EINVAL;
>  
> -	if (nvmem->read_only) {
> +	if (nvmem->reg_read && !nvmem->reg_write) {
>  		if (config->root_only)
>  			nvmem->eeprom = bin_attr_ro_root_nvmem;
>  		else
>  			nvmem->eeprom = bin_attr_ro_nvmem;
> +	} else if (!nvmem->reg_read && nvmem->reg_write) {
> +		if (config->root_only)
> +			nvmem->eeprom = bin_attr_wo_root_nvmem;
> +		else
> +			nvmem->eeprom = bin_attr_wo_nvmem;
>  	} else {
>  		if (config->root_only)
>  			nvmem->eeprom = bin_attr_rw_root_nvmem;
> -- 
> 2.25.1