Re: [PATCH v5 1/1] nvmem: Add support for write-only instances

[Srinivas Kandagatla]

Sorry for such late review, but I did endup finding two new issues with 
this patch.

On 18/03/2020 15:20, Nicholas Johnson wrote:
> There is at least one real-world use-case for write-only nvmem
> instances. Refer to 03cd45d2e219 ("thunderbolt: Prevent crash if
> non-active NVMem file is read").
>
> Add support for write-only nvmem instances by adding attrs for 0200.
>
> Change nvmem_register() to abort if NULL group is returned from
> nvmem_sysfs_get_groups().
>
> Return NULL from nvmem_sysfs_get_groups() in invalid cases.
>
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@xxxxxxxxxxxxxx>
> ---
>   drivers/nvmem/core.c        | 10 +++++--
>   drivers/nvmem/nvmem-sysfs.c | 56 +++++++++++++++++++++++++++++++------
>   2 files changed, 56 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 77d890d36..ddc7be514 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -381,6 +381,14 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>   	nvmem->type = config->type;
>   	nvmem->reg_read = config->reg_read;
>   	nvmem->reg_write = config->reg_write;
> +	nvmem->dev.groups = nvmem_sysfs_get_groups(nvmem, config);
> +	if (!nvmem->dev.groups) {
> +		ida_simple_remove(&nvmem_ida, nvmem->id);
> +		gpiod_put(nvmem->wp_gpio);
> +		kfree(nvmem);
> +		return ERR_PTR(-EINVAL);
> +	}

returning NULL is valid usecase here, consider it like not having 
CONFIG_NVMEM_SYSFS so no need to check the return value.

But a warning/error message from nvmem_sysfs_get_groups would help debug 
in case of invalid configurations.

> +
>   	if (!config->no_of_node)
>   		nvmem->dev.of_node = config->dev->of_node;
>   
> @@ -395,8 +403,6 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>   	nvmem->read_only = device_property_present(config->dev, "read-only") ||
>   			   config->read_only || !nvmem->reg_write;
>   
> -	nvmem->dev.groups = nvmem_sysfs_get_groups(nvmem, config);
> -
Moving this to before read_only flag is set would not work! as 
nvmem_sysfs_get_groups() is going to use read_only flag.

Ideally you do not need to change anything in core.c file and just 
update nvmem-sysfs.c should be enough.


>   	device_initialize(&nvmem->dev);
>   
>   	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> diff --git a/drivers/nvmem/nvmem-sysfs.c b/drivers/nvmem/nvmem-sysfs.c
> index 9e0c429cd..4ceac8680 100644
> --- a/drivers/nvmem/nvmem-sysfs.c
> +++ b/drivers/nvmem/nvmem-sysfs.c
> @@ -196,16 +196,49 @@ 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;
> +	/* Read-only */
> +	if (nvmem->reg_read && (!nvmem->reg_write || nvmem->read_only))
> +		return config->root_only ?
> +			nvmem_ro_root_dev_groups : nvmem_ro_dev_groups;
> +
> +	/* Read-write */
> +	if (nvmem->reg_read && nvmem->reg_write && !nvmem->read_only)
> +		return config->root_only ?
> +			nvmem_rw_root_dev_groups : nvmem_rw_dev_groups;
> +
> +	/* Write-only, do not honour request for global writable entry */
> +	if (!nvmem->reg_read && nvmem->reg_write && !nvmem->read_only)
> +		return config->root_only ? nvmem_wo_root_dev_groups : NULL;
>   
> -	return nvmem->read_only ? nvmem_ro_dev_groups : nvmem_rw_dev_groups;
> +	return NULL;
>   }
>   
>   /*
> @@ -224,17 +257,24 @@ 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 || nvmem->read_only)) {
>   		if (config->root_only)
>   			nvmem->eeprom = bin_attr_ro_root_nvmem;
>   		else
>   			nvmem->eeprom = bin_attr_ro_nvmem;
> -	} else {
> +	} else if (!nvmem->reg_read && nvmem->reg_write && !nvmem->read_only) {
> +		if (config->root_only)
> +			nvmem->eeprom = bin_attr_wo_root_nvmem;
> +		else
> +			return -EINVAL;
> +	} else if (nvmem->reg_read && nvmem->reg_write && !nvmem->read_only) {
>   		if (config->root_only)
>   			nvmem->eeprom = bin_attr_rw_root_nvmem;
>   		else
>   			nvmem->eeprom = bin_attr_rw_nvmem;
> -	}
> +	} else
> +		return -EINVAL;
> +
>   	nvmem->eeprom.attr.name = "eeprom";
>   	nvmem->eeprom.size = nvmem->size;
>   #ifdef CONFIG_DEBUG_LOCK_ALLOC