[RFC][PATCH] device mapper: Add builtin function dm_get_status()

[Roberto Sassu]

> From: Roberto Sassu
> Sent: Wednesday, December 1, 2021 5:37 PM
> Users of the device mapper driver might want to obtain a device status,
> with status types defined in the status_type_t enumerator.
> 
> If a function to get the status is exported by the device mapper, when
> compiled as a module, it is not suitable to use by callers compiled builtin
> in the kernel.
> 
> Introduce the real function to get the status, _dm_get_status(), in the
> device mapper module, and add the stub dm_get_status() in dm-builtin.c, so
> that it can be invoked by builtin callers.
> 
> The stub calls the real function if the device mapper is compiled builtin
> or the module has been loaded. Calls to the real function are safely
> disabled if the module is unloaded. The race condition is avoided by
> incrementing the reference count of the module.
> 
> _dm_get_status() invokes the status() method for each device mapper table,
> which writes a string to the supplied buffer as output. The buffer might
> contain multiple strings concatenated together. If there is not enough
> space available, the string is truncated and a termination character is put
> at the end.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> ---
>  drivers/md/dm-builtin.c       | 13 +++++++
>  drivers/md/dm-core.h          |  5 +++
>  drivers/md/dm.c               | 71 +++++++++++++++++++++++++++++++++++
>  include/linux/device-mapper.h |  3 ++
>  4 files changed, 92 insertions(+)
> 
> diff --git a/drivers/md/dm-builtin.c b/drivers/md/dm-builtin.c
> index 8eb52e425141..cc1e9c27ab41 100644
> --- a/drivers/md/dm-builtin.c
> +++ b/drivers/md/dm-builtin.c
> @@ -47,3 +47,16 @@ void dm_kobject_release(struct kobject *kobj)
>  }
> 
>  EXPORT_SYMBOL(dm_kobject_release);
> +
> +dm_get_status_fn status_fn;
> +EXPORT_SYMBOL(status_fn);
> +
> +ssize_t dm_get_status(dev_t dev, status_type_t type, const char
> *target_name,
> +		      u8 *buf, size_t buf_len)
> +{
> +	if (status_fn)
> +		return status_fn(dev, type, target_name, buf, buf_len);
> +
> +	return -EOPNOTSUPP;
> +}
> +EXPORT_SYMBOL(dm_get_status);
> diff --git a/drivers/md/dm-core.h b/drivers/md/dm-core.h
> index b855fef4f38a..6600ec260558 100644
> --- a/drivers/md/dm-core.h
> +++ b/drivers/md/dm-core.h
> @@ -259,4 +259,9 @@ extern atomic_t dm_global_event_nr;
>  extern wait_queue_head_t dm_global_eventq;
>  void dm_issue_global_event(void);
> 
> +typedef ssize_t (*dm_get_status_fn)(dev_t dev, status_type_t type,
> +				    const char *target_name, u8 *buf,
> +				    size_t buf_len);
> +
> +extern dm_get_status_fn status_fn;
>  #endif
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 662742a310cb..55e59a4e3661 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -192,6 +192,74 @@ static unsigned dm_get_numa_node(void)
>  					 DM_NUMA_NODE,
> num_online_nodes() - 1);
>  }
> 
> +static ssize_t _dm_get_status(dev_t dev, status_type_t type,
> +			      const char *target_name, u8 *buf, size_t buf_len)
> +{
> +	struct mapped_device *md;
> +	struct dm_table *table;
> +	u8 *buf_ptr = buf;
> +	ssize_t len, res = 0;
> +	int srcu_idx, num_targets, i;
> +
> +	if (buf_len > INT_MAX)
> +		return -EINVAL;
> +
> +	if (!buf_len)
> +		return buf_len;
> +
> +	if (!try_module_get(THIS_MODULE))
> +		return -EBUSY;
> +
> +	md = dm_get_md(dev);
> +	if (!md) {
> +		res = -ENOENT;
> +		goto out_module;
> +	}
> +
> +	table = dm_get_live_table(md, &srcu_idx);
> +	if (!table) {
> +		res = -ENOENT;
> +		goto out_md;
> +	}
> +
> +	memset(buf, 0, buf_len);
> +
> +	num_targets = dm_table_get_num_targets(table);
> +
> +	for (i = 0; i < num_targets; i++) {
> +		struct dm_target *ti = dm_table_get_target(table, i);
> +
> +		if (!ti)
> +			continue;
> +
> +		if (target_name && strcmp(ti->type->name, target_name))
> +			continue;
> +
> +		if (!ti->type->status)
> +			continue;
> +
> +		ti->type->status(ti, type, 0, buf_ptr, buf + buf_len - buf_ptr);
> +
> +		if (!*buf_ptr)
> +			continue;
> +
> +		len = strlen(buf_ptr);
> +		buf_ptr += len + 1;
> +
> +		if (buf_ptr == buf + buf_len)
> +			break;
> +
> +		res += len + 1;

The line above before the check.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Zhong Ronghua

> +	}
> +
> +	dm_put_live_table(md, srcu_idx);
> +out_md:
> +	dm_put(md);
> +out_module:
> +	module_put(THIS_MODULE);
> +	return res;
> +}
> +
>  static int __init local_init(void)
>  {
>  	int r;
> @@ -275,6 +343,7 @@ static int __init dm_init(void)
>  			goto bad;
>  	}
> 
> +	status_fn = _dm_get_status;
>  	return 0;
>  bad:
>  	while (i--)
> @@ -287,6 +356,8 @@ static void __exit dm_exit(void)
>  {
>  	int i = ARRAY_SIZE(_exits);
> 
> +	status_fn = NULL;
> +
>  	while (i--)
>  		_exits[i]();
> 
> diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
> index a7df155ea49b..d97b296d3104 100644
> --- a/include/linux/device-mapper.h
> +++ b/include/linux/device-mapper.h
> @@ -487,6 +487,9 @@ int dm_report_zones(struct block_device *bdev,
> sector_t start, sector_t sector,
>  		    struct dm_report_zones_args *args, unsigned int nr_zones);
>  #endif /* CONFIG_BLK_DEV_ZONED */
> 
> +ssize_t dm_get_status(dev_t dev, status_type_t type, const char
> *target_name,
> +		      u8 *buf, size_t buf_len);
> +
>  /*
>   * Device mapper functions to parse and create devices specified by the
>   * parameter "dm-mod.create="
> --
> 2.32.0