[lvm-devel] [PATCH 1/2] dev names: Add dm_device_get_name to acquire device name for given major:minor pair

[Zdenek Kabelac]

Dne 7.11.2011 13:49, Peter Rajnoha napsal(a):
> The code to acquire device's kernel name using sysfs is already upstream,
> this is just a tiny refactor that exports this functionality as libdevmapper fn.
> In addition, it adds a possibility to read the device-mapper name from sysfs
> using given major:minor pair (which is a quick operation).
> 
> The new fn "int dm_device_get_name(uint32_t major, uint32_t minor, int prefer_kernel_name, char *buf, size_t buf_size)"
> has a "pefer_kernel_name" switch with which it's possible to prefer kernel "dm-X"
> names over device-mapper names if this is a device-mapper device (for non-dm
> devices, only kernel name is possible).
> 
> Note:
> Since the /sys/block/major:minor/dm/name item was added later (kernel 2.6.29 iirc),
> there's a fallback to return kernel "dm-X" name if we're not able to get the
> dm name via sysfs (with some log_debug in place).
> 
> Peter
> ---
>  libdm/libdevmapper.h |    9 ++++
>  libdm/libdm-common.c |  122 ++++++++++++++++++++++++++++++++++++++++---------
>  2 files changed, 108 insertions(+), 23 deletions(-)
> 
> diff --git a/libdm/libdevmapper.h b/libdm/libdevmapper.h
> index c865f6f..d71d61e 100644
> --- a/libdm/libdevmapper.h
> +++ b/libdm/libdevmapper.h
> @@ -267,6 +267,15 @@ const char *dm_sysfs_dir(void);
>  int dm_is_dm_major(uint32_t major);
>  
>  /*
> + * Get device name for given major and minor number. If prefer_kernel_name
> + * is set, kernel name is preferred over dm name for dm devices. If sysfs
> + * is not used (or configured incorrectly), returns 0.
> + */
> +int dm_device_get_name(uint32_t major, uint32_t minor,
> +		       int prefer_kernel_name,
> +		       char *buf, size_t buf_size);
> +
> +/*
>   * Determine whether a device has any holders (devices
>   * using this device). If sysfs is not used (or configured
>   * incorrectly), returns 0.
> diff --git a/libdm/libdm-common.c b/libdm/libdm-common.c
> index 23539d7..5b9ba0b 100644
> --- a/libdm/libdm-common.c
> +++ b/libdm/libdm-common.c
> @@ -1056,6 +1056,103 @@ const char *dm_sysfs_dir(void)
>  	return _sysfs_dir;
>  }
>  
> +static int _sysfs_get_dm_name(uint32_t major, uint32_t minor, char *buf, size_t buf_size)
> +{
> +	char sysfs_path[PATH_MAX];
> +	char temp_buf[PATH_MAX];
> +	FILE *fp;
> +
> +	if (dm_snprintf(sysfs_path, PATH_MAX, "%sdev/block/%" PRIu32 ":%" PRIu32

I'd prefer to use  'sizeof(sysfs_path)' in these case - even though in this
case it doesn't really matter.



> +			"/dm/name", _sysfs_dir, major, minor) < 0) {
> +		log_error("_sysfs_get_dm_name: dm_snprintf failed");
> +		return 0;
> +	}
> +
> +	if (!(fp = fopen(sysfs_path, "r"))) {
> +		if (errno != ENOENT)
> +			log_sys_error("fopen", sysfs_path);
> +		else
> +			log_sys_debug("fopen", sysfs_path);
> +		return 0;
> +	}
> +
> +	if (!fgets(temp_buf, sizeof(temp_buf), fp)) {
> +		log_sys_debug("fgets", sysfs_path);

log_sys_error()

> +		return 0;

Well we should have 2 kinds of errors here - but since there is fallback,
it's not that important.

> +	}
> +	temp_buf[strlen(temp_buf) - 1] = '\0';

Maybe we should check here, the character is really '\n' before removing it.
But the behavior on the border PATH_MAX size might be tricky.


> +
> +	if (fclose(fp))
> +		log_sys_error("fclose", sysfs_path);
> +
> +	if (buf_size < strlen(temp_buf) + 1) {
> +		log_error("_sysfs_get_dm_name: output buffer too small");
> +		return 0;
> +	}
> +
> +	strcpy(buf, temp_buf);
+newline
> +	return 1;
> +}
> +
> +static int _sysfs_get_kernel_name(uint32_t major, uint32_t minor, char *buf, size_t buf_size)
> +{
> +	char sysfs_path[PATH_MAX];
> +	char temp_buf[PATH_MAX];
> +	ssize_t size;
> +	char *name;
> +
> +	if (dm_snprintf(sysfs_path, PATH_MAX, "%sdev/block/%" PRIu32 ":%" PRIu32,
> +			_sysfs_dir, major, minor) < 0) {
> +		log_error("_sysfs_get_kerne_name: dm_snprintf failed");
> +		return 0;
> +	}
> +
> +	if ((size = readlink(sysfs_path, temp_buf, PATH_MAX)) < 0) {
> +		if (errno != ENOENT)
> +			log_sys_error("readlink", sysfs_path);
> +		else
> +			log_sys_debug("readlink", sysfs_path);
> +		return 0;
> +	}
> +	temp_buf[size] = '\0';
> +
> +	if (!(name = strrchr(temp_buf, '/'))) {
> +		log_error("Could not locate device kernel name in sysfs path %s", temp_buf);
> +		return 0;
> +	}
> +	name += 1;
> +
> +	if (buf_size < strlen(name) + 1) {
> +		log_error("_sysfs_get_kernel_name: output buffer too small");
> +		return 0;
> +	}
> +
> +	strcpy(buf, name);
> +	return 1;
> +}
> +
> +int dm_device_get_name(uint32_t major, uint32_t minor, int prefer_kernel_name,
> +		       char *buf, size_t buf_size)
> +{
> +	if (!*_sysfs_dir)
> +		return 0;
> +
> +	/*
> +	 * device-mapper devices and prefer_kernel_name = 0
> +	 * get dm name by reading /sys/dev/block/major:minor/dm/name,
> +	 * fallback to _sysfs_get_kernel_name if not successful
> +	 */
> +	if (dm_is_dm_major(major) && !prefer_kernel_name &&
> +	    _sysfs_get_dm_name(major, minor, buf, buf_size))
> +		return 1;

+ stack; for fail case

> +
> +	/*
> +	 * non-device-mapper devices or prefer_kernel_name = 1
> +	 * get kernel name using readlink /sys/dev/block/major:minor -> .../dm-X
> +	 */
> +	return _sysfs_get_kernel_name(major, minor, buf, buf_size);
> +}
> +
>  int dm_device_has_holders(uint32_t major, uint32_t minor)
>  {
>  	char sysfs_path[PATH_MAX];
> @@ -1126,32 +1223,11 @@ static int _mounted_fs_on_device(const char *kernel_dev_name)
>  
>  int dm_device_has_mounted_fs(uint32_t major, uint32_t minor)
>  {
> -	char sysfs_path[PATH_MAX];
> -	char temp_path[PATH_MAX];
> -	char *kernel_dev_name;
> -	ssize_t size;
> -
> -	if (!*_sysfs_dir)
> -		return 0;
> +	char kernel_dev_name[PATH_MAX];
>  
>  	/* Get kernel device name first */
> -	if (dm_snprintf(sysfs_path, PATH_MAX, "%sdev/block/%" PRIu32 ":%" PRIu32,
> -			_sysfs_dir, major, minor) < 0) {
> -		log_error("sysfs_path dm_snprintf failed");
> -		return 0;
> -	}
> -
> -	if ((size = readlink(sysfs_path, temp_path, PATH_MAX)) < 0) {
> -		log_sys_error("readlink", sysfs_path);
> +	if (!dm_device_get_name(major, minor, 1, kernel_dev_name, PATH_MAX))
>  		return 0;
> -	}
> -	temp_path[size] = '\0';
> -
> -	if (!(kernel_dev_name = strrchr(temp_path, '/'))) {
> -		log_error("Could not locate device kernel name in sysfs path %s", temp_path);
> -		return 0;
> -	}
> -	kernel_dev_name += 1;
>  
>  	/* Check /sys/fs/<fs_name>/<kernel_dev_name> presence */
>  	return _mounted_fs_on_device(kernel_dev_name);
> 
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