blkid: use /sys/block/dm-<N>/dm/name

The Linux kernel (since 2.6.29, patch 784aae735d9b0bba3f8b9faef4c8b30df3bf0128)
exports the real DM device names in /sys/block/<ptname>/dm/name.

The sysfs based solution is nicer and faster than scan for devno in
/dev/mapper/.

CC: Milan Broz <mbroz@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/lib/blkid/devname.c b/lib/blkid/devname.c
index ec19a2e..b151354 100644 (file)
--- a/lib/blkid/devname.c
+++ b/lib/blkid/devname.c
@@ -153,6 +153,30 @@ static int is_dm_leaf(const char *devname)
 }
 
 /*
+ * Since 2.6.29 (patch 784aae735d9b0bba3f8b9faef4c8b30df3bf0128) kernel sysfs
+ * provides the real DM device names in /sys/block/<ptname>/dm/name
+ */
+static char *get_dm_name(const char *ptname)
+{
+       FILE    *f;
+       size_t  sz;
+       char    path[256], name[256], *res = NULL;
+
+       snprintf(path, sizeof(path), "/sys/block/%s/dm/name", ptname);
+       if ((f = fopen(path, "r")) == NULL)
+               return NULL;
+
+       /* read "<name>\n" from sysfs */
+       if (fgets(name, sizeof(name), f) && (sz = strlen(name)) > 1) {
+               name[sz - 1] = '\0';
+               snprintf(path, sizeof(path), "/dev/mapper/%s", name);
+               res = blkid_strdup(path);
+       }
+       fclose(f);
+       return res;
+}
+
+/*
  * Probe a single block device to add to the device cache.
  */
 static void probe_one(blkid_cache cache, const char *ptname,
@@ -183,7 +207,9 @@ static void probe_one(blkid_cache cache, const char *ptname,
         * to standard /dev/mapper/<name>.
         */
        if (!strncmp(ptname, "dm-", 3) && isdigit(ptname[3])) {
-               blkid__scan_dir("/dev/mapper", devno, 0, &devname);
+               devname = get_dm_name(ptname);
+               if (!devname)
+                       blkid__scan_dir("/dev/mapper", devno, 0, &devname);
                if (devname)
                        goto get_dev;
        }
@@ -211,6 +237,8 @@ static void probe_one(blkid_cache cache, const char *ptname,
        }
        /* Do a short-cut scan of /dev/mapper first */
        if (!devname)
+               devname = get_dm_name(ptname);
+       if (!devname)
                blkid__scan_dir("/dev/mapper", devno, 0, &devname);
        if (!devname) {
                devname = blkid_devno_to_devname(devno);