From: Zdenek Kabelac <zkabelac@redhat.com>
Date: Fri, 17 Apr 2026 14:55:43 +0000 (+0200)
Subject: libblkid: introduce sysfs_devno_is_dm_hidden() for pre-open check
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=a42fbbb5a446f8f97a7b401653c98042be45d3df;p=thirdparty%2Futil-linux.git

libblkid: introduce sysfs_devno_is_dm_hidden() for pre-open check

Stratis reuses the DM "private" UUID convention ("stratis-1-private")
but its devices are legitimately opened by tools like mkfs.xfs to
obtain device geometry.  The pre-open check added in the previous
commit (d05a84b22) used sysfs_devno_is_dm_private() which also
blocks Stratis devices, breaking mkfs.xfs on Stratis pools.

Introduce sysfs_devno_is_dm_hidden() which only checks for LVM
internal devices (the "LVM-<uuid>-<name>" UUID pattern).  A "hidden"
DM device is one that exists solely as a building block for another
DM target -- no user-space tool should normally open or write to it.

Use sysfs_devno_is_dm_hidden() in blkid_new_probe_from_filename()
so that Stratis devices remain accessible while LVM internal devices
are still skipped before open().

Signed-off-by: Zdenek Kabelac <zkabelac@redhat.com>
---

diff --git a/include/sysfs.h b/include/sysfs.h
index 758340702..37f3b71ca 100644
--- a/include/sysfs.h
+++ b/include/sysfs.h
@@ -95,6 +95,7 @@ int sysfs_blkdev_get_wholedisk( struct path_cxt *pc,
 
 int sysfs_devno_to_wholedisk(dev_t dev, char *diskname,
                              size_t len, dev_t *diskdevno);
+int sysfs_devno_is_dm_hidden(dev_t devno, char **uuid);
 int sysfs_devno_is_dm_private(dev_t devno, char **uuid);
 int sysfs_devno_is_wholedisk(dev_t devno);
 
diff --git a/lib/sysfs.c b/lib/sysfs.c
index c2ffd429e..a856854bd 100644
--- a/lib/sysfs.c
+++ b/lib/sysfs.c
@@ -652,6 +652,48 @@ int sysfs_devno_to_wholedisk(dev_t devno, char *diskname,
 	return rc;
 }
 
+/*
+ * Returns 1 if the device is a hidden device-mapper device -- i.e. a device
+ * that exists only as a building block for another DM target and should not
+ * be opened by user-space tools (blkid, mkfs, etc.) during udev processing.
+ *
+ * Unlike sysfs_devno_is_dm_private(), this does NOT flag Stratis devices.
+ * Stratis places its own UUID ("stratis-1-private*") in the DM uuid field,
+ * but its devices are legitimately opened by tools such as mkfs.xfs to
+ * obtain device geometry.
+ *
+ * The @uuid (if not NULL) returns DM device UUID, use free() to deallocate.
+ */
+int sysfs_devno_is_dm_hidden(dev_t devno, char **uuid)
+{
+	struct path_cxt *pc = NULL;
+	char *id = NULL;
+	int rc = 0;
+
+	pc = ul_new_sysfs_path(devno, NULL, NULL);
+	if (!pc)
+		goto done;
+	if (ul_path_read_string(pc, &id, "dm/uuid") <= 0 || !id)
+		goto done;
+
+	/* Private LVM devices use "LVM-<uuid>-<name>" uuid format (important
+	 * is the "LVM" prefix and "-<name>" postfix).
+	 */
+	if (strncmp(id, "LVM-", 4) == 0) {
+		char *p = strrchr(id + 4, '-');
+
+		if (p && *(p + 1))
+			rc = 1;
+	}
+done:
+	ul_unref_path(pc);
+	if (uuid)
+		*uuid = id;
+	else
+		free(id);
+	return rc;
+}
+
 /*
  * Returns 1 if the device is private device mapper device. The @uuid
  * (if not NULL) returns DM device UUID, use free() to deallocate.
diff --git a/libblkid/src/probe.c b/libblkid/src/probe.c
index 84cffa40f..bbe6d8c46 100644
--- a/libblkid/src/probe.c
+++ b/libblkid/src/probe.c
@@ -219,13 +219,17 @@ blkid_probe blkid_new_probe_from_filename(const char *filename)
 	struct stat sb;
 
 	/*
-	 * Check for private device-mapper devices (LVM internals, etc.)
+	 * Check for hidden device-mapper devices (LVM internals, etc.)
 	 * before open() to avoid bumping the kernel open count.  A racing
 	 * DM_DEVICE_REMOVE would otherwise see EBUSY.
+	 *
+	 * Use sysfs_devno_is_dm_hidden() rather than _dm_private() so that
+	 * Stratis devices remain accessible to tools like mkfs.xfs that
+	 * need to probe device geometry.
 	 */
 	if (stat(filename, &sb) == 0 && S_ISBLK(sb.st_mode) &&
-	    sysfs_devno_is_dm_private(sb.st_rdev, NULL)) {
-		DBG(LOWPROBE, ul_debug("ignore private device mapper device"));
+	    sysfs_devno_is_dm_hidden(sb.st_rdev, NULL)) {
+		DBG(LOWPROBE, ul_debug("ignore hidden device mapper device"));
 		errno = EINVAL;
 		return NULL;
 	}