From: Karel Zak <kzak@redhat.com>
Date: Tue, 12 Jan 2021 10:43:31 +0000 (+0100)
Subject: lib/loopdev: make is_loopdev() more robust
X-Git-Tag: v2.37-rc1~188
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fbc2fe824dd3450df122d8416da8689f82fb0b4c;p=thirdparty%2Futil-linux.git

lib/loopdev: make is_loopdev() more robust

It seems the current kernel can create a loop devices with a different
major number. For example

  # losetup /dev/loop12345678 file.img
  # lsblk /dev/loop12345678
  NAME          MAJ:MIN    RM SIZE RO TYPE MOUNTPOINT
  loop12345678   15:811342  0   5M  0 loop

We need a way how to verify the device is loopdev also when the device is
not associated with any backing file -- in this case there is no "loop"
directory in /sys/dev/block/<maj:min>/, but we can cannonicalize this sysfs
symlink as it points to /sys/devices/virtual/block/loop<n> (see "loop" in
the path).

Note that without this change losetup is not able to list and delete
the loop device.

Addresses: https://github.com/karelzak/util-linux/issues/1202
Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/lib/loopdev.c b/lib/loopdev.c
index 2b909084e7..b946acf319 100644
--- a/lib/loopdev.c
+++ b/lib/loopdev.c
@@ -41,6 +41,7 @@
 #include "canonicalize.h"
 #include "blkdev.h"
 #include "debug.h"
+#include "fileutils.h"
 
 /*
  * Debug stuff (based on include/debug.h)
@@ -634,14 +635,30 @@ done:
 int is_loopdev(const char *device)
 {
 	struct stat st;
+	int rc = 0;
 
-	if (device && stat(device, &st) == 0 &&
-		S_ISBLK(st.st_mode) &&
-		major(st.st_rdev) == LOOPDEV_MAJOR)
-		return 1;
+	if (!device || stat(device, &st) != 0 || !S_ISBLK(st.st_mode))
+		rc = 0;
+	else if (major(st.st_rdev) == LOOPDEV_MAJOR)
+		rc = 1;
+	else {
+		/* It's possible that kernel creates a device with a different
+		 * major number ... check by /sys it's really loop device.
+		 */
+		char name[PATH_MAX], *cn, *p = NULL;
+
+		snprintf(name, sizeof(name), _PATH_SYS_DEVBLOCK "/%d:%d",
+				major(st.st_rdev), minor(st.st_rdev));
+		cn = canonicalize_path(name);
+		if (cn)
+			p = stripoff_last_component(cn);
+		rc = p && startswith(p, "loop");
+		free(cn);
+	}
 
-	errno = ENODEV;
-	return 0;
+	if (rc == 0)
+		errno = ENODEV;
+	return rc;
 }
 
 /*