btrfs-progs: Fix partitioned loop devices resolving

When using partitions on a loop device, the device's name can be
e.g. /dev/loop0p1 or similar, and no relevant entry exists in the /sys
filesystem, so the current resolve_loop_device function fails.

Instead of using string functions to extract the device name and reading
this file, this patch uses the loop device API through ioctl to get the
correct backing file.

Signed-off-by: Florian Margaine <florian@platform.sh>
[ changed checks of error values from open and ioctl ]
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 30 insertions, 1 deletions

diff --git a/utils.c b/utils.c
index 17f47574..4f3fbbae 100644
--- a/utils.c
+++ b/utils.c
@@ -1172,6 +1172,28 @@ static int is_loop_device (const char* device) {
 		MAJOR(statbuf.st_rdev) == LOOP_MAJOR);
 }
 
+/*
+ * Takes a loop device path (e.g. /dev/loop0) and returns
+ * the associated file (e.g. /images/my_btrfs.img) using
+ * loopdev API
+ */
+static int resolve_loop_device_with_loopdev(const char* loop_dev, char* loop_file)
+{
+	int fd;
+	struct loop_info64 lo64;
+
+	fd = open(loop_dev, O_RDONLY | O_NONBLOCK);
+	if (fd < 0)
+		return -errno;
+	if (ioctl(fd, LOOP_GET_STATUS64, &lo64) < 0)
+		return -errno;
+
+	memcpy(loop_file, lo64.lo_file_name, strlen(lo64.lo_file_name) + 1);
+	if (close(fd) < 0)
+		return -errno;
+
+	return 0;
+}
 
 /* Takes a loop device path (e.g. /dev/loop0) and returns
  * the associated file (e.g. /images/my_btrfs.img) */
@@ -1187,8 +1209,15 @@ static int resolve_loop_device(const char* loop_dev, char* loop_file,
 	if (!realpath(loop_dev, real_loop_dev))
 		return -errno;
 	snprintf(p, PATH_MAX, "/sys/block/%s/loop/backing_file", strrchr(real_loop_dev, '/'));
-	if (!(f = fopen(p, "r")))
+	if (!(f = fopen(p, "r"))) {
+		if (errno == ENOENT)
+			/*
+			 * It's possibly a partitioned loop device, which is
+			 * resolvable with loopdev API.
+			 */
+			return resolve_loop_device_with_loopdev(loop_dev, loop_file);
 		return -errno;
+	}
 
 	snprintf(fmt, 20, "%%%i[^\n]", max_len-1);
 	ret = fscanf(f, fmt, loop_file);