Use /dev/loop-control if it exists

Loop devices can be added on the fly when needed, they're
not always created beforehand. The loop-control device can
be used to find and allocate the next available number
instead of going through the /dev directory contents (which
is now only a fallback mechanism).

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/lxc/bdev.c

@@ -1868,7 +1868,7 @@ static int loop_detect(const char *path)
 	return 0;
 }
 
-static int find_free_loopdev(int *retfd, char *namep)
+static int find_free_loopdev_no_control(int *retfd, char *namep)
 {
 	struct dirent dirent, *direntp;
 	struct loop_info64 lo;
@@ -1908,6 +1908,26 @@ static int find_free_loopdev(int *retfd, char *namep)
 	return 0;
 }
 
+static int find_free_loopdev(int *retfd, char *namep)
+{
+	int rc, fd = -1;
+	int ctl = open("/dev/loop-control", O_RDWR);
+	if (ctl < 0)
+		return find_free_loopdev_no_control(retfd, namep);
+	rc = ioctl(ctl, LOOP_CTL_GET_FREE);
+	if (rc >= 0) {
+		snprintf(namep, 100, "/dev/loop%d", rc);
+		fd = open(namep, O_RDWR);
+	}
+	close(ctl);
+	if (fd == -1) {
+		ERROR("No loop device found");
+		return -1;
+	}
+	*retfd = fd;
+	return 0;
+}
+
 static int loop_mount(struct bdev *bdev)
 {
 	int lfd, ffd = -1, ret = -1;