losetup: use loop-control to explicitly ask for device

Now we use LOOP_CTL_GET_FREE ioctl to ask for free device, for example

  losetup -f foo.img

Unfortunately, losetup(8) allows to ask for specified device

  losetup /dev/loop100 foo.img

and in this case we assume that the device already exists in the
system. This is incorrect, we should be able to use loop-control
LOOP_CTL_ADD ioctl to ask for the specified device.

Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/lib/loopdev.c b/lib/loopdev.c
index 9789feb88..27276bfab 100644
--- a/lib/loopdev.c
+++ b/lib/loopdev.c
@@ -1297,6 +1297,36 @@ int loopcxt_delete_device(struct loopdev_cxt *lc)
 	return 0;
 }
 
+int loopcxt_add_device(struct loopdev_cxt *lc)
+{
+	int rc = -EINVAL;
+	int ctl, nr = -1;
+	const char *p, *dev = loopcxt_get_device(lc);
+
+	if (!dev)
+		goto done;
+
+	if (!(lc->flags & LOOPDEV_FL_CONTROL)) {
+		rc = -ENOSYS;
+		goto done;
+	}
+
+	p = strrchr(dev, '/');
+	if (!p || (sscanf(p, "/loop%d", &nr) != 1 && sscanf(p, "/%d", &nr) != 1)
+	       || nr < 0)
+		goto done;
+
+	ctl = open(_PATH_DEV_LOOPCTL, O_RDWR|O_CLOEXEC);
+	if (ctl >= 0) {
+		DBG(lc, loopdev_debug("add_device %d", nr));
+		rc = ioctl(ctl, LOOP_CTL_ADD, nr);
+		close(ctl);
+	}
+done:
+	DBG(lc, loopdev_debug("add_device done [rc=%d]", rc));
+	return rc;
+}
+
 /*
  * Note that LOOP_CTL_GET_FREE ioctl is supported since kernel 3.1. In older
  * kernels we have to check all loop devices to found unused one.