[RFC PATCH 3/7] Implement diskseq checks in blkback

[Demi Marie Obenour <demi@invisiblethingslab.com>]

From: Demi Marie Obenour <demiobenour@gmail.com>

This allows specifying a disk sequence number in XenStore.  If it does
not match the disk sequence number of the underlying device, the device
will not be exported and a warning will be logged.  Userspace can use
this to eliminate race conditions due to major/minor number reuse.
Older kernels will ignore this, so it is safe for userspace to set it
unconditionally.

This also makes physical-device parsing stricter.  I do not believe this
will break any extant userspace tools.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/block/xen-blkback/xenbus.c | 137 +++++++++++++++++++++--------
 1 file changed, 100 insertions(+), 37 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 4807af1d58059394d7a992335dabaf2bc3901721..2c43bfc7ab5ba6954f11d4b949a5668660dbd290 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -24,6 +24,7 @@ struct backend_info {
 	struct xenbus_watch	backend_watch;
 	unsigned		major;
 	unsigned		minor;
+	unsigned long long	diskseq;
 	char			*mode;
 };
 
@@ -479,7 +480,7 @@ static void xen_vbd_free(struct xen_vbd *vbd)
 
 static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 			  unsigned major, unsigned minor, int readonly,
-			  int cdrom)
+			  bool cdrom, u64 diskseq)
 {
 	struct xen_vbd *vbd;
 	struct block_device *bdev;
@@ -507,6 +508,25 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 		xen_vbd_free(vbd);
 		return -ENOENT;
 	}
+
+	if (diskseq) {
+		struct gendisk *disk = bdev->bd_disk;
+		if (unlikely(disk == NULL)) {
+			pr_err("xen_vbd_create: device %08x has no gendisk\n",
+			       vbd->pdevice);
+			xen_vbd_free(vbd);
+			return -EFAULT;
+		}
+
+		if (unlikely(disk->diskseq != diskseq)) {
+			pr_warn("xen_vbd_create: device %08x has incorrect sequence "
+				"number 0x%llx (expected 0x%llx)\n",
+				vbd->pdevice, disk->diskseq, diskseq);
+			xen_vbd_free(vbd);
+			return -ENODEV;
+		}
+	}
+
 	vbd->size = vbd_sz(vbd);
 
 	if (cdrom || disk_to_cdi(vbd->bdev->bd_disk))
@@ -690,6 +710,55 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 	return err;
 }
 
+static bool read_physical_device(struct xenbus_device *dev,
+				 unsigned long long *diskseq,
+				 unsigned *major, unsigned *minor)
+{
+	char *physical_device, *problem;
+	int i, physical_device_length;
+	char junk;
+
+	physical_device = xenbus_read(XBT_NIL, dev->nodename, "physical-device",
+				      &physical_device_length);
+
+	if (IS_ERR(physical_device)) {
+		int err = PTR_ERR(physical_device);
+		/*
+		 * Since this watch will fire once immediately after it is
+		 * registered, we expect "does not exist" errors.  Ignore
+		 * them and wait for the hotplug scripts.
+		 */
+		if (unlikely(!XENBUS_EXIST_ERR(err)))
+			xenbus_dev_fatal(dev, err, "reading physical-device");
+		return false;
+	}
+
+	for (i = 0; i < physical_device_length; ++i)
+		if (unlikely(physical_device[i] <= 0x20 || physical_device[i] >= 0x7F)) {
+			problem = "bad byte in physical-device";
+			goto fail;
+		}
+
+	if (sscanf(physical_device, "%16llx@%8x:%8x%c",
+		   diskseq, major, minor, &junk) == 3) {
+		if (*diskseq == 0) {
+			problem = "diskseq 0 is invalid";
+			goto fail;
+		}
+	} else if (sscanf(physical_device, "%8x:%8x%c", major, minor, &junk) == 2) {
+		*diskseq = 0;
+	} else {
+		problem = "invalid physical-device";
+		goto fail;
+	}
+	kfree(physical_device);
+	return true;
+fail:
+	kfree(physical_device);
+	xenbus_dev_fatal(dev, -EINVAL, problem);
+	return false;
+}
+
 /*
  * Callback received when the hotplug scripts have placed the physical-device
  * node.  Read it and the mode node, and create a vbd.  If the frontend is
@@ -707,28 +776,17 @@ static void backend_changed(struct xenbus_watch *watch,
 	int cdrom = 0;
 	unsigned long handle;
 	char *device_type;
+	unsigned long long diskseq;
 
 	pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
-
-	err = xenbus_scanf(XBT_NIL, dev->nodename, "physical-device", "%x:%x",
-			   &major, &minor);
-	if (XENBUS_EXIST_ERR(err)) {
-		/*
-		 * Since this watch will fire once immediately after it is
-		 * registered, we expect this.  Ignore it, and wait for the
-		 * hotplug scripts.
-		 */
+	if (!read_physical_device(dev, &diskseq, &major, &minor))
 		return;
-	}
-	if (err != 2) {
-		xenbus_dev_fatal(dev, err, "reading physical-device");
-		return;
-	}
 
-	if (be->major | be->minor) {
-		if (be->major != major || be->minor != minor)
-			pr_warn("changing physical device (from %x:%x to %x:%x) not supported.\n",
-				be->major, be->minor, major, minor);
+	if (be->major | be->minor | be->diskseq) {
+		if (be->major != major || be->minor != minor || be->diskseq != diskseq)
+			pr_warn("changing physical device (from %x:%x:%llx to %x:%x:%llx)"
+				" not supported.\n",
+				be->major, be->minor, be->diskseq, major, minor, diskseq);
 		return;
 	}
 
@@ -756,29 +814,34 @@ static void backend_changed(struct xenbus_watch *watch,
 
 	be->major = major;
 	be->minor = minor;
+	be->diskseq = diskseq;
 
 	err = xen_vbd_create(be->blkif, handle, major, minor,
-			     !strchr(be->mode, 'w'), cdrom);
-
-	if (err)
-		xenbus_dev_fatal(dev, err, "creating vbd structure");
-	else {
-		err = xenvbd_sysfs_addif(dev);
-		if (err) {
-			xen_vbd_free(&be->blkif->vbd);
-			xenbus_dev_fatal(dev, err, "creating sysfs entries");
-		}
-	}
+			     !strchr(be->mode, 'w'), cdrom, diskseq);
 
 	if (err) {
-		kfree(be->mode);
-		be->mode = NULL;
-		be->major = 0;
-		be->minor = 0;
-	} else {
-		/* We're potentially connected now */
-		xen_update_blkif_status(be->blkif);
+		xenbus_dev_fatal(dev, err, "creating vbd structure");
+		goto fail;
 	}
+
+	err = xenvbd_sysfs_addif(dev);
+	if (err) {
+		xenbus_dev_fatal(dev, err, "creating sysfs entries");
+		goto free_vbd;
+	}
+
+	/* We're potentially connected now */
+	xen_update_blkif_status(be->blkif);
+	return;
+
+free_vbd:
+	xen_vbd_free(&be->blkif->vbd);
+fail:
+	kfree(be->mode);
+	be->mode = NULL;
+	be->major = 0;
+	be->minor = 0;
+	be->diskseq = 0;
 }
 
 /*
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab