[PATCH 1/2] xen-blkback: Implement diskseq checks

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

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.
Old kernels do not support the new syntax, but a later patch will allow
userspace to discover that the new syntax is supported.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/block/xen-blkback/xenbus.c | 112 +++++++++++++++++++++++------
 1 file changed, 89 insertions(+), 23 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index 
4807af1d58059394d7a992335dabaf2bc3901721..9c3eb148fbd802c74e626c3d7bcd69dcb09bd921
 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 +508,26 @@ 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("%s: device %08x has no gendisk\n",
+                              __func__, vbd->pdevice);
+                       xen_vbd_free(vbd);
+                       return -EFAULT;
+               }
+
+               if (unlikely(disk->diskseq != diskseq)) {
+                       pr_warn("%s: device %08x has incorrect sequence "
+                               "number 0x%llx (expected 0x%llx)\n",
+                               __func__, 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))
@@ -707,6 +728,9 @@ static void backend_changed(struct xenbus_watch *watch,
        int cdrom = 0;
        unsigned long handle;
        char *device_type;
+       char *diskseq_str = NULL;
+       int diskseq_len;
+       unsigned long long diskseq;
 
        pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
 
@@ -725,10 +749,46 @@ static void backend_changed(struct xenbus_watch *watch,
                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);
+       diskseq_str = xenbus_read(XBT_NIL, dev->nodename, "diskseq", 
&diskseq_len);
+       if (IS_ERR(diskseq_str)) {
+               int err = PTR_ERR(diskseq_str);
+               diskseq_str = NULL;
+
+               /*
+                * If this does not exist, it means legacy userspace that does 
not
+                * support diskseq.
+                */
+               if (unlikely(!XENBUS_EXIST_ERR(err))) {
+                       xenbus_dev_fatal(dev, err, "reading diskseq");
+                       return;
+               }
+               diskseq = 0;
+       } else if (diskseq_len <= 0) {
+               xenbus_dev_fatal(dev, -EFAULT, "diskseq must not be empty");
+               goto fail;
+       } else if (diskseq_len > 16) {
+               xenbus_dev_fatal(dev, -ERANGE, "diskseq too long: got %d but 
limit is 16",
+                                diskseq_len);
+               goto fail;
+       } else if (diskseq_str[0] == '0') {
+               xenbus_dev_fatal(dev, -ERANGE, "diskseq must not start with 
'0'");
+               goto fail;
+       } else {
+               char *diskseq_end;
+               diskseq = simple_strtoull(diskseq_str, &diskseq_end, 16);
+               if (diskseq_end != diskseq_str + diskseq_len) {
+                       xenbus_dev_fatal(dev, -EINVAL, "invalid diskseq");
+                       goto fail;
+               }
+               kfree(diskseq_str);
+               diskseq_str = NULL;
+       }
+
+       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 +816,35 @@ 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(diskseq_str);
+       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